diff --git a/core/lib/Drupal/Core/KeyValueStore/DatabaseStorageSortedSet.php b/core/lib/Drupal/Core/KeyValueStore/DatabaseStorageSortedSet.php
index c9b83e3..bbd56bd 100644
--- a/core/lib/Drupal/Core/KeyValueStore/DatabaseStorageSortedSet.php
+++ b/core/lib/Drupal/Core/KeyValueStore/DatabaseStorageSortedSet.php
@@ -5,6 +5,12 @@
 use Drupal\Component\Serialization\SerializationInterface;
 use Drupal\Core\Database\Connection;
 
+/**
+ * Defines the default key/value implementation for sorted sets.
+ *
+ * This key/value store implementation uses the database to store key/value
+ * data in a sorted set.
+ */
 class DatabaseStorageSortedSet implements KeyValueStoreSortedSetInterface {
 
   /**
@@ -27,6 +33,16 @@ class DatabaseStorageSortedSet implements KeyValueStoreSortedSetInterface {
    */
   protected $table;
 
+  /**
+   * @param string $collection
+   *   The name of the collection holding key and value pairs.
+   * @param \Drupal\Component\Serialization\SerializationInterface $serializer
+   *   The serialization class to use.
+   * @param \Drupal\Core\Database\Connection $connection
+   *   The database connection to use.
+   * @param string $table
+   *   The name of the SQL table to use, defaults to key_value_sorted.
+   */
   public function __construct($collection, SerializationInterface $serializer, Connection $connection,  $table = 'key_value_sorted') {
     $this->collection = $collection;
     $this->serializer = $serializer;
@@ -93,6 +109,9 @@ public function getRange($start, $stop = NULL) {
     return $values;
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function getMaxScore() {
     $query = $this->connection->select($this->table);
     $query->condition('collection', $this->collection, '=');
@@ -100,6 +119,9 @@ public function getMaxScore() {
     return $query->execute()->fetchField();
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function getMinScore() {
     $query = $this->connection->select($this->table);
     $query->condition('collection', $this->collection, '=');
diff --git a/core/lib/Drupal/Core/KeyValueStore/KeyValueDatabaseSortedSetFactory.php b/core/lib/Drupal/Core/KeyValueStore/KeyValueDatabaseSortedSetFactory.php
index 583254c..071a45d 100644
--- a/core/lib/Drupal/Core/KeyValueStore/KeyValueDatabaseSortedSetFactory.php
+++ b/core/lib/Drupal/Core/KeyValueStore/KeyValueDatabaseSortedSetFactory.php
@@ -5,21 +5,32 @@
 use Drupal\Component\Serialization\SerializationInterface;
 use Drupal\Core\Database\Connection;
 
+/**
+ * Defines the key/value store factory for the database backend.
+ */
 class KeyValueDatabaseSortedSetFactory implements KeyValueSortedSetFactoryInterface {
 
   /**
+   * The serialization class to use.
+   *
    * @var \Drupal\Component\Serialization\SerializationInterface
    */
   protected $serializer;
 
   /**
+   * The database connection.
+   *
    * @var \Drupal\Core\Database\Connection
    */
   protected $connection;
 
   /**
+   * Constructs this factory object.
+   *
    * @param \Drupal\Component\Serialization\SerializationInterface $serializer
+   *   The serialization class to use.
    * @param \Drupal\Core\Database\Connection $connection
+   *   The Connection object containing the key-value tables.
    */
   function __construct(SerializationInterface $serializer, Connection $connection) {
     $this->serializer = $serializer;
diff --git a/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactory.php b/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactory.php
index adeadd6..70ef0bd 100644
--- a/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactory.php
+++ b/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactory.php
@@ -2,6 +2,9 @@
 
 namespace Drupal\Core\KeyValueStore;
 
+/**
+ * Defines the key/value store factory.
+ */
 class KeyValueSortedSetFactory extends KeyValueFactory implements KeyValueSortedSetFactoryInterface {
 
   const DEFAULT_SERVICE = 'keyvalue.sorted_set.database';
diff --git a/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactoryInterface.php b/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactoryInterface.php
index 56d4ab3..415b888 100644
--- a/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactoryInterface.php
+++ b/core/lib/Drupal/Core/KeyValueStore/KeyValueSortedSetFactoryInterface.php
@@ -2,12 +2,19 @@
 
 namespace Drupal\Core\KeyValueStore;
 
+/**
+ * Defines the sorted set key/value store factory interface.
+ */
 interface KeyValueSortedSetFactoryInterface {
 
   /**
+   * Constructs a new sorted set key/value store for a given collection name.
+   *
    * @param string $collection
+   *   The name of the collection holding key and value pairs.
    *
    * @return \Drupal\Core\KeyValueStore\KeyValueStoreSortedSetInterface
+   *   An sorted set key/value store implementation for the given $collection.
    */
   public function get($collection);
 
diff --git a/core/lib/Drupal/Core/KeyValueStore/KeyValueStoreSortedSetInterface.php b/core/lib/Drupal/Core/KeyValueStore/KeyValueStoreSortedSetInterface.php
index 5529285..b59fe6f 100644
--- a/core/lib/Drupal/Core/KeyValueStore/KeyValueStoreSortedSetInterface.php
+++ b/core/lib/Drupal/Core/KeyValueStore/KeyValueStoreSortedSetInterface.php
@@ -2,39 +2,55 @@
 
 namespace Drupal\Core\KeyValueStore;
 
+/**
+ * Defines the interface for sorted data in a key/value store.
+ *
+ * An example would be an index of items, which have an order, where the last
+ * item or a range of items may be required.
+ */
 interface KeyValueStoreSortedSetInterface {
 
   /**
    * @param float $score
+   *   The key for the item, for example microtime(), which can be used to
+   *   generate a sequential value.
    * @param mixed $member
+   *   The value of the item.
    */
   public function add($score, $member);
 
   /**
    * @param array $pairs
+   *   An array of key/value pairs to add.
    */
   public function addMultiple(array $pairs);
 
   /**
    * @return float
+   *   The highest key in the collection.
    */
   public function getMaxScore();
 
   /**
    * @return float
+   *   The lowest key in the collection.
    */
   public function getMinScore();
 
   /**
    * @return integer
+   *   The number of items in a collection.
    */
   public function getCount();
 
   /**
    * @param float $start
+   *   The first key in the range.
    * @param float $stop
+   *   The last key in the range.
    *
    * @return array
+   *   An array of items within the given range.
    */
   public function getRange($start, $stop = NULL);