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From Apache Wiki <wikidi...@apache.org>
Subject [Jackrabbit Wiki] Update of "Journal based Async Indexer" by ChetanMehrotra
Date Wed, 09 Aug 2017 14:31:53 GMT
Dear Wiki user,

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The "Journal based Async Indexer" page has been changed by ChetanMehrotra:

New page:
JIRA Issue [[https://issues.apache.org/jira/browse/OAK-6513|OAK-6513]]

Current async indexer design is based on NodeState diff. This has served us fine so far however
off late it is not able to perform well if rate of repository writes is high. When changes
happen faster than index-update can process them, larger and larger diffs will happen. These
make index-updates slower, which again lead to the next diff being ever larger than the one
before (assuming a constant ingestion rate).

In current diff based flow the indexer performs complete diff for all changes happening between
2 cycle. It may happen that lots of writes happens but not much indexable content is written.
So doing diff there is a wasted effort.

In 1.6 release for NRT Indexing we implemented a journal based indexing for external changes(OAK-4808,
OAK-5430). That approach can be generalized and used for async indexing.

Before talking about the journal based approach lets see how IndexEditor work currently

== IndexEditor ==

Currently any IndexEditor performs 2 tasks

 1. Identify which node is to be indexed based on some index definition. The Editor gets invoked
as part of content diff where it determines which NodeState is to be indexed
 1. Update the index based on node to be indexed

For e.g. in oak-lucene we have LuceneIndexEditor which identifies the NodeStates to be indexed
and LuceneDocumentMaker which constructs the Lucene Document from NodeState to be indexed.
For journal based approach we can decouple these 2 parts and thus have

 1. IndexEditor - Identifies which all paths need to be indexed for given index definition
 1. IndexUpdater - Updates the index based on given NodeState and its path

== Proposal ==

 1. Session Commit Flow
   1. Each index type would provide a IndexEditor which would be invoked as part of commit
(like sync indexes). These IndexEditor would just determine which paths needs to be indexed.
   1. As part of commit the paths to be indexed would be written to journal.
 1. AsyncIndexUpdate flow
    1. AsyncIndexUpdate would query this journal to fetch all such indexed paths between the
2 checkpoints
    1. Based on the index path data it would invoke the IndexUpdater to update the index for
that path
    1. Merge the index updates

=== Benefits ===

Such a design would have following impact

 1. More work done as part of write
 1. Marking of indexable content is distributed hence at indexing time lesser work to be done
 1. Indexing can progress in batches
 1. The indexers can be called in parallel

=== Journal Implementation ===

DocumentNodeStore currently has an in built journal which is being used for NRT Indexing.
That feature can be exposed as an api.

For scaling index this design is mostly required for cluster case. So we can possibly have
both indexing support implemented and use the journal based support for DocumentNodeStore
setups. Or we can look into implementing such a journal for SegmentNodeStore setups also

=== Open Points ===

 1. Journal support in SegmentNodeStore
 1. Handling deletes.

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