diff --git a/search_api_db/tests/src/Kernel/BackendTest.php b/search_api_db/tests/src/Kernel/BackendTest.php
index 2263168..1c150f5 100644
--- a/search_api_db/tests/src/Kernel/BackendTest.php
+++ b/search_api_db/tests/src/Kernel/BackendTest.php
@@ -4,30 +4,17 @@ namespace Drupal\Tests\search_api_db\Kernel;
 
 use Drupal\Component\Render\FormattableMarkup;
 use Drupal\Core\Database\Database;
-use Drupal\Core\Field\FieldStorageDefinitionInterface;
-use Drupal\Core\StringTranslation\StringTranslationTrait;
-use Drupal\field\Entity\FieldConfig;
-use Drupal\field\Entity\FieldStorageConfig;
-use Drupal\KernelTests\KernelTestBase;
-use Drupal\search_api\Entity\Index;
 use Drupal\search_api\Entity\Server;
-use Drupal\search_api\IndexInterface;
-use Drupal\search_api\Query\QueryInterface;
-use Drupal\search_api\Query\ResultSetInterface;
 use Drupal\search_api\SearchApiException;
-use Drupal\search_api\Tests\ExampleContentTrait;
-use Drupal\search_api\Utility;
 use Drupal\search_api_db\Plugin\search_api\backend\Database as BackendDatabase;
+use Drupal\Tests\search_api\Kernel\BackendTestBase;
 
 /**
  * Tests index and search capabilities using the Database search backend.
  *
  * @group search_api
  */
-class BackendTest extends KernelTestBase {
-
-  use ExampleContentTrait;
-  use StringTranslationTrait;
+class BackendTest extends BackendTestBase {
 
   /**
    * Modules to enable for this test.
@@ -65,68 +52,13 @@ class BackendTest extends KernelTestBase {
   public function setUp() {
     parent::setUp();
 
-    $this->installSchema('search_api', array('search_api_item', 'search_api_task'));
-    $this->installSchema('system', array('router'));
-    $this->installSchema('user', array('users_data'));
-    $this->installEntitySchema('entity_test');
-
-    // Do not use a batch for tracking the initial items after creating an
-    // index when running the tests via the GUI. Otherwise, it seems Drupal's
-    // Batch API gets confused and the test fails.
-    \Drupal::state()->set('search_api_use_tracking_batch', FALSE);
-
-    $this->setUpExampleStructure();
-
     $this->installConfig(array('search_api_test_db'));
   }
 
   /**
-   * Tests various indexing scenarios for the Database search backend.
-   *
-   * Uses a single method to save time.
-   */
-  public function testFramework() {
-    $this->insertExampleContent();
-    $this->checkDefaultServer();
-    $this->checkServerTables();
-    $this->checkDefaultIndex();
-    $this->updateIndex();
-    $this->searchNoResults();
-    $this->indexItems($this->indexId);
-    $this->checkMultiValuedInfo();
-    $this->searchSuccess1();
-    $this->checkFacets();
-    $this->regressionTests();
-
-    $this->editServerPartial();
-    $this->searchSuccessPartial();
-
-    $this->editServer();
-    $this->searchSuccess2();
-    $this->clearIndex();
-
-    $this->enableHtmlFilter();
-    $this->indexItems($this->indexId);
-    $this->disableHtmlFilter();
-    $this->clearIndex();
-
-    $this->searchNoResults();
-    $this->regressionTests2();
-    $this->checkModuleUninstall();
-  }
-
-  /**
-   * Tests the server that was installed through default configuration files.
-   */
-  protected function checkDefaultServer() {
-    $server = $this->getServer();
-    $this->assertTrue((bool) $server, 'The server was successfully created.');
-  }
-
-  /**
    * Tests that all tables and all columns have been created.
    */
-  protected function checkServerTables() {
+  protected function checkServerBackend() {
     $db_info = $this->getIndexDbInfo();
     $normalized_storage_table = $db_info['index_table'];
     $field_tables = $db_info['field_tables'];
@@ -154,20 +86,6 @@ class BackendTest extends KernelTestBase {
   }
 
   /**
-   * Tests the index that was installed through default configuration files.
-   */
-  protected function checkDefaultIndex() {
-    $index = $this->getIndex();
-    $this->assertTrue((bool) $index, 'The index was successfully created.');
-
-    $this->assertEquals(array("entity:entity_test"), $index->getDatasourceIds(), 'Datasources are set correctly.');
-    $this->assertEquals('default', $index->getTrackerId(), 'Tracker is set correctly.');
-
-    $this->assertEquals(5, $index->getTrackerInstance()->getTotalItemsCount(), 'Correct item count.');
-    $this->assertEquals(0, $index->getTrackerInstance()->getIndexedItemsCount(), 'All items still need to be indexed.');
-  }
-
-  /**
    * Checks whether changes to the index's fields are picked up by the server.
    */
   protected function updateIndex() {
@@ -197,80 +115,6 @@ class BackendTest extends KernelTestBase {
   }
 
   /**
-   * Enables the "HTML Filter" processor for the index.
-   */
-  protected function enableHtmlFilter() {
-    /** @var \Drupal\search_api\IndexInterface $index */
-    $index = $this->getIndex();
-
-    $processor = \Drupal::getContainer()
-      ->get('plugin.manager.search_api.processor')
-      ->createInstance('html_filter');
-
-    $index->addProcessor($processor);
-    $index->save();
-
-    $this->assertArrayHasKey('html_filter', $index->getProcessors(), 'HTML filter processor is added.');
-  }
-
-  /**
-   * Disables the "HTML Filter" processor for the index.
-   */
-  protected function disableHtmlFilter() {
-    /** @var \Drupal\search_api\IndexInterface $index */
-    $index = $this->getIndex();
-    $index->removeField('body');
-    $index->removeProcessor('html_filter');
-    $index->save();
-
-    $this->assertArrayNotHasKey('html_filter', $index->getProcessors(), 'HTML filter processor is removed.');
-    $this->assertArrayNotHasKey('body', $index->getFields(), 'Body field is removed.');
-  }
-
-  /**
-   * Builds a search query for testing purposes.
-   *
-   * Used as a helper method during testing.
-   *
-   * @param string|array|null $keys
-   *   (optional) The search keys to set, if any.
-   * @param string[] $conditions
-   *   (optional) Conditions to set on the query, in the format "field,value".
-   * @param string[]|null $fields
-   *   (optional) Fulltext fields to search for the keys.
-   *
-   * @return \Drupal\search_api\Query\QueryInterface
-   *   A search query on the test index.
-   */
-  protected function buildSearch($keys = NULL, array $conditions = array(), array $fields = NULL) {
-    $query = $this->getIndex()->query();
-    if ($keys) {
-      $query->keys($keys);
-      if ($fields) {
-        $query->setFulltextFields($fields);
-      }
-    }
-    foreach ($conditions as $condition) {
-      list($field, $value) = explode(',', $condition, 2);
-      $query->addCondition($field, $value);
-    }
-    $query->range(0, 10);
-
-    return $query;
-  }
-
-  /**
-   * Tests that a search on the index doesn't have any results.
-   */
-  protected function searchNoResults() {
-    $results = $this->buildSearch('test')->execute();
-    $this->assertEquals(0, $results->getResultCount(), 'No search results returned without indexing.');
-    $this->assertEquals(array(), array_keys($results->getResultItems()), 'No search results returned without indexing.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-  }
-
-  /**
    * Verifies that the stored information about multi-valued fields is correct.
    */
   protected function checkMultiValuedInfo() {
@@ -291,368 +135,9 @@ class BackendTest extends KernelTestBase {
   }
 
   /**
-   * Tests whether some test searches have the correct results.
+   * Second server.
    */
-  protected function searchSuccess1() {
-    $results = $this->buildSearch('test')->range(1, 2)->sort('id', QueryInterface::SORT_ASC)->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Search for »test« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(2, 3)), array_keys($results->getResultItems()), 'Search for »test« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $ids = $this->getItemIds(array(2));
-    $id = reset($ids);
-    $this->assertEquals($id, key($results->getResultItems()));
-    $this->assertEquals($id, $results->getResultItems()[$id]->getId());
-    $this->assertEquals('entity:entity_test', $results->getResultItems()[$id]->getDatasourceId());
-
-    $results = $this->buildSearch('test foo')->sort('id', QueryInterface::SORT_ASC)->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Search for »test foo« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4)), array_keys($results->getResultItems()), 'Search for »test foo« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('foo', array('type,item'))->sort('id', QueryInterface::SORT_ASC)->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Search for »foo« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2)), array_keys($results->getResultItems()), 'Search for »foo« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $keys = array(
-      '#conjunction' => 'AND',
-      'test',
-      array(
-        '#conjunction' => 'OR',
-        'baz',
-        'foobar',
-      ),
-      array(
-        '#conjunction' => 'OR',
-        '#negation' => TRUE,
-        'bar',
-        'fooblob',
-      ),
-    );
-    $results = $this->buildSearch($keys)->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Complex search 1 returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(4)), array_keys($results->getResultItems()), 'Complex search 1 returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch()->sort('id');
-    $conditions = $query->createConditionGroup('OR');
-    $conditions->addCondition('name', 'bar');
-    $conditions->addCondition('body', 'bar');
-    $query->addConditionGroup($conditions);
-    $results = $query->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Search with multi-field fulltext filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3, 5)), array_keys($results->getResultItems()), 'Search with multi-field fulltext filter returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch()->addCondition('keywords', array('grape', 'apple'), 'IN')->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Query with IN filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(2, 4, 5)), array_keys($results->getResultItems()), 'Query with IN filter field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch()->addCondition('keywords', array('grape', 'apple'), 'NOT IN')->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Query with NOT IN filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 3)), array_keys($results->getResultItems()), 'Query with NOT IN filter field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch()->addCondition('width', array('0.9', '1.5'), 'BETWEEN')->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Query with BETWEEN filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(4)), array_keys($results->getResultItems()), 'Query with BETWEEN filter field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-  }
-
-  /**
-   * Tests whether facets work correctly.
