I'm sql coder and new in Cassandra, which version should I choose, Cassandra 3.X or Cassandra 2.X?
We already have cassandra-2.1.17 but it could not support any aggregate function at all, I want to upgrade to cassandra 3.X but I don't know much about the difference, just want to get some advice, appreciate.
Usually the best it's to go with latest versions.
Cassandra 2.x: You should have in mind that if you need stability, go for 2.2.x/2.1.x, but those will be out of support when Cassandra 4.0 will be out. The latest releases are 2.2.11 (changes, release notes) and 2.1.19 (changes, release notes), both of them released in 2017.10.05. Go for 2.2.11 if you need something that goes well and you like playing it safe. 2.2.11 is low risk.
Cassandra 3.0: Probably it's better to skip this, because it's pretty buggy, especially on the storage. But you definitely should go with 3.0.15 if you plan to pick 3.0. You can check the changes and the release notes. Also have in mind that materialized view are still in experimental phase or, better to say, the community might start marking those as experimental. Probably it's best to skip 3.0 and go directly to 3.11.x.
Cassandra 3.x: Probably the latest version should be more stable. 3.11.1 has been released on 2017.10.10. You can check the changes and the release notes.
Final thoughts: at the end it's you who decides based on your use cases and data or if you know that are features that you need/don't need or if there are bugs that you can cope with.
The latest stable release of Cassandra is 3.11.1. It would be good to upgrade yourself with it. I have tested and used 3.11.0 with satisfaction. You can check the latest release and changelog here - http://cassandra.apache.org/
While updgrating to 3.0.15 or 3.11.1, please check below open jira tickes and see if it does not effect your usecase.
3.0.15 :
CASSANDRA-13900(Major) Massive GC suspension increase after updating to 3.0.14 from 2.1.18
3.11.1:
CASSANDRA-13929(Major) BTree$Builder / io.netty.util.Recycler$Stack leaking memory CASSANDRA-13221(Minor) Connection reset handling not working in case of protocol errors
