Frequently Asked Questions

Can mongosync run on its own hardware?

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.

Should I increase the size of the oplog in the source cluster?

The oplog in the source cluster must be large enough to track events that happen during the time it takes to complete the initial sync to the destination cluster.

If you anticipate syncing a large data set, or if you plan to pause synchronization for an extended period of time, increase the size of the replica set oplog in the source cluster.

The proper oplog size depends on system hardware, network speed, and other factors including system workload. However, assuming network transfer speeds of 30-50GB per hour, a rough formula to estimate the required oplog size is:

minimumRetentionHours = dataSizeInGB / 30

To estimate the size of oplog needed for initial synchronization, see: oplog Sizing.

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

Which connection string options does mongosync allow?

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.

Which security and authentication options are supported?

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.

Can I configure mongosync for high availability?

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.

Can the source or destination be a replica set with arbiters?

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.

What if I see a Slow Operation Warning?

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.