-   */
-  protected function checkFacets() {
-    $query = $this->buildSearch();
-    $conditions = $query->createConditionGroup('OR', array('facet:' . 'category'));
-    $conditions->addCondition('category', 'article_category');
-    $query->addConditionGroup($conditions);
-    $facets['category'] = array(
-      'field' => 'category',
-      'limit' => 0,
-      'min_count' => 1,
-      'missing' => TRUE,
-      'operator' => 'or',
-    );
-    $query->setOption('search_api_facets', $facets);
-    $results = $query->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'OR facets query returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(4, 5)), array_keys($results->getResultItems()));
-    $expected = array(
-      array('count' => 2, 'filter' => '"article_category"'),
-      array('count' => 2, 'filter' => '"item_category"'),
-      array('count' => 1, 'filter' => '!'),
-    );
-    $category_facets = $results->getExtraData('search_api_facets')['category'];
-    usort($category_facets, array($this, 'facetCompare'));
-    $this->assertEquals($expected, $category_facets, 'Correct OR facets were returned');
-
-    $query = $this->buildSearch();
-    $conditions = $query->createConditionGroup('OR', array('facet:' . 'category'));
-    $conditions->addCondition('category', 'article_category');
-    $query->addConditionGroup($conditions);
-    $conditions = $query->createConditionGroup('AND');
-    $conditions->addCondition('category', NULL, '<>');
-    $query->addConditionGroup($conditions);
-    $facets['category'] = array(
-      'field' => 'category',
-      'limit' => 0,
-      'min_count' => 1,
-      'missing' => TRUE,
-      'operator' => 'or',
-    );
-    $query->setOption('search_api_facets', $facets);
-    $results = $query->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'OR facets query returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(4, 5)), array_keys($results->getResultItems()));
-    $expected = array(
-      array('count' => 2, 'filter' => '"article_category"'),
-      array('count' => 2, 'filter' => '"item_category"'),
-    );
-    $category_facets = $results->getExtraData('search_api_facets')['category'];
-    usort($category_facets, array($this, 'facetCompare'));
-    $this->assertEquals($expected, $category_facets, 'Correct OR facets were returned');
-  }
-
-  /**
-   * Edits the server to enable partial matches.
-   *
-   * @param bool $enable
-   *   (optional) Whether partial matching should be enabled or disabled.
-   */
-  protected function editServerPartial($enable = TRUE) {
-    $server = $this->getServer();
-    $backend_config = $server->getBackendConfig();
-    $backend_config['partial_matches'] = $enable;
-    $server->setBackendConfig($backend_config);
-    $this->assertTrue((bool) $server->save(), 'The server was successfully edited.');
-    $this->resetEntityCache();
-  }
-
-  /**
-   * Tests whether partial searches work.
-   */
-  protected function searchSuccessPartial() {
-    $results = $this->buildSearch('foobaz')->range(0, 1)->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Partial search for »foobaz« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Partial search for »foobaz« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('foo')
-      ->sort('search_api_relevance', QueryInterface::SORT_DESC)
-      ->sort('id')
-      ->execute();
-    $this->assertEquals(5, $results->getResultCount(), 'Partial search for »foo« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4, 3, 5)), array_keys($results->getResultItems()), 'Partial search for »foo« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('foo tes')->sort('id')->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Partial search for »foo tes« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Partial search for »foo tes« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('oob est')->sort('id')->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Partial search for »oob est« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3)), array_keys($results->getResultItems()), 'Partial search for »oob est« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('foo nonexistent')->execute();
-    $this->assertEquals(0, $results->getResultCount(), 'Partial search for »foo nonexistent« returned correct number of results.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('bar nonexistent')->execute();
-    $this->assertEquals(0, $results->getResultCount(), 'Partial search for »foo nonexistent« returned correct number of results.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $keys = array(
-      '#conjunction' => 'AND',
-      'oob',
-      array(
-        '#conjunction' => 'OR',
-        'est',
-        'nonexistent',
-      ),
-    );
-    $results = $this->buildSearch($keys)->sort('id')->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Partial search for complex keys returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3)), array_keys($results->getResultItems()), 'Partial search for complex keys returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('foo', array('category,item_category'))
-      ->sort('id', QueryInterface::SORT_DESC)
-      ->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Partial search for »foo« with additional filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(2, 1)), array_keys($results->getResultItems()), 'Partial search for »foo« with additional filter returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch()->sort('id');
-    $conditions = $query->createConditionGroup('OR');
-    $conditions->addCondition('name', 'test');
-    $conditions->addCondition('body', 'test');
-    $query->addConditionGroup($conditions);
-    $results = $query->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Partial search with multi-field fulltext filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Partial search with multi-field fulltext filter returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-  }
-
-  /**
-   * Edits the server to change the "Minimum word length" setting.
-   */
-  protected function editServer() {
-    $server = $this->getServer();
-    $backend_config = $server->getBackendConfig();
-    $backend_config['min_chars'] = 4;
-    $backend_config['partial_matches'] = FALSE;
-    $server->setBackendConfig($backend_config);
-    $success = (bool) $server->save();
-    $this->assertTrue($success, 'The server was successfully edited.');
-
-    $this->clearIndex();
-    $this->indexItems($this->indexId);
-
-    $this->resetEntityCache();
-  }
-
-  /**
-   * Tests the results of some test searches with minimum word length of 4.
-   */
-  protected function searchSuccess2() {
-    $results = $this->buildSearch('test')->range(1, 2)->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Search for »test« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(4, 1)), array_keys($results->getResultItems()), 'Search for »test« returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch()->sort('id');
-    $conditions = $query->createConditionGroup('OR');
-    $conditions->addCondition('name', 'test');
-    $conditions->addCondition('body', 'test');
-    $query->addConditionGroup($conditions);
-    $results = $query->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Search with multi-field fulltext filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Search with multi-field fulltext filter returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch(NULL, array('body,test foobar'))->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Search with multi-term fulltext filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Search with multi-term fulltext filter returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('test foo')->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Search for »test foo« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(2, 4, 1, 3)), array_keys($results->getResultItems()), 'Search for »test foo« returned correct result.');
-    $this->assertIgnored($results, array('foo'), 'Short key was ignored.');
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('foo', array('type,item'))->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Search for »foo« returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3)), array_keys($results->getResultItems()), 'Search for »foo« returned correct result.');
-    $this->assertIgnored($results, array('foo'), 'Short key was ignored.');
-    $this->assertWarnings($results, array((string) $this->t('No valid search keys were present in the query.')), '"No valid keys" warning was displayed.');
-
-    $keys = array(
-      '#conjunction' => 'AND',
-      'test',
-      array(
-        '#conjunction' => 'OR',
-        'baz',
-        'foobar',
-      ),
-      array(
-        '#conjunction' => 'OR',
-        '#negation' => TRUE,
-        'bar',
-        'fooblob',
-      ),
-    );
-    $results = $this->buildSearch($keys)->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Complex search 1 returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Complex search 1 returned correct result.');
-    $this->assertIgnored($results, array('baz', 'bar'), 'Correct keys were ignored.');
-    $this->assertWarnings($results);
-
-    $keys = array(
-      '#conjunction' => 'AND',
-      'test',
-      array(
-        '#conjunction' => 'OR',
-        'baz',
-        'foobar',
-      ),
-      array(
-        '#conjunction' => 'OR',
-        '#negation' => TRUE,
-        'bar',
-        'fooblob',
-      ),
-    );
-    $results = $this->buildSearch($keys)->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Complex search 2 returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Complex search 2 returned correct result.');
-    $this->assertIgnored($results, array('baz', 'bar'), 'Correct keys were ignored.');
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch(NULL, array('keywords,orange'))->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Filter query 1 on multi-valued field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 5)), array_keys($results->getResultItems()), 'Filter query 1 on multi-valued field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $conditions = array(
-      'keywords,orange',
-      'keywords,apple',
-    );
-    $results = $this->buildSearch(NULL, $conditions)->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Filter query 2 on multi-valued field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(2)), array_keys($results->getResultItems()), 'Filter query 2 on multi-valued field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch()->addCondition('keywords', 'orange', '<>')->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Negated filter on multi-valued field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3, 4)), array_keys($results->getResultItems()), 'Negated filter on multi-valued field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch()->addCondition('keywords', NULL)->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Query with NULL filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Query with NULL filter returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch()->addCondition('keywords', NULL, '<>')->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Query with NOT NULL filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'Query with NOT NULL filter returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-  }
-
-  /**
-   * Executes regression tests for issues that were already fixed.
