Replica Set Data Synchronization

Logical Initial Sync Process

When you perform a logical initial sync, MongoDB:

1. Clones all databases except the local database. To clone, the mongod scans every collection in each source database and inserts all data into its own copies of these collections.

2. Builds all collection indexes as the documents are copied for each collection.

3. Pulls newly added oplog records during the data copy. Ensure that the target member has enough disk space in the local database to temporarily store these oplog records for the duration of this data copy stage.

4. Applies all changes to the data set. Using the oplog from the source, the mongod updates its data set to reflect the current state of the replica set.

When the initial sync finishes, the member transitions from STARTUP2 to SECONDARY.

To perform an initial sync, see Resync a Member of a Replica Set.

File Copy Based Initial Sync

Available in MongoDB Enterprise only.

File copy based initial sync runs the initial sync process by copying and moving files on the file system. This sync method can be faster than logical initial sync.

Fault Tolerance

If a secondary performing initial sync encounters a non-transient (i.e. persistent) network error during the sync process, the secondary restarts the initial sync process from the beginning.

Starting in MongoDB 4.4, a secondary performing initial sync can attempt to resume the sync process if interrupted by a transient (i.e. temporary) network error, collection drop, or collection rename. The sync source must also run MongoDB 4.4 to support resumable initial sync. If the sync source runs MongoDB 4.2 or earlier, the secondary must restart the initial sync process as if it encountered a non-transient network error.

By default, the secondary tries to resume initial sync for 24 hours. MongoDB 4.4 adds the initialSyncTransientErrorRetryPeriodSeconds server parameter for controlling the amount of time the secondary attempts to resume initial sync. If the secondary cannot successfully resume the initial sync process during the configured time period, it selects a new healthy source from the replica set and restarts the initial synchronization process from the beginning.

The secondary attempts to restart the initial sync up to 10 times before returning a fatal error.

Initial Sync Source Selection

Initial sync source selection depends on the value of the mongod startup parameter initialSyncSourceReadPreference (new in 4.4):

• For initialSyncSourceReadPreference set to primary (default if chaining is disabled), select the primary as the sync source. If the primary is unavailable or unreachable, log an error and periodically check for primary availability.

• For initialSyncSourceReadPreference set to primaryPreferred (default for voting replica set members), attempt to select the primary as the sync source. If the primary is unavailable or unreachable, perform sync source selection from the remaining replica set members.

• For all other supported read modes, perform sync source selection from the replica set members.

Members performing initial sync source selection make two passes through the list of all replica set members:

If the member cannot select an initial sync source after two passes, it logs an error and waits 1 second before restarting the selection process. The secondary mongod can restart the initial sync source selection process up to 10 times before exiting with an error.

Oplog Window

The oplog window must be long enough so that a secondary can fetch any new oplog entries that occur between the start and end of the Logical Initial Sync Process. If the window isn't long enough, there is a risk that some entries may fall off the oplog before the secondary can apply them.

It is recommended that you size the oplog for additional time to fetch any new oplog entries. This allows for changes that may occur during initial syncs.

For more information, see Oplog Size.

Streaming Replication

Starting in MongoDB 4.4, sync from sources send a continuous stream of oplog entries to their syncing secondaries. Streaming replication mitigates replication lag in high-load and high-latency networks. It also:

• Reduces staleness for reads from secondaries.

• Reduces risk of losing write operations with w: 1 due to primary failover.

• Reduces latency on write operations with w: "majority" and w: >1 (that is, any write concern that requires waiting for replication).

Prior to MongoDB 4.4, secondaries fetched batches of oplog entries by issuing a request to their sync from source and waiting for a response. This required a network roundtrip for each batch of oplog entries. MongoDB 4.4 adds the oplogFetcherUsesExhaust startup parameter for disabling streaming replication and using the older replication behavior. Set the oplogFetcherUsesExhaust parameter to false only if there are any resource constraints on the sync from source or if you wish to limit MongoDB's usage of network bandwidth for replication.

Multithreaded Replication

MongoDB applies write operations in batches using multiple threads to improve concurrency. MongoDB groups batches by document ID (WiredTiger) and simultaneously applies each group of operations using a different thread. MongoDB always applies write operations to a given document in their original write order.

Read operations that target secondaries and are configured with a read concern level of "local" or "majority" read from a WiredTiger snapshot of the data if the read takes place on a secondary where replication batches are being applied.

Reading from a snapshot guarantees a consistent view of the data, and allows the read to occur simultaneously with the ongoing replication without the need for a lock. As a result, secondary reads requiring these read concern levels no longer need to wait for replication batches to be applied, and can be handled as they are received.

Flow Control

Starting in MongoDB 4.2, administrators can limit the rate at which the primary applies its writes with the goal of keeping the majority committed lag under a configurable maximum value flowControlTargetLagSeconds.

By default, flow control is enabled.

For more information, see Flow Control.

Replication Sync Source Selection

Replication sync source selection depends on the replica set chaining setting:

• With chaining enabled (default), perform sync source selection from the replica set members.

• With chaining disabled, select the primary as the sync source. If the primary is unavailable or unreachable, log an error and periodically check for primary availability.

Members performing replication sync source selection make two passes through the list of all replica set members:

If the member cannot select a sync source after two passes, it logs an error and waits 1 second before restarting the selection process.

The number of times a source can be changed per hour is configurable by setting the maxNumSyncSourceChangesPerHour parameter.