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  • Validate Unique Indexes
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Reverses the direction of a committed sync operation.

For example:

  • You have a COMMITTED sync operation.

  • cluster0 is the source and cluster1 is the destination.

  • After the sync operation is COMMITTED, new writes occur only on the destination cluster. The source cluster will not accept new writes.

In this scenario, you can use the reverse endpoint to sync writes from cluster1 to cluster0.

To use the reverse endpoint:

  • mongosync must be configured when the initial sync begins. The call to the /start API endpoint must set:

    • reversible to true

    • enableUserWriteBlocking to true

    You cannot update these options after the sync starts.

  • mongosync must be in the COMMITTED state.

  • The destination cluster oplog must not roll over between mongosync reaching the COMMITTED state and receiving the /reverse request.

  • Unique Indexes require proper formatting. Collections with indexes initially created on MongoDB 4.2 may not have the proper formatting.

    To validate that collection indexes use the proper formatting, see Validate Unique Indexes.

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

  • The user specified in the mongosync connection string must have the required permissions on the source and destination clusters. The permissions vary depending on your environment and if you want to run a write-blocking or reverse sync.


When you configure multiple mongosync instances to sync between sharded clusters, you must send identical API endpoint commands to each mongosync instance.

For more information, see Reverse Multiple Mongosyncs.

In order to reverse direction, mongosync requires that all unique indexes use the correct formatting. Clusters that began with MongoDB 4.2 or older and were since upgraded may include unique indexes that are not properly formatted.

To correct indexes, you can resync all nodes in the original source cluster. To resync all nodes:


For a tutorial on resyncing nodes, see Resync a Member of a Replica Set.


Alternatively, to avoid resyncing, you can use the db.collection.validate() method with full = false on each collection with unique indexes on all nodes to determine whether each collection contains improperly formatted unique indexes. If db.collection.validate() does not return a warning about a unique index, you can skip resyncing.

POST /api/v1/reverse

This endpoint does not use HTTP request body parameters. However, you must specify the --data option with an empty object { }.

When the request is successful, this value is true.
If an error occurred, indicates the name of the error. This field is omitted from the response when success is true.
Detailed description of the error that occurred. This field is omitted from the response when success is true.

The following example reverses the direction of a committed sync operation.

curl localhost:27182/api/v1/reverse -XPOST --data '{ }'

If the reverse request is successful, mongosync enters the RUNNING state. The synchronization continues in the reverse direction from the original sync job. You do not need to restart the entire sync process to copy the original data.

To view the mapping direction for the synchronization of the source and destination clusters, use the progress endpoint and check the directionMapping object.

mongosync does not protect the reverse endpoint. However, by default the API binds to localhost only and does not accept calls from other sources. Additionally, the reverse call does not expose connection credentials or user data.

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