-   */
-  protected function regressionTests() {
+  protected function regressionTestSecondServer() {
     /** @var \Drupal\search_api\ServerInterface $second_server */
     $second_server = Server::create(array(
       'id' => 'test2',
@@ -666,473 +151,10 @@ class BackendTest extends KernelTestBase {
     try {
       $second_server->search($query);
       $this->fail('Could execute a query for an index on a different server.');
-    }
-    catch (SearchApiException $e) {
+    } catch (SearchApiException $e) {
       $this->assertTrue(TRUE, 'Executing a query for an index on a different server throws an exception.');
     }
     $second_server->delete();
-
-    // Regression tests for #2007872.
-    $results = $this->buildSearch('test')->sort('id', QueryInterface::SORT_ASC)->sort('type', QueryInterface::SORT_ASC)->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Sorting on field with NULLs returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Sorting on field with NULLs returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch();
-    $conditions = $query->createConditionGroup('OR');
-    $conditions->addCondition('id', 3);
-    $conditions->addCondition('type', 'article');
-    $query->addConditionGroup($conditions);
-    $query->sort('id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'OR filter on field with NULLs returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3, 4, 5)), array_keys($results->getResultItems()), 'OR filter on field with NULLs returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    // Regression tests for #1863672.
-    $query = $this->buildSearch();
-    $conditions = $query->createConditionGroup('OR');
-    $conditions->addCondition('keywords', 'orange');
-    $conditions->addCondition('keywords', 'apple');
-    $query->addConditionGroup($conditions);
-    $query->sort('id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'OR filter on multi-valued field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'OR filter on multi-valued field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch();
-    $conditions = $query->createConditionGroup('OR');
-    $conditions->addCondition('keywords', 'orange');
-    $conditions->addCondition('keywords', 'strawberry');
-    $query->addConditionGroup($conditions);
-    $conditions = $query->createConditionGroup('OR');
-    $conditions->addCondition('keywords', 'apple');
-    $conditions->addCondition('keywords', 'grape');
-    $query->addConditionGroup($conditions);
-    $query->sort('id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Multiple OR filters on multi-valued field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(2, 4, 5)), array_keys($results->getResultItems()), 'Multiple OR filters on multi-valued field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch();
-    $conditions1 = $query->createConditionGroup('OR');
-    $conditions = $query->createConditionGroup('AND');
-    $conditions->addCondition('keywords', 'orange');
-    $conditions->addCondition('keywords', 'apple');
-    $conditions1->addConditionGroup($conditions);
-    $conditions = $query->createConditionGroup('AND');
-    $conditions->addCondition('keywords', 'strawberry');
-    $conditions->addCondition('keywords', 'grape');
-    $conditions1->addConditionGroup($conditions);
-    $query->addConditionGroup($conditions1);
-    $query->sort('id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'Complex nested filters on multi-valued field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(2, 4, 5)), array_keys($results->getResultItems()), 'Complex nested filters on multi-valued field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    // Regression tests for #2040543.
-    $query = $this->buildSearch();
-    $facets['category'] = array(
-      'field' => 'category',
-      'limit' => 0,
-      'min_count' => 1,
-      'missing' => TRUE,
-    );
-    $query->setOption('search_api_facets', $facets);
-    $query->range(0, 0);
-    $results = $query->execute();
-    $expected = array(
-      array('count' => 2, 'filter' => '"article_category"'),
-      array('count' => 2, 'filter' => '"item_category"'),
-      array('count' => 1, 'filter' => '!'),
-    );
-    $type_facets = $results->getExtraData('search_api_facets')['category'];
-    usort($type_facets, array($this, 'facetCompare'));
-    $this->assertEquals($expected, $type_facets, 'Correct facets were returned');
-
-    $query = $this->buildSearch();
-    $facets['category']['missing'] = FALSE;
-    $query->setOption('search_api_facets', $facets);
-    $query->range(0, 0);
-    $results = $query->execute();
-    $expected = array(
-      array('count' => 2, 'filter' => '"article_category"'),
-      array('count' => 2, 'filter' => '"item_category"'),
-    );
-    $type_facets = $results->getExtraData('search_api_facets')['category'];
-    usort($type_facets, array($this, 'facetCompare'));
-    $this->assertEquals($expected, $type_facets, 'Correct facets were returned');
-
-    // Regression tests for #2111753.
-    $keys = array(
-      '#conjunction' => 'OR',
-      'foo',
-      'test',
-    );
-    $query = $this->buildSearch($keys, array(), array('name'));
-    $query->sort('id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(3, $results->getResultCount(), 'OR keywords returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4)), array_keys($results->getResultItems()), 'OR keywords returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch($keys, array(), array('name', 'body'));
-    $query->range(0, 0);
-    $results = $query->execute();
-    $this->assertEquals(5, $results->getResultCount(), 'Multi-field OR keywords returned correct number of results.');
-    $this->assertFalse($results->getResultItems(), 'Multi-field OR keywords returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $keys = array(
-      '#conjunction' => 'OR',
-      'foo',
-      'test',
-      array(
-        '#conjunction' => 'AND',
-        'bar',
-        'baz',
-      ),
-    );
-    $query = $this->buildSearch($keys, array(), array('name'));
-    $query->sort('id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Nested OR keywords returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'Nested OR keywords returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $keys = array(
-      '#conjunction' => 'OR',
-      array(
-        '#conjunction' => 'AND',
-        'foo',
-        'test',
-      ),
-      array(
-        '#conjunction' => 'AND',
-        'bar',
-        'baz',
-      ),
-    );
-    $query = $this->buildSearch($keys, array(), array('name', 'body'));
-    $query->sort('id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'Nested multi-field OR keywords returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'Nested multi-field OR keywords returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    // Regression tests for #2127001.
-    $keys = array(
-      '#conjunction' => 'AND',
-      '#negation' => TRUE,
-      'foo',
-      'bar',
-    );
-    $results = $this->buildSearch($keys)->sort('search_api_id', QueryInterface::SORT_ASC)->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Negated AND fulltext search returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3, 4)), array_keys($results->getResultItems()), 'Negated AND fulltext search returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $keys = array(
-      '#conjunction' => 'OR',
-      '#negation' => TRUE,
-      'foo',
-      'baz',
-    );
-    $results = $this->buildSearch($keys)->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Negated OR fulltext search returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Negated OR fulltext search returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $keys = array(
-      '#conjunction' => 'AND',
-      'test',
-      array(
-        '#conjunction' => 'AND',
-        '#negation' => TRUE,
-        'foo',
-        'bar',
-      ),
-    );
-    $results = $this->buildSearch($keys)->sort('search_api_id', QueryInterface::SORT_ASC)->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Nested NOT AND fulltext search returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3, 4)), array_keys($results->getResultItems()), 'Nested NOT AND fulltext search returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    // Regression tests for #2136409.
-    $query = $this->buildSearch();
-    $query->addCondition('category', NULL);
-    $query->sort('search_api_id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'NULL filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'NULL filter returned correct result.');
-
-    $query = $this->buildSearch();
-    $query->addCondition('category', NULL, '<>');
-    $query->sort('search_api_id', QueryInterface::SORT_ASC);
-    $results = $query->execute();
-    $this->assertEquals(4, $results->getResultCount(), 'NOT NULL filter returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'NOT NULL filter returned correct result.');
-
-    // Regression tests for #1658964.
-    $query = $this->buildSearch();
-    $facets['type'] = array(
-      'field' => 'type',
-      'limit' => 0,
-      'min_count' => 0,
-      'missing' => TRUE,
-    );
-    $query->setOption('search_api_facets', $facets);
-    $query->addCondition('type', 'article');
-    $query->range(0, 0);
-    $results = $query->execute();
-    $expected = array(
-      array('count' => 2, 'filter' => '"article"'),
-      array('count' => 0, 'filter' => '!'),
-      array('count' => 0, 'filter' => '"item"'),
-    );
-    $facets = $results->getExtraData('search_api_facets', array())['type'];
-    usort($facets, array($this, 'facetCompare'));
-    $this->assertEquals($expected, $facets, 'Correct facets were returned');
-
-    // Regression tests for #2469547.
-    $query = $this->buildSearch();
-    $facets = array();
-    $facets['body'] = array(
-      'field' => 'body',
-      'limit' => 0,
-      'min_count' => 1,
-      'missing' => FALSE,
-    );
-    $query->setOption('search_api_facets', $facets);
-    $query->addCondition('id', 5, '<>');
-    $query->range(0, 0);
-    $results = $query->execute();
-    $expected = array(
-      array('count' => 4, 'filter' => '"test"'),
-      array('count' => 2, 'filter' => '"Case"'),
-      array('count' => 2, 'filter' => '"casE"'),
-      array('count' => 1, 'filter' => '"bar"'),
-      array('count' => 1, 'filter' => '"case"'),
-      array('count' => 1, 'filter' => '"foobar"'),
-    );
-    // We can't guarantee the order of returned facets, since "bar" and "foobar"
-    // both occur once, so we have to manually sort the returned facets first.
-    $facets = $results->getExtraData('search_api_facets', array())['body'];
-    usort($facets, array($this, 'facetCompare'));
-    $this->assertEquals($expected, $facets, 'Correct facets were returned for a fulltext field.');
-
-    // Regression tests for #1403916.
