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[b1ron]

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1. MongoServerSelectionError - fixed by setting directConnection=true in MONGO_OPLOG_URL
2. MongoServerError (FieldPath field names may not start with '$') - fixed in FerretDB/FerretDB#3638
3. when the oplog is detected it will open a tailable cursor with awaitData true FerretDB/FerretDB#2283
4. it uses a deprecated command oplogReplay in conjunction with a filter on ts to replay oplog entries
5. seems to be optional FerretDB/FerretDB#2984
6. FailedToParse -"find: unknown field "$clusterTime"" FerretDB/FerretDB#3667

[b1ron]

re 4.

Deprecated

Changed in version 4.4.

Starting in MongoDB 4.4, the oplogReplay field is deprecated. find fields that use the $gte, $gt, or $eq filter predicated on the ts field will automatically utilize the storage format of the replica set's oplog to execute the command more efficiently. If specified, the server accepts the oplogReplay flag for backwards compatibility, but the flag has no effect.