Is it OK to have nodes with mixed major versions running for months?

We are planning to upgrade to Cassandra 4 (currently on 3.x).

The question is -> Is it ok for Cassandra cluster to run for extended periods (months) in a mixed mode. Like some datacenters Cassandra 4.0.x and some Cassandra 4.1.x? If not allowed, what is the worst thing can happen?

Thanks in advance.

We do not recommended running a cluster in "mixed mode", specifically where nodes are running different major versions of Cassandra for longer than 1 or 2 days.

There is a long list of operational limitations you will run into while the cluster is in mixed mode. They include the following:

• Inability to perform schema changes since schema versions between major versions are incompatible.

• Inability to TRUNCATE tables.

• Inability to perform any operation that requires streaming including repairs, topology changes (node additions, decommissions) and bulkloads with sstableloader.

These limitations will be very challenging in a production environment since they will hamper your ability to maintain the optimal performance of your cluster. For example, if you encounter a hardware failure then you will not be able to bootstrap the node back into the cluster until all nodes have been upgraded.

We recommend that you at least upgrade the binaries on all nodes in all DCs. You can then delay the SSTable upgrades until such time that you are able to schedule them. Cheers!

StackExchange Database Administrators Q#330282, answer score: 7