Limitations

General Limitations

• The minimum supported server version is MongoDB 6.0.4.

• mongosync does not support MongoDB rapid releases such as 6.1 or 6.2. Only major MongoDB releases such as 6.0 or 7.0. are supported. For more information on MongoDB versioning, see MongoDB Versioning.

• The source and destination clusters must have the same release version.

• The minimum supported Feature Compatibility Version is 6.0.

• The source and destination clusters must have the same Feature Compatibility Version.

• The destination cluster must be empty.

• mongosync does not validate that the clusters or the environment are properly configured.

• Other clients must not write to the destination cluster while mongosync is running.

• If write blocking is disabled, the client must prevent writes to the source cluster before starting the commit process.

• Network compression is not supported.

• applyOps operations from the source cluster are not supported.

• system.* collections are not replicated.

• Documents that have dollar ($) prefixed field names are not supported. See Field Names with Periods and Dollar Signs.

• Serverless clusters are not supported.

• The MongoDB Shared Tier is not supported.

• Queryable Encryption is not supported.

• After you replace the mongosync binary during an upgrade or a downgrade, you should drop all non-system databases in the destination cluster before starting the new binary. Syncing operations will restart from the beginning when you start the new processes.

• You cannot sync a collection that has a unique index and a non-unique index defined on the same field(s).

MongoDB Community Edition

Cluster-to-Cluster Sync supports a limited number of operations with MongoDB Community Edition. Please contact a sales representative to discuss your requirements.

Unsupported Collection Types

• Time-series collections are not supported.

• Capped collections are not supported.

• Clustered collections with expireAfterSeconds set are not supported.

Sharded Clusters

• mongosync does not support sync from a sharded cluster to a replica set.

• Sync from a replica set to a sharded cluster has the following limitations:

  • Collections included in the sharding.shardingEntries option for the start command cannot be renamed to use a different database while mongosync is running.

  • When using the sharding.createSupportingIndexes option to create indexes supporting shard keys on the destination cluster during sync, you cannot create these indexes afterwards on the source cluster.

    The index must either exist before mongosync starts or be created after the migration is complete and mongosync has stopped.

• Within a collection, the _id field must be unique across all of the shards in the cluster. See Sharded Clusters and Unique Indexes for more details.

• The movePrimary command cannot be used to reassign the primary shard while syncing.

• There is no replication for zone configuration. mongosync replicates data, it does not inherit zones.

• Shards cannot be added or removed while syncing.

• Only indexes which exist on all shards are synced.

• The shard key cannot be refined while syncing.

• The shard key cannot be modified using reshardCollection during syncing.

• The maximum number of shard key indexes is one lower than normal, 63 instead of 64.

Reversing

• If the old source has unique indexes which are partially distributed across shards, reversing may cause failures. Ensure that unique indexes exist on all shards before reversing.

• The source and destination clusters must have the same number of shards. You cannot reverse sync when the clusters have different topologies.

Multiple Clusters

• Syncing multiple source clusters to one destination cluster is not supported.

• Syncing one source cluster to many destination clusters is not supported.

Filtered Sync

• Filtering is not supported with reversible sync.

• The destination cluster must not contain user data prior to starting.

• The destination cluster must not contain the mongosync_reserved_for_internal_use system database prior to starting.

• You cannot modify a filter that is in use. To create a new filter, see: Replace an Existing Filter.

• You can only rename collections in certain situations. For more details see: Adding and Renaming Collections.

• If a filter includes a view but not the base collection, only the view is replicated.

• You cannot specify system collections or system databases in a filter.

• Operations that use the $out aggregation stage are only supported if the entire database is specified in the filter. You cannot limit the filter to a collection within the database. See: Filtering with $out.

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.