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  1. Core Server
  2. SERVER-32185

Freshly synced secondaries respond to queries before their "sync time"

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Details

    • Bug
    • Status: Closed
    • Major - P3
    • Resolution: Works as Designed
    • None
    • None
    • Replication
    • None
    • ALL
    • v3.6

    Description

      The last phase of a secondary performing initial sync is to apply oplog operations up through some time `T` representing when the collection cloning phase completed. It's incorrect for a secondary to respond to majority read/at a timestamp queries before time T.

      When a secondary comes out of initial sync, it will still have a notion of the replica sets majority commit time. Because the majority commit time is translated to a "read at a timestamp", the secondary will incorrectly respond to a query, but with a view of inconsistent data.

      A couple starting points for solutions:

      1. An API was introduced for recover to a stable timestamp known as the "initial data timestamp" that replication sets when initial sync completes. This represents the timestamp at which the data is in a consistent state. This could be used to reject/block incoming majority reads/read at a timestamp requests.
      2. Alternatively, a secondary can refuse to come out of initial sync until the majority commit point passes `T`. Currently there is no mechanism to tell drivers which timestamps a node can service reads for. This solution would be a way to signal to drivers to not send majority reads the node cannot service, at the cost of not participating in reads `>= T`.

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              backlog-server-repl Backlog - Replication Team
              daniel.gottlieb@mongodb.com Daniel Gottlieb
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