[LUCENE-3569] Consolidate IndexWriter's optimize, maybeMerge and expungeDeletes under one merge(MP) method

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: core/index
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Shai Erera
• Created: 09/Nov/11 08:35
• Updated: 28/Aug/22 13:01

Description

Today, IndexWriter exposes 3 methods for 'cleaning up' / 'compacting' / 'optimizing' your index:

• optimize() – merges as much segments as possible (down to 1 segment), and is discouraged in many cases because of its performance implications.
• maybeMerge() – runs 'subtle' merges. Attempts to balance the index by not leaving too many segments, yet not merging large segments if unneeded.
• expungeDeletes() – cleans up deleted documents from segments and on the go merges them.
• a default MP that can be set on IndexWriterConfig, for ongoing merges IW performs (i.e. as a result of flushing a new segment).

These methods are confusing in several levels:

• Their names are misleading, see LUCENE-3454.
• Why does expungeDeletes need to merge segments?
• Eventually, they really do what the MergePolicy decides that should be done. I.e., one could write an MP that always merges all segments, and therefore calling maybeMerge would not be so subtle anymore. On the other hand, one could write an MP that never merges large segments (we in fact have several of those), and therefore calling optimize(1) would not end up with one segment.

So the proposal is to replace all these methods with a single one merge(MergePolicy) (more on the names later). MergePolicy will have only one method findSegmentsForMerge and the caller will be responsible to configure it in order to perform the needed merges. We will provide ready-to-use MPs:

• LightMergePolicy – for setting on IWC and doing the ongoing merges IW executes. This one will pick segments respecting various parameters such as mergeFactor, segmentSizes etc.
• HeavyMergePolicy – for doing the optimize()-style merges.
• ExpungeDeletesMergePolicy – for expunging deletes (my proposal is to drop segment merging from it, by default).

Now about the names:

• I think that it will be good, API-backcompat wise and in general, if we name that method doMaintenance (as expungeDeletes does not have to merge anything).
• Instead of MergePolicy we call it MaintenancePolicy and similarly its single method findSegmentsForMaintenance, or getMaintenanceSpecification.
• I called the MPs Light and Heavy just for the text, I think a better name should be found, but nothing comes up to mind now.

It will allow us to use this on 3.x, by deprecating MP and all related methods.