Cassandra 5.0 Unified Compaction Strategy (UCS)

[pjfanning]

Discussed in #363

^{Originally posted by EmadAlblueshi January 27, 2026}
Hello Everyone,

The current compaction strategy employed for the Apache Cassandra tables is SizeTieredCompactionStrategy which is fine for version 4 but version 5 is using UnifiedCompactionStrategy as the default strategy and recommended

The UnifiedCompactionStrategy (UCS) is recommended for most workloads, whether read-heavy, write-heavy, mixed read-write, or time-series. There is no need to use legacy compaction strategies, because UCS can be configured to behave like any of them.

Do we have any plans to update the current tables to use UnifiedCompactionStrategy in line with version 5 best practices?

[pjfanning]

Seems like a useful addition. Would anyone be interested in taking this on and doing a PR?

[pjfanning]

I've done a small bit of reading about this and am still not an expert but the docs in 5.0 still recommend SizeTieredCompactionStrategy and does have 1 mention of the Unified Compaction Strategy but I'm not sure how this works in CQL.

https://cassandra.apache.org/doc/5.0/cassandra/developing/cql/indexing/sai/operations/configuring.html#compaction-strategies

So the main issue would be to try to work out the CQL for the Unified Compaction Strategy because that's what the Pekko classes for compaction strategies set.

[danischroeter]

Seems the documentation has been updated - looks reasonable to change the table generation to ucs for cassandra 5

see
https://cassandra.apache.org/doc/stable/cassandra/managing/operating/compaction/stcs.html

[pjfanning]

I'm not recommending anything in this comment and if anyone wants to try any changes to their Cassandra tables, then they need to take great care and thoroughly test it. It does appear like it should be feasible for users to modify their Casandra tables to change the compaction strategy themselves.
pekko-persistence-cassandra seems to only really care about the config settings when it creates the tables.

We should make changes in pekko-persistence-cassandra but it seems probable that users can modify the existing tables themselves.

I suspect that if we make pekko-persistence-cassandra changes, that we won't likely be modifying the existing tables for existing users - that if we change, it is likely to be that UCS would be supported when creating new tables.

[danischroeter]

@pjfanning fully agree.
We should just provide the new strategy for initial table creation and document the recommendation to optionally alter the existing table strategy (manually).