I've tried to install apache Cassandra on latest Debian (bullseye), to use as a storage backend of RESTBase instead of sqlite, to see if it improves storage usage. It has been an impossible task and very time consuming. So please, don't try yourself unless you're adventurous and have a lot of time to spend on this!
I started by installing the latest Cassandra (version 4). Cassandra works. After configuring RESTBase to use Cassandra as the backend, RESTBase doesn't start. Or rather, it starts, but it never reaches the "startup finished" state... It connects to Cassandra, initializes everything, and then it seems to get stuck somewhere to the point that it never listens to the configured port.
Then I decided to install version 2.2.x of Cassandra, to see if it was a compatibility problem. WMF seems to be using 2.2.x (OMG very old version!). But neither it or 3.11.x works on latest Debian. Cassandra won't start. It fails with errors that require editing several config files, removing configurations not supported on recent Java versions... and after making it run, looks like some tools like nodetool will refuse to work, with cryptic errors that apparently are caused by not using Java 8.
Yes, Cassandra 2.x and 3.x only supports Java 8, which is not included by default in Debian. Even cassandra 4.x says support for Java 11 is experimental.
Is it worth the hassle? No
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