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Replica Set Data Synchronization

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In order to maintain up-to-date copies of the shared data set, secondary members of a replica set sync or replicate data from other members. MongoDB uses two forms of data synchronization: initial sync to populate new members with the full data set, and replication to apply ongoing changes to the entire data set.

Initial Sync

Initial sync copies all the data from one member of the replica set to another member. See Initial Sync Source Selection for more information on initial sync source selection criteria.

Starting in MongoDB 4.4, you can specify the preferred initial sync source using the initialSyncSourceReadPreference parameter. This parameter can only be specified when starting the mongod.

Process

When you perform an 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.

    Changed in version 3.4: Initial sync builds all collection indexes as the documents are copied for each collection. In earlier versions of MongoDB, only the _id indexes are built during this stage.

    Changed in version 3.4: Initial sync 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.

  2. 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.

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:

The member applies the following criteria to each replica set member when making the first pass for selecting a initial sync source:

  • The sync source must be in the PRIMARY or SECONDARY replication state.
  • The sync source must be online and reachable.
  • If initialSyncSourceReadPreference is secondary or secondaryPreferred, the sync source must be a secondary.
  • The sync source must be visible.
  • The sync source must be within 30 seconds of the newest oplog entry on the primary.
  • If the member builds indexes, the sync source must build indexes.
  • If the member votes in replica set elections, the sync source must also vote.
  • If the member is not a delayed member, the sync source must not be delayed.
  • If the member is a delayed member, the sync source must have a shorter configured delay.
  • The sync source must be faster (i.e. lower latency) than the current best sync source.

If no candidate sync sources remain after the first pass, the member performs a second pass with relaxed criteria. See Sync Source Selection (Second Pass).

The member applies the following criteria to each replica set member when making the second pass for selecting a initial sync source:

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.

Replication

Secondary members replicate data continuously after the initial sync. Secondary members copy the oplog from their sync from source and apply these operations in an asynchronous process. [1]

Secondaries may automatically change their sync from source as needed based on changes in the ping time and state of other members’ replication. See Replication Sync Source Selection for more information on sync source selection criteria.

[1]Starting in version 4.2 (also available starting in 4.0.6), secondary members of a replica set now log oplog entries that take longer than the slow operation threshold to apply. These slow oplog messages are logged for the secondaries in the diagnostic log under the REPL component with the text applied op: <oplog entry> took <num>ms. These slow oplog entries depend only on the slow operation threshold. They do not depend on the log levels (either at the system or component level), or the profiling level, or the slow operation sample rate. The profiler does not capture slow oplog entries.

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.

Changed in version 4.0.

Starting in MongoDB 4.0, read operations that target secondaries and are configured with a read concern level of "local" or "majority" will now 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.

Note

For flow control to engage, the replica set/sharded cluster must have: featureCompatibilityVersion (FCV) of 4.2 and read concern majority enabled. That is, enabled flow control has no effect if FCV is not 4.2 or if read concern majority is disabled.

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:

The member applies the following criteria to each replica set member when making the first pass for selecting a replication sync source:

  • The sync source must be in the PRIMARY or SECONDARY replication state.
  • The sync source must be online and reachable.
  • The sync source must have newer oplog entries than the member (i.e. the sync source is ahead of the member).
  • The sync source must be visible.
  • The sync source must be within 30 seconds of the newest oplog entry on the primary.
  • If the member builds indexes, the sync source must build indexes.
  • If the member votes in replica set elections, the sync source must also vote.
  • If the member is not a delayed member, the sync source must not be delayed.
  • If the member is a delayed member, the sync source must have a shorter configured delay.
  • The sync source must be faster (i.e. lower latency) than the current best sync source.

If no candidate sync sources remain after the first pass, the member performs a second pass with relaxed criteria. See the Sync Source Selection (Second Pass).

The member applies the following criteria to each replica set member when making the second pass for selecting a replication sync source:

  • The sync source must be in the PRIMARY or SECONDARY replication state.
  • The sync source must be online and reachable.
  • If the member builds indexes, the sync source must build indexes.
  • The sync source must be faster (i.e. lower latency) than the current best sync source.

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

Note

Starting in MongoDB 4.4, the startup parameter initialSyncSourceReadPreference takes precedence over the replica set’s settings.chainingAllowed setting when selecting an initial sync source. After a replica set member successfully performs initial sync, it defers to the value of chainingAllowed when selecting a replication sync source.

See Initial Sync Source Selection for more information on initial sync source selection.

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