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Replication Processes

Members of a replica set replicate data continuously. First, a member uses initial sync to capture the data set. Then the member continuously records and applies every operation that modifies the data set. Every member records operations in its oplog, which is a capped collection.

Replica Set Oplog
The oplog records all operations that modify the data in the replica set.
Replica Set Data Synchronization
Secondaries must replicate all changes accepted by the primary. This process is the basis of replica set operations.