-    $query = $this->buildSearch('test foo');
-    $facets = array();
-    $facets['type'] = array(
-      'field' => 'type',
-      'limit' => 0,
-      'min_count' => 1,
-      'missing' => TRUE,
-    );
-    $query->setOption('search_api_facets', $facets);
-    $query->range(0, 0);
-    $results = $query->execute();
-    $expected = array(
-      array('count' => 2, 'filter' => '"item"'),
-      array('count' => 1, 'filter' => '"article"'),
-    );
-    $facets = $results->getExtraData('search_api_facets', array())['type'];
-    usort($facets, array($this, 'facetCompare'));
-    $this->assertEquals($expected, $facets, 'Correct facets were returned');
-
-    // Regression tests for #2557291.
-    $results = $this->buildSearch('case')->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Search for lowercase "case" returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Search for lowercase "case" returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('Case')->sort('search_api_id')->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Search for capitalized "Case" returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 3)), array_keys($results->getResultItems()), 'Search for capitalized "Case" returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('CASE')->execute();
-    $this->assertEquals(0, $results->getResultCount(), 'Search for non-existent uppercase version of "CASE" returned correct number of results.');
-    $this->assertEquals(array(), array_keys($results->getResultItems()), 'Search for non-existent uppercase version of "CASE" returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('föö')->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Search for keywords with umlauts returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Search for keywords with umlauts returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch('smile' . json_decode('"\u1F601"'))->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Search for keywords with umlauts returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Search for keywords with umlauts returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $results = $this->buildSearch()->addCondition('keywords', 'grape', '<>')->execute();
-    $this->assertEquals(2, $results->getResultCount(), 'Negated filter on multi-valued field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(1, 3)), array_keys($results->getResultItems()), 'Negated filter on multi-valued field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    // Regression tests for #2511860.
-    $query = $this->buildSearch();
-    $query->addCondition('body', 'ab xy');
-    $results = $query->execute();
-    $this->assertEquals(5, $results->getResultCount(), 'Fulltext filters on short words do not change the result.');
-
-    $query = $this->buildSearch();
-    $query->addCondition('body', 'ab ab');
-    $results = $query->execute();
-    $this->assertEquals(5, $results->getResultCount(), 'Fulltext filters on duplicate short words do not change the result.');
-  }
-
-  /**
-   * Compares two facet filters to determine their order.
-   *
-   * Used as a callback for usort() in regressionTests().
-   *
-   * Will first compare the counts, ranking facets with higher count first, and
-   * then by filter value.
-   *
-   * @param array $a
-   *   The first facet filter.
-   * @param array $b
-   *   The second facet filter.
-   *
-   * @return int
-   *   -1 or 1 if the first filter should, respectively, come before or after
-   *   the second; 0 if both facet filters are equal.
-   */
-  protected function facetCompare(array $a, array $b) {
-    if ($a['count'] != $b['count']) {
-      return $b['count'] - $a['count'];
-    }
-    return strcmp($a['filter'], $b['filter']);
-  }
-
-  /**
-   * Clears the test index.
-   */
-  protected function clearIndex() {
-    $this->getIndex()->clear();
-  }
-
-  /**
-   * Executes regression tests which are unpractical to run in between.
-   */
-  protected function regressionTests2() {
-    // Create a "prices" field on the test entity type.
-    FieldStorageConfig::create(array(
-      'field_name' => 'prices',
-      'entity_type' => 'entity_test',
-      'type' => 'decimal',
-      'cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
-    ))->save();
-    FieldConfig::create(array(
-      'field_name' => 'prices',
-      'entity_type' => 'entity_test',
-      'bundle' => 'item',
-      'label' => 'Prices',
-    ))->save();
-
-    // Regression test for #1916474.
-    /** @var \Drupal\search_api\IndexInterface $index */
-    $index = $this->getIndex();
-    $this->addField($index, 'prices', 'decimal');
-    $success = $index->save();
-    $this->assertTrue($success, 'The index field settings were successfully changed.');
-
-    // Reset the static cache so the new values will be available.
-    $this->resetEntityCache('server');
-    $this->resetEntityCache();
-
-    \Drupal::entityTypeManager()
-      ->getStorage('entity_test')
-      ->create(array(
-        'id' => 6,
-        'prices' => array('3.5', '3.25', '3.75', '3.5'),
-        'type' => 'item',
-      ))->save();
-
-    $this->indexItems($this->indexId);
-
-    $query = $this->buildSearch(NULL, array('prices,3.25'));
-    $results = $query->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Filter on decimal field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(6)), array_keys($results->getResultItems()), 'Filter on decimal field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $query = $this->buildSearch(NULL, array('prices,3.5'));
-    $results = $query->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Filter on decimal field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(6)), array_keys($results->getResultItems()), 'Filter on decimal field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    // Regression test for #2284199.
-    \Drupal::entityTypeManager()
-      ->getStorage('entity_test')
-      ->create(array(
-        'id' => 7,
-        'type' => 'item',
-      ))->save();
-
-    $count = $this->indexItems($this->indexId);
-    $this->assertEquals(1, $count, 'Indexing an item with an empty value for a non string field worked.');
-
-    // Regression test for #2471509.
-    $this->addField($index, 'body');
-    $index->save();
-    $this->indexItems($this->indexId);
-
-    \Drupal::entityTypeManager()
-      ->getStorage('entity_test')
-      ->create(array(
-        'id' => 8,
-        'name' => 'Article with long body',
-        'type' => 'article',
-        'body' => 'astringlongerthanfiftycharactersthatcantbestoredbythedbbackend',
-      ))->save();
-    $count = $this->indexItems($this->indexId);
-    $this->assertEquals(1, $count, 'Indexing an item with a word longer than 50 characters worked.');
-
-    // Regression test for #2616268.
-    $index = $this->getIndex();
-    $field = $index->getField('body')->setType('string');
-    $index->addField($field)->save();
-    $count = $this->indexItems($this->indexId);
-    $this->assertEquals(8, $count, 'Switching type from text to string worked.');
-
-    // For a string field, 50 characters shouldn't be a problem.
-    $query = $this->buildSearch(NULL, array('body,astringlongerthanfiftycharactersthatcantbestoredbythedbbackend'));
-    $results = $query->execute();
-    $this->assertEquals(1, $results->getResultCount(), 'Filter on new string field returned correct number of results.');
-    $this->assertEquals($this->getItemIds(array(8)), array_keys($results->getResultItems()), 'Filter on new string field returned correct result.');
-    $this->assertIgnored($results);
-    $this->assertWarnings($results);
-
-    $index->removeField('body');
-    $index->save();
   }
 
   /**
@@ -1187,92 +209,6 @@ class BackendTest extends KernelTestBase {
   }
 
   /**
-   * Asserts ignored fields from a set of search results.
-   *
-   * @param \Drupal\search_api\Query\ResultSetInterface $results
-   *   The results to check.
-   * @param array $ignored
-   *   (optional) The ignored keywords that should be present, if any.
-   * @param string $message
-   *   (optional) The message to be displayed with the assertion.
-   */
-  protected function assertIgnored(ResultSetInterface $results, array $ignored = array(), $message = 'No keys were ignored.') {
-    $this->assertEquals($ignored, $results->getIgnoredSearchKeys(), $message);
-  }
-
-  /**
-   * Asserts warnings from a set of search results.
-   *
-   * @param \Drupal\search_api\Query\ResultSetInterface $results
-   *   The results to check.
-   * @param array $warnings
-   *   (optional) The ignored warnings that should be present, if any.
-   * @param string $message
-   *   (optional) The message to be displayed with the assertion.
-   */
-  protected function assertWarnings(ResultSetInterface $results, array $warnings = array(), $message = 'No warnings were displayed.') {
-    $this->assertEquals($warnings, $results->getWarnings(), $message);
-  }
-
-  /**
-   * Retrieves the search server used by this test.
-   *
-   * @return \Drupal\search_api\ServerInterface
-   *   The search server.
-   */
-  protected function getServer() {
-    return Server::load($this->serverId);
-  }
-
-  /**
-   * Retrieves the search index used by this test.
-   *
-   * @return \Drupal\search_api\IndexInterface
-   *   The search index.
-   */
-  protected function getIndex() {
-    return Index::load($this->indexId);
-  }
-
-  /**
-   * Adds a field to a search index.
-   *
-   * The index will not be saved automatically.
-   *
-   * @param \Drupal\search_api\IndexInterface $index
-   *   The search index.
-   * @param string $property_name
-   *   The property's name.
-   * @param string $type
-   *   (optional) The field type.
-   */
-  protected function addField(IndexInterface $index, $property_name, $type = 'text') {
-    $field_info = array(
-      'label' => $property_name,
-      'type' => $type,
-      'datasource_id' => 'entity:entity_test',
-      'property_path' => $property_name,
-    );
-    $field = Utility::createField($index, $property_name, $field_info);
-    $index->addField($field);
-    $index->save();
-  }
-
-  /**
-   * Resets the entity cache for the specified entity.
-   *
-   * @param string $type
-   *   (optional) The type of entity whose cache should be reset. Either "index"
-   *   or "server".
-   */
-  protected function resetEntityCache($type = 'index') {
-    $entity_type_id = 'search_api_' . $type;
-    \Drupal::entityTypeManager()
-      ->getStorage($entity_type_id)
-      ->resetCache(array($this->{$type . 'Id'}));
-  }
-
-  /**
    * Retrieves the database information for the test index.
    *
    * @return array
diff --git a/tests/src/Kernel/BackendTestBase.php b/tests/src/Kernel/BackendTestBase.php
new file mode 100644
index 0000000..54cbdc7
--- /dev/null
+++ b/tests/src/Kernel/BackendTestBase.php
@@ -0,0 +1,1222 @@
+<?php
+
+namespace Drupal\Tests\search_api\Kernel;
+
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\search_api\Entity\Index;
+use Drupal\search_api\Entity\Server;
+use Drupal\search_api\IndexInterface;
+use Drupal\search_api\Query\QueryInterface;
+use Drupal\search_api\Query\ResultSetInterface;
+use Drupal\search_api\Tests\ExampleContentTrait;
+use Drupal\search_api\Utility;
+
+/**
+ * Basic tests for index and search capabilities for any search backend.
+ */
+abstract class BackendTestBase extends KernelTestBase {
+
+  use ExampleContentTrait;
+  use StringTranslationTrait;
+
+  /**
+   * Modules to enable for this test.
+   *
+   * @var string[]
+   */
+  public static $modules = array(
+    'field',
+    'search_api',
+    'user',
+    'system',
+    'entity_test',
+    'text',
+  );
+
+  /**
+   * A search server ID.
+   *
+   * @var string
+   */
+  protected $serverId = 'search_server';
+
+  /**
+   * A search index ID.
+   *
+   * @var string
+   */
+  protected $indexId = 'search_index';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    $this->installSchema('search_api', array('search_api_item', 'search_api_task'));
+    $this->installSchema('system', array('router'));
+    $this->installSchema('user', array('users_data'));
+    $this->installEntitySchema('entity_test');
+
+    // Do not use a batch for tracking the initial items after creating an
+    // index when running the tests via the GUI. Otherwise, it seems Drupal's
+    // Batch API gets confused and the test fails.
+    \Drupal::state()->set('search_api_use_tracking_batch', FALSE);
+
+    $this->setUpExampleStructure();
+  }
+
+  /**
+   * Tests various indexing scenarios for the search backend.
+   *
+   * Uses a single method to save time.
+   */
+  public function testFramework() {
+    $this->insertExampleContent();
+    $this->checkDefaultServer();
+    $this->checkServerBackend();
+    $this->checkDefaultIndex();
+    $this->updateIndex();
+    $this->searchNoResults();
+    $this->indexItems($this->indexId);
+    $this->checkMultiValuedInfo();
+    $this->searchSuccess();
+    $this->checkFacets();
+    $this->regressionTests();
+
+    $this->editServerPartial();
+    $this->searchSuccessPartial();
+
+    $this->editServerMinChars();
+    $this->searchSuccessMinChars();
+    $this->clearIndex();
+
+    $this->enableHtmlFilter();
+    $this->indexItems($this->indexId);
+    $this->disableHtmlFilter();
+    $this->clearIndex();
+
+    $this->searchNoResults();
+    $this->regressionTests2();
+    $this->checkModuleUninstall();
+  }
+
+  /**
+   * Tests the server that was installed through default configuration files.
+   */
+  protected function checkDefaultServer() {
+    $server = $this->getServer();
+    $this->assertTrue((bool) $server, 'The server was successfully created.');
+  }
+
+  /**
+   * Tests that all tables and all columns have been created.
+   */
+  abstract protected function checkServerBackend();
+
+  /**
+   * Tests the index that was installed through default configuration files.
+   */
+  protected function checkDefaultIndex() {
+    $index = $this->getIndex();
+    $this->assertTrue((bool) $index, 'The index was successfully created.');
+
+    $this->assertEquals(array("entity:entity_test"), $index->getDatasourceIds(), 'Datasources are set correctly.');
+    $this->assertEquals('default', $index->getTrackerId(), 'Tracker is set correctly.');
+
+    $this->assertEquals(5, $index->getTrackerInstance()->getTotalItemsCount(), 'Correct item count.');
+    $this->assertEquals(0, $index->getTrackerInstance()->getIndexedItemsCount(), 'All items still need to be indexed.');
+  }
+
+  /**
+   * Checks whether changes to the index's fields are picked up by the server.
+   */
+  abstract protected function updateIndex();
+
+  /**
+   * Enables the "HTML Filter" processor for the index.
+   */
+  protected function enableHtmlFilter() {
+    /** @var \Drupal\search_api\IndexInterface $index */
+    $index = $this->getIndex();
+
+    $processor = \Drupal::getContainer()
+      ->get('plugin.manager.search_api.processor')
+      ->createInstance('html_filter');
+
+    $index->addProcessor($processor);
+    $index->save();
+
+    $this->assertArrayHasKey('html_filter', $index->getProcessors(), 'HTML filter processor is added.');
+  }
+
+  /**
+   * Disables the "HTML Filter" processor for the index.
+   */
+  protected function disableHtmlFilter() {
+    /** @var \Drupal\search_api\IndexInterface $index */
+    $index = $this->getIndex();
+    $index->removeField('body');
+    $index->removeProcessor('html_filter');
+    $index->save();
+
+    $this->assertArrayNotHasKey('html_filter', $index->getProcessors(), 'HTML filter processor is removed.');
+    $this->assertArrayNotHasKey('body', $index->getFields(), 'Body field is removed.');
+  }
+
+  /**
+   * Builds a search query for testing purposes.
+   *
+   * Used as a helper method during testing.
+   *
+   * @param string|array|null $keys
+   *   (optional) The search keys to set, if any.
+   * @param string[] $conditions
+   *   (optional) Conditions to set on the query, in the format "field,value".
+   * @param string[]|null $fields
+   *   (optional) Fulltext fields to search for the keys.
+   *
+   * @return \Drupal\search_api\Query\QueryInterface
+   *   A search query on the test index.
+   */
+  protected function buildSearch($keys = NULL, array $conditions = array(), array $fields = NULL) {
+    $query = $this->getIndex()->query();
+    if ($keys) {
+      $query->keys($keys);
+      if ($fields) {
+        $query->setFulltextFields($fields);
+      }
+    }
+    foreach ($conditions as $condition) {
+      list($field, $value) = explode(',', $condition, 2);
+      $query->addCondition($field, $value);
+    }
+    $query->range(0, 10);
+
+    return $query;
+  }
+
+  /**
+   * Tests that a search on the index doesn't have any results.
+   */
+  protected function searchNoResults() {
+    $results = $this->buildSearch('test')->execute();
+    $this->assertEquals(0, $results->getResultCount(), 'No search results returned without indexing.');
+    $this->assertEquals(array(), array_keys($results->getResultItems()), 'No search results returned without indexing.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Verifies that the stored information about multi-valued fields is correct.
+   */
+  abstract protected function checkMultiValuedInfo();
+
+  /**
+   * Tests whether some test searches have the correct results.
+   */
+  protected function searchSuccess() {
+    $results = $this->buildSearch('test')->range(1, 2)->sort('id', QueryInterface::SORT_ASC)->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Search for »test« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(2, 3)), array_keys($results->getResultItems()), 'Search for »test« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $ids = $this->getItemIds(array(2));
+    $id = reset($ids);
+    $this->assertEquals($id, key($results->getResultItems()));
+    $this->assertEquals($id, $results->getResultItems()[$id]->getId());
+    $this->assertEquals('entity:entity_test', $results->getResultItems()[$id]->getDatasourceId());
+
+    $results = $this->buildSearch('test foo')->sort('id', QueryInterface::SORT_ASC)->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Search for »test foo« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4)), array_keys($results->getResultItems()), 'Search for »test foo« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('foo', array('type,item'))->sort('id', QueryInterface::SORT_ASC)->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Search for »foo« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2)), array_keys($results->getResultItems()), 'Search for »foo« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $keys = array(
+      '#conjunction' => 'AND',
+      'test',
+      array(
+        '#conjunction' => 'OR',
+        'baz',
+        'foobar',
+      ),
+      array(
+        '#conjunction' => 'OR',
+        '#negation' => TRUE,
+        'bar',
+        'fooblob',
+      ),
+    );
+    $results = $this->buildSearch($keys)->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Complex search 1 returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(4)), array_keys($results->getResultItems()), 'Complex search 1 returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch()->sort('id');
+    $conditions = $query->createConditionGroup('OR');
+    $conditions->addCondition('name', 'bar');
+    $conditions->addCondition('body', 'bar');
+    $query->addConditionGroup($conditions);
+    $results = $query->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Search with multi-field fulltext filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3, 5)), array_keys($results->getResultItems()), 'Search with multi-field fulltext filter returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch()->addCondition('keywords', array('grape', 'apple'), 'IN')->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Query with IN filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(2, 4, 5)), array_keys($results->getResultItems()), 'Query with IN filter field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch()->addCondition('keywords', array('grape', 'apple'), 'NOT IN')->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Query with NOT IN filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 3)), array_keys($results->getResultItems()), 'Query with NOT IN filter field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch()->addCondition('width', array('0.9', '1.5'), 'BETWEEN')->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Query with BETWEEN filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(4)), array_keys($results->getResultItems()), 'Query with BETWEEN filter field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Tests whether facets work correctly.
