Thomas Mueller created OAK-8343:
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Summary: Allow queries to be delayed until an index is available
Key: OAK-8343
URL: https://issues.apache.org/jira/browse/OAK-8343
Project: Jackrabbit Oak
Issue Type: Improvement
Components: lucene, query
Reporter: Thomas Mueller
Assignee: Thomas Mueller
Currently, indexes are built asynchronously. That is, if an index definition is
added, the index is eventually built, but it's quite hard to say when it is
ready for queries. This can be specially a problem right after the initial
repository initialization, or after an upgrade.
In theory, system startup could be delayed until all indexes are ready (e.g.
set the "reindex" flag for important indexes, and at startup, wait until the
"reindex" flag is set to "false"). However, doing that would block threads that
_don't_ need an index. It would be better to only block threads that actually
do run queries. That would make startup deterministic, without delaying other
threads unnecessarily.
To solve the problem, we can add a property "waitForIndex" in the index
definition (just Lucene indexes is fine for now, as those are the important
asynchronous ones). If set, then queries that potentially use those indexes are
delayed, until the indexes are ready for sure. Reindex would need to remove
that property (the same as it removes e.g. refresh or sets reindex to false).
For added security, queries are only blocked as long as "reindex" is also set
to true (this ensures that waitForIndex is removed eventually), and waiting
should time out after 2 minutes, to ensure the feature doesn't block startup
forever if indexing fails for some reason.
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