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normal operation of the replica set, particularly failover (page 580) in the form of rollbacks (page 584) as well as general read consistency (page 585). 

To help avoid this issue, you can customize the write concern (page 78) to return confirmation of the write operation to another member 5 of the replica set every 100 or 1,000 operations. This provides an opportunity for secondaries to catch up with the primary. Write concern can slow the overall progress of write operations but ensure that the secondaries can maintain a largely current state with respect to the primary. 

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For more information on replica sets and write operations, see Replica Acknowledged (page 80), Oplog Size (page 593), and Change the Size of the Oplog (page 628). 

