REP-6088 Tolerate high numbers of mismatches #117
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tdq45gj
requested changes
Jun 3, 2025
| return errors.Wrapf(err, "starting session") | ||
| } | ||
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| sctx := mongo.NewSessionContext(ctx, sess) |
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Are we reading in a session just to get the cluster time?
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Yes. That, per the driver team, is the approved way to do this.
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Could we add a comment to explain the purpose of a session here?
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Previously all document mismatches were recorded directly in the verification task. This meant, though, that if a task encompassed a large number of mismatched or missing documents, the verifier could fail to persist all of the mismatches, which caused a crash.
(The usual cause of excess mismatched/missing documents is starting migration-verifier before initial sync finishes, but it can also reasonably happen without REP-6129’s fix for queries against pre-v5 servers. See HELP-75910.)
This changeset makes the verifier save mismatches to a dedicated collection instead, one document per mismatch.
This change upends some familiar workflows for investigating mismatches: it’s no longer sufficient just to query the
verification_taskscollection for mismatch information since the actual mismatches are recorded in a separate collection. To address this, the documentation now gives an aggregation pipeline that yields a similarly-useful result.This entails a metadata version change. Because that’s happening, this also changes the task type
verifytoverifyDocuments. (That required some sorting workarounds in tests, which were tight-coupled to the task type strings.)