REP-6882 Track mismatch length #186
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One of this tool’s longstanding pain points is that it is quite common for mismatches to appear in only 1 generation. Because of this it’s not been feasible to distinguish “real” mismatches from “ephemeral” ones.
This changeset teaches Migration Verifier to track how long a given mismatch has been seen. It does so by recording the mismatch’s first-seen & latest-seen time; the delta between those times is the mismatch’s “duration”. We now log that value as part of the table of mismatches, and we sort the mismatches on it (descending). Thus, the top line in the mismatch table is the longest-lived mismatch, which is the one most likely to warrant manual intervention.
This makes no effort to hide “ephemeral” mismatches because determining what counts as “ephemeral” requires knowledge of how far behind the replicator (e.g., mongosync) lags the source cluster. If the lag is 30 minutes, for example, then ideally the logs would hide (or, at least, just mention without surfacing) any mismatch newer than that lag time. This is a possible area for follow-up improvement.