[Labs-l] Database replica drift

[Merlijn van Deen (valhallasw)]

Hello all,

As you may have noticed, the database replicas sometimes return weird and
incorrect results. The cause seems to be that certain queries, under
certain circumstances, get different results on labs and production.
Because it's the /queries/ that get replicated (and not the underlying
data), this causes the replicated databases to drift from the production
databases.
The issue seems to be mostly related to deletion/undeletion, where
revisions get moved to and from the archive table. For more details, please
see
https://wikitech.wikimedia.org/wiki/Help:Tool_Labs/Database/Replica_drift.

The current (short-time) workaround is regular re-imports of data into the
replicas. To do so effectively, it helps to have an overview of queries
that do not yield the expected result. If you encounter an odd result,
please report it in a comment at https://phabricator.wikimedia.org/T138967.

The more long-term solution lies in new database servers with a different
server configuration (and in changes to how MediaWiki runs queries). The
new database servers have been ordered, but installation and configuration
will take some time. We hope to have them set up somewhere in the coming
months.

Best,
Merlijn
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