DataStax Upgrade Guide

1. Familiarize yourself with the changes and features in this release:
   • General upgrade advice and Cassandra features in NEWS.txt. If you are upgrading from an earlier release, read NEWS.txt all the way back to your current version.
   • Cassandra changes in CHANGES.txt.
2. When your Cassandra upgrade includes a major version (for example, from Cassandra 1.2 to 2.0) or a major point release (for example, from Cassandra 2.0 to 2.1), upgrade the SSTables on each node to ensure that all SSTables are on the current version. If the SSTables are already on the current version, the command returns immediately and no action is taken.

   $ nodetool upgradesstables

3. Run nodetool drain before shutting down the existing Cassandra service. This prevents overcounts of counter data, and also speeds up restart post-upgrade.
4. Stop the node.
5. Back up your configuration files.

   Depending on how you install the product, these files might get overwritten with default values during the installation. After backing up your configuration, follow the appropriate installation instructions depending on your current installation type.

6. Install the new version of Cassandra:
   • Upgrading Debian-based installations
   • Upgrading RHEL-based installations
   • Upgrading Tarball installations
7. Configure the new product.
   Using the backups you made of your configuration files, merge any modifications you have previously made into the new configuration files for the new version. Configuration options change often, so be sure to double check the version restrictions for additional steps and changes regarding configuration.
   Ensure that the latest default values from cassandra-env.sh match your local cassandra-env.sh file.
8. Start the node.
   An explicit compatibility check is performed on startup. If you skipped any upgrade step, startup fails. Changes from the new release are not modified, and SSTables are not upgraded to the new format. Review the error messages to correct the problem and then start the node.
9. One each node after the upgrade is performed, run $ nodetool upgradesstables.
   Upgrading SSTables is required for Cassandra upgrades that include a major version (for example, from Cassandra 1.2 to 2.0) or a major point release (for example, from Cassandra 2.0 to 2.1).
10. Check the logs for warnings, errors, and exceptions.
11. Repeat these upgrade steps on each node in the cluster.

Cassandra configuration files

Source : https://docs.datastax.com/en/upgrade/doc/upgrade/cassandra/upgradeCassandraDetails.html