Cassandra 3.x (currently 3.10 / 3.0.12) has a number of significant improvements, including completely rewritten storage, materialized views, and time-windowed compaction.
As part of {T156199}, we are considering a migration strategy that would downsize the current cluster to free up enough hardware nodes that we could setup a secondary cluster, deploy the updated storage module, and migrate use-cases to it incrementally. If we decide on this approach, it would provide a unique opportunity to deploy this new cluster with Cassandra 3.x, and avoid the high costs associated with an in-place upgrade later.
The first step is to research outstanding issues, attempt to identify potential risks to our use-cases, and select a version to begin testing with.
(WARNING) WIP; What follows is still evolving...
== Cassandra Releases ==
=== 2.2.x ===
We are currently using 2.2.6 in production (the latest is 2.2.9). The 2.2 series took quite a bit longer to release than was planned; It was held up by ambitious features that took longer to get completed than expected (most notably, [[ https://issues.apache.org/jira/browse/CASSANDRA-8099 | a complete rewrite of storage ]]). At some point, a decision to release with the (stable) features that had accumulated was made, rather than to make users continue to wait. The net result is that the 2.2 series carried fewer new features than previous major releases; It's basically a bridge between 2.1 and 3.x (which is what should have been the next major release).
The 2.2 series is set to be EOL with the release of 4.0 (TBD).
=== 3.0.x ===
Starting with v3, the Cassandra project moved to a [[ http://www.intel.com/content/www/us/en/silicon-innovations/intel-tick-tock-model-general.html | tick-tock release cycle ]]. Releases are made monthly, even-numbered releases contain new features and bug fixes, odd-numbered releases contain bug fixes only. The idea was to keep trunk as close to releasable at all times, and to get new features (and fixes) out to users for testing more incrementally. Since it was recognized that this might take some time to achieve, the 3.0 release was special-cased to receive all bug-fixes from the 3.x series.
The 3.0 series (currently at 3.0.12) is set to be EOL 6 months after 4.0 (TBD).
=== 3.x (tick-tock releases) ===