Flow has a $wgFlowCacheVersion value. We change it if we change Flow's cache format, and we can bump it to brute-force a cache invalidation.

But I think all WMF wikis share the cache, even though some are running an earlier MediaWiki and thus a different version of the Flow code.
a) If two WMF wikis are using different $wgFlowCacheVersion , they won't see each other's Flow updates.
b) If two WMF wikis are using the same $wgFlowCacheVersion with different code, they may make incompatible cache updates.

So we have to backport incompatible cache code changes to previous releases in production. We may be able to get away without doing this until we support cross-wiki Flow activity.

Currently a) can't happen because we force all wikis to use the same cache value by also setting $wgFlowCacheVersion in operations/mediawiki-config wmf-config/CommonSettings.php

To start, someone who really understands Flow caching should add a comment about this to Flow.php.

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