Docs Menu

Docs HomeMongoDB Cluster-to-Cluster Sync

Frequently Asked Questions

On this page

  • Can mongosync run on its own hardware?
  • Should I increase the size of the oplog in the source cluster?
  • Which connection string options does mongosync allow?
  • Which security and authentication options are supported?
  • Can I configure mongosync for high availability?
  • Can the source or destination be a replica set with arbiters?
  • What if I see a Slow Operation Warning?

This page provides answers to some frequently asked questions we have encountered. If you have additional questions please contact MongoDB Support.

Yes, mongosync can run on its own hardware. mongosync does not have to run on the servers that host your MongoDB instances. When mongosync runs on its own hardware, it can use an operating system (OS) that is different than the OS on the source or destination clusters.

The oplog is a capped collection that keeps a rolling record of all operations that modify the data stored in your databases.

mongosync applies operations in the oplog on the source cluster to the data on the destination cluster. When operations that mongosync has not applied roll off the oplog on the source cluster, the sync fails and mongosync exits.

During the initial sync, mongosync may apply operations at a slower rate due to the load imposed by copying documents concurrently. After mongosync completes the initial sync, it applies changes faster and is more likely to maintain a position in the oplog that is close to the real-time writes occuring on the source cluster.

If you anticipate syncing a large data set, or if you plan to pause synchronization for an extended period of time, you might exceed the oplog window. Use the oplogSizeMB setting to increase the size of the oplog on the source cluster.

To learn more about how to increase the size of the oplog, see: Change the Size of the Oplog.

mongosync requires readConcern: "majority" and writeConcern: "majority".

If the readConcern is not majority, mongosync returns an error:

Invalid URI option, read concern must be majority

If the writeConcern is not majority, mongosync returns an error:

Invalid URI option, write concern must be majority

mongosync accepts all other connection string options.

mongosync uses a standard MongoDB connection string to connect to the source and destination clusters.

LDAP and X509 are supported. For available authentication options, see Authentication.

There is no automatic failover built into mongosync. However you can write a script or use your operating system's process managers, systemd for example, to restart the mongosync process.

The mongosync binary is stateless. The metadata for restarting is stored on the destination cluster.

A mongosync operation can be resumed if mongosync becomes unavailable during synchronization. When mongosync becomes available again, restart the mongosync process with the same parameters. mongosync resumes the operation from where it stopped when mongosync became unavailable.

Yes, the replica set can have arbiters. The source replica set must have more than 2 non-arbiter nodes and you must sync from a non-arbiter node. Use the source cluster's connection string to specify a read preference for a non-arbiter, data-bearing node.

Slow operation warnings can occur during the initial sync or the application of a change event when there is a slow read operation on the source cluster or a slow write operation on the destination cluster. The warning may indicate network congestion or resource strain on the source or destination cluster.

While these warnings do not indicate failures in themselves, slow operations can cause operation timeout errors in mongosync and migration failures.

If you see slow operation warnings, check CPU, memory, and network usage on the source and destination clusters. If the clusters are underprovisioned for your needs, consider upgrading the cluster hardware.

←  Release Notes for mongosync 0.9
Share Feedback
© 2023 MongoDB, Inc.


  • Careers
  • Investor Relations
  • Legal Notices
  • Privacy Notices
  • Security Information
  • Trust Center
© 2023 MongoDB, Inc.