`mongosync` Behavior

The mongosync binary is the primary process used in Cluster-to-Cluster Sync. mongosync migrates data from one cluster to another and can keep the clusters in continuous sync.

For an overview of the mongosync process, see About mongosync.

To get started with mongosync, refer to the Quick Start Guide.

For more detailed information, refer to the Installation or Connecting mongosync page that best fits your situation.

Settings

Cluster Independence

mongosync syncs collection data between a source cluster and destination cluster. mongosync does not synchronize users or roles. As a result, you can create users with different access permissions on each cluster.

Configuration File

Options for mongosync can be set in a YAML configuration file. Use the --config option. For example:

$ mongosync --config /etc/mongosync.conf

For information on available settings, see Configuration.

Cluster and Collection Types

Sharded Clusters

Cluster-to-Cluster Sync supports replication between sharded clusters. Individual shards are replicated in parallel from the source cluster to the destination cluster, however a chunk migration or similar source update could move documents to a new source shard during replication.

Even if documents move between source shards during replication, Cluster-to-Cluster Sync maintains the eventual consistency guarantee on the destination cluster. For more information, see Sync Sharded Clusters.

Multiple Clusters

To sync a source cluster to multiple destination clusters, use one mongosync instance for each destination cluster. For more information, see Multiple Clusters Limitations.

Capped Collections

Starting in 1.3.0, Cluster-to-Cluster Sync supports capped collections with some limitations.

convertToCapped is not supported. If you run convertToCapped, mongosync exits with an error.
cloneCollectionAsCapped is not supported.

Capped collections on the source cluster work normally during sync.

Capped collections on the destination cluster have temporary changes during sync:

There is no maximum number of documents.
The maximum collection size is 1PB.

mongosync restores the original values for maximum number of documents and maximum document size during commit.

Reads and Writes

Write Blocking

mongosync does not enable write-blocking by default. If you enable write-blocking, mongosync blocks writes:

On the destination cluster during sync.
On the source cluster when commit is received.

To enable write-blocking, use the start API to set enableUserWriteBlocking to true. You cannot enable write-blocking after the sync starts.

You must enable write-blocking when you start mongosync if you want to use reverse synchronization later.

User Permissions

To set enableUserWriteBlocking, the mongosync user must have a role that includes the setUserWriteBlockMode and bypassWriteBlockingMode ActionTypes.

Note

When using enableUserWriteBlocking, writes are only blocked for users that do not have the bypassWriteBlockingMode ActionType. Users who have this ActionType are able to perform writes.

Permissible Reads

Read operations on the source cluster are always permitted.

When the /progress endpoint reports canWrite is true, the data on the source and destination clusters is consistent.

Permissible Writes

To see what state mongosync is in, call the /progress API endpoint. The /progress output includes a boolean value, canWrite.

When canWrite is true, it is safe to write to the destination cluster.
When canWrite is false, do not write to the destination cluster.

You can safely write to the source cluster while mongosync is syncing. Do not write to the destination cluster unless canWrite is true.

Read and Write Concern

By default, mongosync sets the read concern level to "majority" for reads on the source cluster. For writes on the destination cluster, mongosync sets the write concern level to "majority" with j: true.

For more information on read and write concern configuration and behavior, see Read Concern and Write Concern.

Read Preference

mongosync requires the primary read preference when connecting to the source and destination clusters. For more information, see Read Preference Options.

Considerations for Continuous Sync

For any continuous synchronization use cases with mongosync, ensure that mongosync commits before cutting over from the source to the destination.

If the source cluster shuts down before mongosync can commit, such as in a disaster scenario, the destination cluster might not have a consistent snapshot of the source data. To learn more, see Consistency.

Note

After commit, you can't resume continuous sync between two clusters since mongosync can only sync into empty destination clusters. If you need to use the same two clusters after cutover, you can call the reverse endpoint to keep the clusters in sync. Otherwise, start a new continuous sync operation by using a new empty destination cluster.

Sharded Clusters

When running continuous sync with sharded clusters, you must stop balancers on both the source and the destination for the lifetime of the sync until commit.

Temporary Changes to Collection Characteristics

mongosync temporarily alters the following collection characteristics during synchronization. The original values are restored during the commit process.

Change	Description
Unique Indexes	Unique indexes on the source cluster are synced as non-unique indexes on the destination cluster.
TTL Indexes	Synchronization sets `expireAfterSeconds` to the value of `MAX_INT` on the destination cluster.
Hidden Indexes	Synchronization replicates hidden indexes as non-hidden.
Write Blocking	If you enable write-blocking, `mongosync` blocks writes: On the destination cluster during sync. On the source cluster when `commit` is received. To learn more, see Write Blocking.
Capped Collections	Synchronization sets capped collections to the maximum allowable size.
Dummy Indexes	In some cases, synchronization may create dummy indexes on the destination to support writes on sharded or collated collections.

Destination Clusters

Consistency

mongosync supports eventual consistency on the destination cluster. Read consistency is not guaranteed on the destination cluster until commit. Before committing, the source and destination clusters may differ at a given point in time. To learn more, see Considerations for Continuous Sync.

While mongosync is syncing, mongosync may reorder or combine writes as it relays them from source to destination. For a given document, the total number of writes may differ between source and destination.

Transactions might not appear atomically on the destination cluster. Retryable writes may not be retryable on the destination cluster.

Profiling

If profiling is enabled on a source database, MongoDB creates a special collection named <db>.system.profile. After synchronization is complete, Cluster-to-Cluster Sync will not drop the <db>.system.profile collection from the destination even if the source database is dropped at a later time. The <db>.system.profile collection will not change the accuracy of user data on the destination.

Views

If a database with views is dropped on the source, the destination may show an empty system.views collection in that database. The empty system.views collection will not change the accuracy of user data on the destination.

System Collections

Cluster-to-Cluster Sync does not replicate system collections to the destination cluster.

If you issue a dropDatabase command on the source cluster, this change is not directly applied on the destination cluster. Instead, Cluster-to-Cluster Sync drops user collections and views in the database on the destination cluster, but it does not drop system collections on that database.

For example, on the destination cluster:

The drop operation does not affect a user-created system.js collection.
If you enable profiling, the system.profile collection remains.
If you create views on the source cluster and then drop the database, replicating the drop removes the views, but leaves an empty system.views collection.

In these cases, the replication of dropDatabase removes all user-created collections from the database, but leaves its system collections on the destination cluster.

UUIDs

mongosync creates collections with new UUIDs on the destination cluster. There is no relationship between UUIDs on the source cluster and the destination cluster. If applications contain hard-coded UUIDs (which MongoDB does not recommend), you may need to update those applications before they work properly with the migrated cluster.

Sorting

mongosync inserts documents on the destination cluster in an undefined order which does not preserve natural sort order from the source cluster. If applications depend on document order but don't have a defined sort method, you may need to update those applications to specify the expected sort order before the applications work properly with the migrated cluster.

Performance

Resilience

mongosync is resilient and able to handle non-fatal errors. Logs that contain the word "error" or "failure" do not indicate that mongosync is failing or corrupting data. For example, if a network error occurs, the mongosync log may contain the word "error' but mongosync is still able to complete the sync. In the case that a sync does not complete, mongosync writes a fatal log entry.

Data Definition Language (DDL) Operations

Using DDL operations (operations that act on collections or databases such as db.createCollection() and db.dropDatabase()) during sync increase the risk of migration failure and may negatively impact mongosync performance. For best performance, refrain from performing DDL operations on the source cluster while the sync is in progress.

For more information on DDL operations, see Pending DDL Operations and Transactions.

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