+   */
+  protected function checkFacets() {
+    $query = $this->buildSearch();
+    $conditions = $query->createConditionGroup('OR', array('facet:' . 'category'));
+    $conditions->addCondition('category', 'article_category');
+    $query->addConditionGroup($conditions);
+    $facets['category'] = array(
+      'field' => 'category',
+      'limit' => 0,
+      'min_count' => 1,
+      'missing' => TRUE,
+      'operator' => 'or',
+    );
+    $query->setOption('search_api_facets', $facets);
+    $results = $query->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'OR facets query returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(4, 5)), array_keys($results->getResultItems()));
+    $expected = array(
+      array('count' => 2, 'filter' => '"article_category"'),
+      array('count' => 2, 'filter' => '"item_category"'),
+      array('count' => 1, 'filter' => '!'),
+    );
+    $category_facets = $results->getExtraData('search_api_facets')['category'];
+    usort($category_facets, array($this, 'facetCompare'));
+    $this->assertEquals($expected, $category_facets, 'Correct OR facets were returned');
+
+    $query = $this->buildSearch();
+    $conditions = $query->createConditionGroup('OR', array('facet:' . 'category'));
+    $conditions->addCondition('category', 'article_category');
+    $query->addConditionGroup($conditions);
+    $conditions = $query->createConditionGroup('AND');
+    $conditions->addCondition('category', NULL, '<>');
+    $query->addConditionGroup($conditions);
+    $facets['category'] = array(
+      'field' => 'category',
+      'limit' => 0,
+      'min_count' => 1,
+      'missing' => TRUE,
+      'operator' => 'or',
+    );
+    $query->setOption('search_api_facets', $facets);
+    $results = $query->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'OR facets query returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(4, 5)), array_keys($results->getResultItems()));
+    $expected = array(
+      array('count' => 2, 'filter' => '"article_category"'),
+      array('count' => 2, 'filter' => '"item_category"'),
+    );
+    $category_facets = $results->getExtraData('search_api_facets')['category'];
+    usort($category_facets, array($this, 'facetCompare'));
+    $this->assertEquals($expected, $category_facets, 'Correct OR facets were returned');
+  }
+
+  /**
+   * Edits the server to enable partial matches.
+   *
+   * @param bool $enable
+   *   (optional) Whether partial matching should be enabled or disabled.
+   */
+  protected function editServerPartial($enable = TRUE) {
+    $server = $this->getServer();
+    $backend_config = $server->getBackendConfig();
+    $backend_config['partial_matches'] = $enable;
+    $server->setBackendConfig($backend_config);
+    $this->assertTrue((bool) $server->save(), 'The server was successfully edited.');
+    $this->resetEntityCache();
+  }
+
+  /**
+   * Tests whether partial searches work.
+   */
+  protected function searchSuccessPartial() {
+    $results = $this->buildSearch('foobaz')->range(0, 1)->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Partial search for »foobaz« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Partial search for »foobaz« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('foo')
+      ->sort('search_api_relevance', QueryInterface::SORT_DESC)
+      ->sort('id')
+      ->execute();
+    $this->assertEquals(5, $results->getResultCount(), 'Partial search for »foo« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4, 3, 5)), array_keys($results->getResultItems()), 'Partial search for »foo« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('foo tes')->sort('id')->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Partial search for »foo tes« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Partial search for »foo tes« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('oob est')->sort('id')->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Partial search for »oob est« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3)), array_keys($results->getResultItems()), 'Partial search for »oob est« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('foo nonexistent')->execute();
+    $this->assertEquals(0, $results->getResultCount(), 'Partial search for »foo nonexistent« returned correct number of results.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('bar nonexistent')->execute();
+    $this->assertEquals(0, $results->getResultCount(), 'Partial search for »foo nonexistent« returned correct number of results.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $keys = array(
+      '#conjunction' => 'AND',
+      'oob',
+      array(
+        '#conjunction' => 'OR',
+        'est',
+        'nonexistent',
+      ),
+    );
+    $results = $this->buildSearch($keys)->sort('id')->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Partial search for complex keys returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3)), array_keys($results->getResultItems()), 'Partial search for complex keys returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('foo', array('category,item_category'))
+      ->sort('id', QueryInterface::SORT_DESC)
+      ->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Partial search for »foo« with additional filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(2, 1)), array_keys($results->getResultItems()), 'Partial search for »foo« with additional filter returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch()->sort('id');
+    $conditions = $query->createConditionGroup('OR');
+    $conditions->addCondition('name', 'test');
+    $conditions->addCondition('body', 'test');
+    $query->addConditionGroup($conditions);
+    $results = $query->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Partial search with multi-field fulltext filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Partial search with multi-field fulltext filter returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Edits the server to change the "Minimum word length" setting.
+   */
+  protected function editServerMinChars() {
+    $server = $this->getServer();
+    $backend_config = $server->getBackendConfig();
+    $backend_config['min_chars'] = 4;
+    $backend_config['partial_matches'] = FALSE;
+    $server->setBackendConfig($backend_config);
+    $success = (bool) $server->save();
+    $this->assertTrue($success, 'The server was successfully edited.');
+
+    $this->clearIndex();
+    $this->indexItems($this->indexId);
+
+    $this->resetEntityCache();
+  }
+
+  /**
+   * Tests the results of some test searches with minimum word length of 4.
+   */
+  protected function searchSuccessMinChars() {
+    $results = $this->buildSearch('test')->range(1, 2)->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Search for »test« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(4, 1)), array_keys($results->getResultItems()), 'Search for »test« returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch()->sort('id');
+    $conditions = $query->createConditionGroup('OR');
+    $conditions->addCondition('name', 'test');
+    $conditions->addCondition('body', 'test');
+    $query->addConditionGroup($conditions);
+    $results = $query->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Search with multi-field fulltext filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Search with multi-field fulltext filter returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch(NULL, array('body,test foobar'))->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Search with multi-term fulltext filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Search with multi-term fulltext filter returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('test foo')->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Search for »test foo« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(2, 4, 1, 3)), array_keys($results->getResultItems()), 'Search for »test foo« returned correct result.');
+    $this->assertIgnored($results, array('foo'), 'Short key was ignored.');
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('foo', array('type,item'))->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Search for »foo« returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3)), array_keys($results->getResultItems()), 'Search for »foo« returned correct result.');
+    $this->assertIgnored($results, array('foo'), 'Short key was ignored.');
+    $this->assertWarnings($results, array((string) $this->t('No valid search keys were present in the query.')), '"No valid keys" warning was displayed.');
+
+    $keys = array(
+      '#conjunction' => 'AND',
+      'test',
+      array(
+        '#conjunction' => 'OR',
+        'baz',
+        'foobar',
+      ),
+      array(
+        '#conjunction' => 'OR',
+        '#negation' => TRUE,
+        'bar',
+        'fooblob',
+      ),
+    );
+    $results = $this->buildSearch($keys)->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Complex search 1 returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Complex search 1 returned correct result.');
+    $this->assertIgnored($results, array('baz', 'bar'), 'Correct keys were ignored.');
+    $this->assertWarnings($results);
+
+    $keys = array(
+      '#conjunction' => 'AND',
+      'test',
+      array(
+        '#conjunction' => 'OR',
+        'baz',
+        'foobar',
+      ),
+      array(
+        '#conjunction' => 'OR',
+        '#negation' => TRUE,
+        'bar',
+        'fooblob',
+      ),
+    );
+    $results = $this->buildSearch($keys)->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Complex search 2 returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Complex search 2 returned correct result.');
+    $this->assertIgnored($results, array('baz', 'bar'), 'Correct keys were ignored.');
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch(NULL, array('keywords,orange'))->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Filter query 1 on multi-valued field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 5)), array_keys($results->getResultItems()), 'Filter query 1 on multi-valued field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $conditions = array(
+      'keywords,orange',
+      'keywords,apple',
+    );
+    $results = $this->buildSearch(NULL, $conditions)->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Filter query 2 on multi-valued field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(2)), array_keys($results->getResultItems()), 'Filter query 2 on multi-valued field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch()->addCondition('keywords', 'orange', '<>')->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Negated filter on multi-valued field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3, 4)), array_keys($results->getResultItems()), 'Negated filter on multi-valued field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch()->addCondition('keywords', NULL)->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Query with NULL filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Query with NULL filter returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch()->addCondition('keywords', NULL, '<>')->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Query with NOT NULL filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'Query with NOT NULL filter returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Executes regression tests for issues that were already fixed.
+   */
+  protected function regressionTests() {
+    $this->regressionTestSecondServer();
+    $this->regressionTest2007872();
+    $this->regressionTest1863672();
+    $this->regressionTest2040543();
+    $this->regressionTest2111753();
+    $this->regressionTest2127001();
+    $this->regressionTest2136409();
+    $this->regressionTest1658964();
+    $this->regressionTest2469547();
+    $this->regressionTest1403916();
+    $this->regressionTest2557291();
+    $this->regressionTest2511860();
+  }
+
+  abstract protected function regressionTestSecondServer();
+
+  /**
+   * Regression tests for #2007872.
+   */
+  protected function regressionTest2007872() {
+    $results = $this->buildSearch('test')
+      ->sort('id', QueryInterface::SORT_ASC)
+      ->sort('type', QueryInterface::SORT_ASC)
+      ->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Sorting on field with NULLs returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 3, 4)), array_keys($results->getResultItems()), 'Sorting on field with NULLs returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch();
+    $conditions = $query->createConditionGroup('OR');
+    $conditions->addCondition('id', 3);
+    $conditions->addCondition('type', 'article');
+    $query->addConditionGroup($conditions);
+    $query->sort('id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'OR filter on field with NULLs returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3, 4, 5)), array_keys($results->getResultItems()), 'OR filter on field with NULLs returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Regression tests for #1863672.
