Sharding support in Percona ClusterSync for MongoDB (Technical Preview)¶

Technical Preview

Sharding support is available starting with Percona ClusterSync for MongoDB 0.7.0 and is currently in technical preview stage. We encourage you to try it out and share your feedback. This will help us improve the feature in future releases.

Percona ClusterSync for MongoDB supports replication from a sharded MongoDB cluster to another sharded cluster or to a replica set.

With a sharded target, you can migrate or synchronize data between sharded deployments with minimal downtime. With a replica set target, Percona ClusterSync for MongoDB copies the data and skips the source sharding configuration.

For details about using a replica set as the target, see Replicate from a sharded cluster to a replica set.

Overview¶

The workflow for sharded clusters is similar to replica sets. See How Percona ClusterSync for MongoDB works for the complete workflow overview. The key difference is the target topology: when the target is a sharded cluster, PCSM connects through mongos on both the source and target. When the target is a replica set, it connects through the source mongos and then the target replica set members instead of a target mongos.

In both cases, the source must be a sharded MongoDB deployment. When the target is a sharded cluster, the source and target can have different numbers of shards. A replica set target does not require a target mongos instance.

Also, PCSM replicates data and not metadata. This means chunk distribution as well as the primary shard name for a collection may differ on source and target clusters.

Prerequisites¶

If the target is a sharded MongoDB deployment, Percona ClusterSync for MongoDB version 0.7.0 or later.
If the target is a replica set, Percona ClusterSync for MongoDB version 0.10.0 or later.
The source must be a sharded MongoDB deployment.
The target can be either a sharded MongoDB deployment or a replica set.
Both clusters must be running the same MongoDB version. Check Version requirements for more information about supported versions.

Connection string format¶

When connecting to a sharded source or a sharded target, use the standard MongoDB connection string format but specify the mongos hostname and port instead of replica set members:

mongodb://user:pwd@mongos-host:port/[authdb]?[options]

When the target is a replica set, specify the target replica set members in the target connection string instead of a mongos URI. PCSM does not require a target mongos instance in that topology.

For detailed information about authentication and connection string configuration, see Configure authentication in MongoDB.

Sharding-specific behavior¶

Initial sync preparation¶

Before starting the initial sync, PCSM checks which collections are sharded on the source cluster and creates corresponding sharded collections on the destination cluster. The only sharding configuration preserved from the source cluster is the sharding key; all other sharding details are handled internally by the destination cluster.

Balancer operation¶

Percona ClusterSync for MongoDB connects to source and target clusters via a mongos instance. Therefore, you do not need to disable the balancer on either the source or target cluster before starting replication. The target cluster’s balancer continues to operate normally and manages chunk distribution according to its own sharding configuration and balancer settings.

Chunk distribution¶

PCSM does not preserve chunk distribution information from the source cluster. The target cluster manages chunk distribution internally through its balancer. This means that after replication, chunks may be distributed differently on the target cluster compared to the source cluster, which is expected behavior.

Since the target cluster already has information about which collections are sharded, it handles sharding internally. PCSM does not interfere with the target cluster’s sharding configuration or chunk distribution.

Usage¶

The commands and API endpoints for sharded cluster replication are the same as for replica set replication. The workflow follows the same stages as replica set replication. See How Percona ClusterSync for MongoDB works for the complete workflow overview and Use Percona ClusterSync for MongoDB for detailed command instructions.

Next steps¶

Last update: September 7, 2026
Created: September 7, 2026