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  1. Core Server
  2. SERVER-54626

Retryable writes may execute more than once in resharding if statements straddle the fetchTimestamp

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    • Fully Compatible
    • ALL
    • Sharding 2021-03-08
    • 1

      Donor 1 Donor 2 Coordinator
      Donor 1 chooses minFetchTimestamp ts=10    
      Donor 1 performs retryable write stmtId 1 at ts=20    
      Donor 1 performs retryable write stmtId 2 at ts=30    
        Donor 2 chooses minFetchTimestamp ts=25  
          Coordinator chooses fetchTimestamp ts=25

      leads to a sequence where the recipients won't have written an incomplete stmtId for the retryable write due to the timestamp for stmtId 2 being greater than the fetchTimestamp. This allows the statements from the retryable write to execute a second time on the recipients after the resharding operation has finished.

      {
    aggregate: "transactions",
    pipeline: [
        {$match: {_id: {$gt: <resume lsid>}}},
        {$sort: {_id: 1}},
        {$match: {"lastWriteOpTime.ts": {$lt: <fetchTimestamp>}}},
    ],
    readConcern: {level: "majority", afterClusterTime: <fetchTimestamp>},
    hint: "_id_",
    cursor: {},
}

      The {"lastWriteOpTime.ts": {$lt: <fetchTimestamp>}} clause is what causes stmtId 2 and therefore the entire retryable write to be skipped over by the recipients.

            Assignee:
            yuhong.zhang@mongodb.com Yuhong Zhang
            Reporter:
            max.hirschhorn@mongodb.com Max Hirschhorn
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              Created:
              Updated:
              Resolved: