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Use fast updates when replica metadata is out of sync but document itself is in sync #11319
When a bucket has replicas with mismatching metadata (i.e. they are out of sync),
This commit lets
It also removes an old (but now presumed unsafe) optimization where Get
…self is in sync When a bucket has replicas with mismatching metadata (i.e. they are out of sync), the distributor will initiate a write-repair for updates to avoid divergence of replica content. This is done by first sending a Get to all diverging replica sets, picking the highest timestamp and applying the update locally. The updated document is then sent out as a Put. This can be very expensive if document Put operations are disproportionally more expensive than partial updates, and also makes the distributor thread part of a contended critical path. This commit lets `TwoPhaseUpdateOperation` restart an update as a "fast path" update (partial updates sent directly to the nodes) if the initial read phase returns the same timestamp for the document across all replicas. It also removes an old (but now presumed unsafe) optimization where Get operations are only sent to replicas marked "trusted" even if others are out of sync with it. Since trustedness is a transient state that does not persist across restarts or bucket handoffs, it's not robust enough to be used for such purposes. Gets will now be sent to all out of sync replica groups regardless of trusted status.