All configuration items loaded multiple times causing performance issue

Problem/Motivation

When viewing the Config Distro updates page with Configuration Synchronizer installed, a query in the following form may be repeated up to hundreds of times:

SELECT cid, data, created, expire, serialized, tags, checksum FROM cache_config WHERE cid IN ( :cids__0, :cids__1, :cids__2, :cids__3, :cids__4, :cids__5, :cids__6, :cids__7, :cids__8, :cids__9, :cids__10, :cids__11, :cids__12, :cids__13, :cids__14, :cids__15, :cids__16, :cids__17, :cids__18, [further CID placeholders omitted] ) ORDER BY cid

This appears to be loading all installed configuration from cache.

This issue results from creating and using multiple storage comparers with the active storage as target. The full config is loaded from cache for each comparer. This occurs only when using the full reset update mode, since only then are we comparing against the active configuration storage.

Proposed resolution

Beyond that, we need the ability to set a source storage on an existing configuration comparer, so that we can reuse the one where records from active are already loaded.

For that:

• Add a SettableStorageComparer class.
• Use that when comparing against active.

Remaining tasks

User interface changes

API changes

Data model changes