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When upgrading to MongoDB 6.0, it’s important to note that starting from MongoDB 5.3, support for multiple arbiters in a replica set is disabled by default. This change can affect quorum and recognition of arbiters in your replica set.

Key Points:

• Single Arbiter Recommendation: Use a single arbiter to avoid data consistency issues. Multiple arbiters can prevent reliable use of the majority write concern, leading to potential data loss if a secondary node falls behind and the cluster is reconfigured.
• Default Behavior: By default, MongoDB 5.3 and later versions do not allow multiple arbiters. If you attempt to add multiple arbiters, the server will return an error unless all nodes are started with the --setParameter 'allowMultipleArbiters=true' option.
• Configuration: To add multiple arbiters, start each node with the allowMultipleArbiters parameter set to true:

mongod --setParameter allowMultipleArbiters=true

Example Error:

If you try to add multiple arbiters without the necessary parameter, you might encounter an error like:

MongoServerError: Multiple arbiters are not allowed unless all nodes were started with --setParameter 'allowMultipleArbiters=true'

Security Considerations:

• Authentication: Arbiters exchange credentials with other set members to authenticate, and this process is encrypted.
• Communication: Votes during elections, heartbeats, and configuration data exchanges are not encrypted unless your deployment uses TLS/SSL.

To ensure smooth operation and avoid quorum issues, adhere to the single arbiter recommendation and configure your replica set accordingly.

Related Resources

Upgrade a Replica Set to 6.0
Docs

6.0 Changelog
Docs

Upgrade a Sharded Cluster to 6.0
Docs

Replica Set Arbiter
Docs

Release Notes for MongoDB 6.0
Docs