+   */
+  protected function regressionTest1863672() {
+    $query = $this->buildSearch();
+    $conditions = $query->createConditionGroup('OR');
+    $conditions->addCondition('keywords', 'orange');
+    $conditions->addCondition('keywords', 'apple');
+    $query->addConditionGroup($conditions);
+    $query->sort('id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'OR filter on multi-valued field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'OR filter on multi-valued field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch();
+    $conditions = $query->createConditionGroup('OR');
+    $conditions->addCondition('keywords', 'orange');
+    $conditions->addCondition('keywords', 'strawberry');
+    $query->addConditionGroup($conditions);
+    $conditions = $query->createConditionGroup('OR');
+    $conditions->addCondition('keywords', 'apple');
+    $conditions->addCondition('keywords', 'grape');
+    $query->addConditionGroup($conditions);
+    $query->sort('id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Multiple OR filters on multi-valued field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(2, 4, 5)), array_keys($results->getResultItems()), 'Multiple OR filters on multi-valued field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch();
+    $conditions1 = $query->createConditionGroup('OR');
+    $conditions = $query->createConditionGroup('AND');
+    $conditions->addCondition('keywords', 'orange');
+    $conditions->addCondition('keywords', 'apple');
+    $conditions1->addConditionGroup($conditions);
+    $conditions = $query->createConditionGroup('AND');
+    $conditions->addCondition('keywords', 'strawberry');
+    $conditions->addCondition('keywords', 'grape');
+    $conditions1->addConditionGroup($conditions);
+    $query->addConditionGroup($conditions1);
+    $query->sort('id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'Complex nested filters on multi-valued field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(2, 4, 5)), array_keys($results->getResultItems()), 'Complex nested filters on multi-valued field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Regression tests for #2040543.
+   */
+  protected function regressionTest2040543() {
+    $query = $this->buildSearch();
+    $facets['category'] = array(
+      'field' => 'category',
+      'limit' => 0,
+      'min_count' => 1,
+      'missing' => TRUE,
+    );
+    $query->setOption('search_api_facets', $facets);
+    $query->range(0, 0);
+    $results = $query->execute();
+    $expected = array(
+      array('count' => 2, 'filter' => '"article_category"'),
+      array('count' => 2, 'filter' => '"item_category"'),
+      array('count' => 1, 'filter' => '!'),
+    );
+    $type_facets = $results->getExtraData('search_api_facets')['category'];
+    usort($type_facets, array($this, 'facetCompare'));
+    $this->assertEquals($expected, $type_facets, 'Correct facets were returned');
+
+    $query = $this->buildSearch();
+    $facets['category']['missing'] = FALSE;
+    $query->setOption('search_api_facets', $facets);
+    $query->range(0, 0);
+    $results = $query->execute();
+    $expected = array(
+      array('count' => 2, 'filter' => '"article_category"'),
+      array('count' => 2, 'filter' => '"item_category"'),
+    );
+    $type_facets = $results->getExtraData('search_api_facets')['category'];
+    usort($type_facets, array($this, 'facetCompare'));
+    $this->assertEquals($expected, $type_facets, 'Correct facets were returned');
+  }
+
+  /**
+   * Regression tests for #2111753.
+   */
+  protected function regressionTest2111753() {
+    $keys = array(
+      '#conjunction' => 'OR',
+      'foo',
+      'test',
+    );
+    $query = $this->buildSearch($keys, array(), array('name'));
+    $query->sort('id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(3, $results->getResultCount(), 'OR keywords returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4)), array_keys($results->getResultItems()), 'OR keywords returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch($keys, array(), array('name', 'body'));
+    $query->range(0, 0);
+    $results = $query->execute();
+    $this->assertEquals(5, $results->getResultCount(), 'Multi-field OR keywords returned correct number of results.');
+    $this->assertFalse($results->getResultItems(), 'Multi-field OR keywords returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $keys = array(
+      '#conjunction' => 'OR',
+      'foo',
+      'test',
+      array(
+        '#conjunction' => 'AND',
+        'bar',
+        'baz',
+      ),
+    );
+    $query = $this->buildSearch($keys, array(), array('name'));
+    $query->sort('id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Nested OR keywords returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'Nested OR keywords returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $keys = array(
+      '#conjunction' => 'OR',
+      array(
+        '#conjunction' => 'AND',
+        'foo',
+        'test',
+      ),
+      array(
+        '#conjunction' => 'AND',
+        'bar',
+        'baz',
+      ),
+    );
+    $query = $this->buildSearch($keys, array(), array('name', 'body'));
+    $query->sort('id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'Nested multi-field OR keywords returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'Nested multi-field OR keywords returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Regression tests for #2127001.
+   */
+  protected function regressionTest2127001() {
+    $keys = array(
+      '#conjunction' => 'AND',
+      '#negation' => TRUE,
+      'foo',
+      'bar',
+    );
+    $results = $this->buildSearch($keys)->sort('search_api_id', QueryInterface::SORT_ASC)->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Negated AND fulltext search returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3, 4)), array_keys($results->getResultItems()), 'Negated AND fulltext search returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $keys = array(
+      '#conjunction' => 'OR',
+      '#negation' => TRUE,
+      'foo',
+      'baz',
+    );
+    $results = $this->buildSearch($keys)->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Negated OR fulltext search returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'Negated OR fulltext search returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $keys = array(
+      '#conjunction' => 'AND',
+      'test',
+      array(
+        '#conjunction' => 'AND',
+        '#negation' => TRUE,
+        'foo',
+        'bar',
+      ),
+    );
+    $results = $this->buildSearch($keys)->sort('search_api_id', QueryInterface::SORT_ASC)->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Nested NOT AND fulltext search returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3, 4)), array_keys($results->getResultItems()), 'Nested NOT AND fulltext search returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Regression tests for #2136409.
+   */
+  protected function regressionTest2136409() {
+    $query = $this->buildSearch();
+    $query->addCondition('category', NULL);
+    $query->sort('search_api_id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'NULL filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(3)), array_keys($results->getResultItems()), 'NULL filter returned correct result.');
+
+    $query = $this->buildSearch();
+    $query->addCondition('category', NULL, '<>');
+    $query->sort('search_api_id', QueryInterface::SORT_ASC);
+    $results = $query->execute();
+    $this->assertEquals(4, $results->getResultCount(), 'NOT NULL filter returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 2, 4, 5)), array_keys($results->getResultItems()), 'NOT NULL filter returned correct result.');
+  }
+
+  /**
+   * Regression tests for #1658964.
+   */
+  protected function regressionTest1658964() {
+    $query = $this->buildSearch();
+    $facets['type'] = array(
+      'field' => 'type',
+      'limit' => 0,
+      'min_count' => 0,
+      'missing' => TRUE,
+    );
+    $query->setOption('search_api_facets', $facets);
+    $query->addCondition('type', 'article');
+    $query->range(0, 0);
+    $results = $query->execute();
+    $expected = array(
+      array('count' => 2, 'filter' => '"article"'),
+      array('count' => 0, 'filter' => '!'),
+      array('count' => 0, 'filter' => '"item"'),
+    );
+    $facets = $results->getExtraData('search_api_facets', array())['type'];
+    usort($facets, array($this, 'facetCompare'));
+    $this->assertEquals($expected, $facets, 'Correct facets were returned');
+  }
+
+  /**
+   * Regression tests for #2469547.
+   */
+  protected function regressionTest2469547() {
+    $query = $this->buildSearch();
+    $facets = array();
+    $facets['body'] = array(
+      'field' => 'body',
+      'limit' => 0,
+      'min_count' => 1,
+      'missing' => FALSE,
+    );
+    $query->setOption('search_api_facets', $facets);
+    $query->addCondition('id', 5, '<>');
+    $query->range(0, 0);
+    $results = $query->execute();
+    $expected = array(
+      array('count' => 4, 'filter' => '"test"'),
+      array('count' => 2, 'filter' => '"Case"'),
+      array('count' => 2, 'filter' => '"casE"'),
+      array('count' => 1, 'filter' => '"bar"'),
+      array('count' => 1, 'filter' => '"case"'),
+      array('count' => 1, 'filter' => '"foobar"'),
+    );
+    // We can't guarantee the order of returned facets, since "bar" and "foobar"
+    // both occur once, so we have to manually sort the returned facets first.
+    $facets = $results->getExtraData('search_api_facets', array())['body'];
+    usort($facets, array($this, 'facetCompare'));
+    $this->assertEquals($expected, $facets, 'Correct facets were returned for a fulltext field.');
+  }
+
+  /**
+   * Regression tests for #1403916.
+   */
+  protected function regressionTest1403916() {
+    $query = $this->buildSearch('test foo');
+    $facets = array();
+    $facets['type'] = array(
+      'field' => 'type',
+      'limit' => 0,
+      'min_count' => 1,
+      'missing' => TRUE,
+    );
+    $query->setOption('search_api_facets', $facets);
+    $query->range(0, 0);
+    $results = $query->execute();
+    $expected = array(
+      array('count' => 2, 'filter' => '"item"'),
+      array('count' => 1, 'filter' => '"article"'),
+    );
+    $facets = $results->getExtraData('search_api_facets', array())['type'];
+    usort($facets, array($this, 'facetCompare'));
+    $this->assertEquals($expected, $facets, 'Correct facets were returned');
+  }
+
+  /**
+   * Regression tests for #2557291.
