Megan Carey created SOLR-14778:
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Summary: Disabling UpdateLog leads to silently lost updates
Key: SOLR-14778
URL: https://issues.apache.org/jira/browse/SOLR-14778
Project: Solr
Issue Type: Bug
Security Level: Public (Default Security Level. Issues are Public)
Components: SolrCloud, update
Affects Versions: 8.6.1
Reporter: Megan Carey
Solr currently "supports" disabling the UpdateLog, though it is "required" for
NRT replicas (per the
[docs|[https://lucene.apache.org/solr/guide/8_6/updatehandlers-in-solrconfig.html#transaction-log]]).
However, when the update log is disabled and a replica is in BUFFERING state
(e.g. during MigrateCmd or SplitShardCmd), updates are *lost silently*. While
most users will likely never consider disabling the updateLog, it seems
pertinent to provide a better support option.
Options as discussed in [ASF
Slack|[https://the-asf.slack.com/archives/CEKUCUNE9/p1598373062262300]:]
# No longer support disabling the updateLog as it is considered a pertinent
feature in SolrCloud. This might be undesirable for use cases where some data
loss is acceptable and the updateLog takes up too much space.
# Improve Solr documentation to explicitly outline the risks of disabling the
updateLog.
# Add logging to indicate when an update is swallowed in this state.
# _My preferred option:_ Support disabling the updateLog by providing
additional replica states so that there is no data loss when updateLog is
disabled and replica goes offline for an operation like split.
## REJECTING: Fail updates so that the client can retry again once the
operation is complete.
## BLOCKING: Stall update until operation is complete, and then execute update.
Feedback is welcome; once we establish a path forward I'll be happy to pick it
up. If others are interested I can document my findings as well.
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