+   */
+  protected function regressionTest2557291() {
+    $results = $this->buildSearch('case')->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Search for lowercase "case" returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Search for lowercase "case" returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('Case')->sort('search_api_id')->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Search for capitalized "Case" returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 3)), array_keys($results->getResultItems()), 'Search for capitalized "Case" returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('CASE')->execute();
+    $this->assertEquals(0, $results->getResultCount(), 'Search for non-existent uppercase version of "CASE" returned correct number of results.');
+    $this->assertEquals(array(), array_keys($results->getResultItems()), 'Search for non-existent uppercase version of "CASE" returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('föö')->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Search for keywords with umlauts returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Search for keywords with umlauts returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch('smile' . json_decode('"\u1F601"'))->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Search for keywords with umlauts returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1)), array_keys($results->getResultItems()), 'Search for keywords with umlauts returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $results = $this->buildSearch()->addCondition('keywords', 'grape', '<>')->execute();
+    $this->assertEquals(2, $results->getResultCount(), 'Negated filter on multi-valued field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(1, 3)), array_keys($results->getResultItems()), 'Negated filter on multi-valued field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Regression tests for #2511860.
+   */
+  protected function regressionTest2511860() {
+    $query = $this->buildSearch();
+    $query->addCondition('body', 'ab xy');
+    $results = $query->execute();
+    $this->assertEquals(5, $results->getResultCount(), 'Fulltext filters on short words do not change the result.');
+
+    $query = $this->buildSearch();
+    $query->addCondition('body', 'ab ab');
+    $results = $query->execute();
+    $this->assertEquals(5, $results->getResultCount(), 'Fulltext filters on duplicate short words do not change the result.');
+  }
+
+  /**
+   * Compares two facet filters to determine their order.
+   *
+   * Used as a callback for usort() in regressionTests().
+   *
+   * Will first compare the counts, ranking facets with higher count first, and
+   * then by filter value.
+   *
+   * @param array $a
+   *   The first facet filter.
+   * @param array $b
+   *   The second facet filter.
+   *
+   * @return int
+   *   -1 or 1 if the first filter should, respectively, come before or after
+   *   the second; 0 if both facet filters are equal.
+   */
+  protected function facetCompare(array $a, array $b) {
+    if ($a['count'] != $b['count']) {
+      return $b['count'] - $a['count'];
+    }
+    return strcmp($a['filter'], $b['filter']);
+  }
+
+  /**
+   * Clears the test index.
+   */
+  protected function clearIndex() {
+    $this->getIndex()->clear();
+  }
+
+  /**
+   * Executes regression tests which are unpractical to run in between.
+   */
+  protected function regressionTests2() {
+    // Create a "prices" field on the test entity type.
+    FieldStorageConfig::create(array(
+      'field_name' => 'prices',
+      'entity_type' => 'entity_test',
+      'type' => 'decimal',
+      'cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
+    ))->save();
+    FieldConfig::create(array(
+      'field_name' => 'prices',
+      'entity_type' => 'entity_test',
+      'bundle' => 'item',
+      'label' => 'Prices',
+    ))->save();
+
+    $this->regressionTest1916474();
+    $this->regressionTest2284199();
+    $this->regressionTest2471509();
+    $this->regressionTest2616268();
+  }
+
+  /**
+   * Regression tests for #1916474.
+   */
+  protected function regressionTest1916474() {
+    /** @var \Drupal\search_api\IndexInterface $index */
+    $index = $this->getIndex();
+    $this->addField($index, 'prices', 'decimal');
+    $success = $index->save();
+    $this->assertTrue($success, 'The index field settings were successfully changed.');
+
+    // Reset the static cache so the new values will be available.
+    $this->resetEntityCache('server');
+    $this->resetEntityCache();
+
+    \Drupal::entityTypeManager()
+      ->getStorage('entity_test')
+      ->create(array(
+        'id' => 6,
+        'prices' => array('3.5', '3.25', '3.75', '3.5'),
+        'type' => 'item',
+      ))->save();
+
+    $this->indexItems($this->indexId);
+
+    $query = $this->buildSearch(NULL, array('prices,3.25'));
+    $results = $query->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Filter on decimal field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(6)), array_keys($results->getResultItems()), 'Filter on decimal field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $query = $this->buildSearch(NULL, array('prices,3.5'));
+    $results = $query->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Filter on decimal field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(6)), array_keys($results->getResultItems()), 'Filter on decimal field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+  }
+
+  /**
+   * Regression tests for #2284199.
+   */
+  protected function regressionTest2284199() {
+    \Drupal::entityTypeManager()
+      ->getStorage('entity_test')
+      ->create(array(
+        'id' => 7,
+        'type' => 'item',
+      ))->save();
+
+    $count = $this->indexItems($this->indexId);
+    $this->assertEquals(1, $count, 'Indexing an item with an empty value for a non string field worked.');
+  }
+
+  /**
+   * Regression tests for #2471509.
+   */
+  protected function regressionTest2471509() {
+    /** @var \Drupal\search_api\IndexInterface $index */
+    $index = $this->getIndex();
+    $this->addField($index, 'body');
+    $index->save();
+    $this->indexItems($this->indexId);
+
+    \Drupal::entityTypeManager()
+      ->getStorage('entity_test')
+      ->create(array(
+        'id' => 8,
+        'name' => 'Article with long body',
+        'type' => 'article',
+        'body' => 'astringlongerthanfiftycharactersthatcantbestoredbythedbbackend',
+      ))->save();
+    $count = $this->indexItems($this->indexId);
+    $this->assertEquals(1, $count, 'Indexing an item with a word longer than 50 characters worked.');
+  }
+
+  /**
+   * Regression tests for #2616268.
+   */
+  protected function regressionTest2616268() {
+    /** @var \Drupal\search_api\IndexInterface $index */
+    $index = $this->getIndex();
+    $field = $index->getField('body')->setType('string');
+    $index->addField($field)->save();
+    $count = $this->indexItems($this->indexId);
+    $this->assertEquals(8, $count, 'Switching type from text to string worked.');
+
+    // For a string field, 50 characters shouldn't be a problem.
+    $query = $this->buildSearch(NULL, array('body,astringlongerthanfiftycharactersthatcantbestoredbythedbbackend'));
+    $results = $query->execute();
+    $this->assertEquals(1, $results->getResultCount(), 'Filter on new string field returned correct number of results.');
+    $this->assertEquals($this->getItemIds(array(8)), array_keys($results->getResultItems()), 'Filter on new string field returned correct result.');
+    $this->assertIgnored($results);
+    $this->assertWarnings($results);
+
+    $index->removeField('body');
+    $index->save();
+  }
+
+  /**
+   * Tests whether removing the configuration again works as it should.
+   */
+  abstract protected function checkModuleUninstall();
+
+  /**
+   * Asserts ignored fields from a set of search results.
+   *
+   * @param \Drupal\search_api\Query\ResultSetInterface $results
+   *   The results to check.
+   * @param array $ignored
+   *   (optional) The ignored keywords that should be present, if any.
+   * @param string $message
+   *   (optional) The message to be displayed with the assertion.
+   */
+  protected function assertIgnored(ResultSetInterface $results, array $ignored = array(), $message = 'No keys were ignored.') {
+    $this->assertEquals($ignored, $results->getIgnoredSearchKeys(), $message);
+  }
+
+  /**
+   * Asserts warnings from a set of search results.
+   *
+   * @param \Drupal\search_api\Query\ResultSetInterface $results
+   *   The results to check.
+   * @param array $warnings
+   *   (optional) The ignored warnings that should be present, if any.
+   * @param string $message
+   *   (optional) The message to be displayed with the assertion.
+   */
+  protected function assertWarnings(ResultSetInterface $results, array $warnings = array(), $message = 'No warnings were displayed.') {
+    $this->assertEquals($warnings, $results->getWarnings(), $message);
+  }
+
+  /**
+   * Retrieves the search server used by this test.
+   *
+   * @return \Drupal\search_api\ServerInterface
+   *   The search server.
+   */
+  protected function getServer() {
+    return Server::load($this->serverId);
+  }
+
+  /**
+   * Retrieves the search index used by this test.
+   *
+   * @return \Drupal\search_api\IndexInterface
+   *   The search index.
+   */
+  protected function getIndex() {
+    return Index::load($this->indexId);
+  }
+
+  /**
+   * Adds a field to a search index.
+   *
+   * The index will not be saved automatically.
+   *
+   * @param \Drupal\search_api\IndexInterface $index
+   *   The search index.
+   * @param string $property_name
+   *   The property's name.
+   * @param string $type
+   *   (optional) The field type.
+   */
+  protected function addField(IndexInterface $index, $property_name, $type = 'text') {
+    $field_info = array(
+      'label' => $property_name,
+      'type' => $type,
+      'datasource_id' => 'entity:entity_test',
+      'property_path' => $property_name,
+    );
+    $field = Utility::createField($index, $property_name, $field_info);
+    $index->addField($field);
+    $index->save();
+  }
+
+  /**
+   * Resets the entity cache for the specified entity.
+   *
+   * @param string $type
+   *   (optional) The type of entity whose cache should be reset. Either "index"
+   *   or "server".
+   */
+  protected function resetEntityCache($type = 'index') {
+    $entity_type_id = 'search_api_' . $type;
+    \Drupal::entityTypeManager()
+      ->getStorage($entity_type_id)
+      ->resetCache(array($this->{$type . 'Id'}));
+  }
+
+}