Cassandra chat table design - cassandra

Question

For my chat table design in cassandra I have the following scheme:
USE zwoop_chat
CREATE TABLE IF NOT EXISTS public_messages (
chatRoomId text,
date timestamp,
fromUserId text,
fromUserNickName text,
message text,
PRIMARY KEY ((chatRoomId, fromUserId), date)
) WITH CLUSTERING ORDER BY (date ASC);
The following query:
SELECT * FROM public_messages WHERE chatroomid=? LIMIT 20
Results in the typical message:
Cannot execute this query as it might involve data filtering and thus
may have unpredictable performance. If you want to execute this query
despite the performance unpredictability, use ALLOW FILTERING;
Obviously I'm doing something wrong with the partitioning here.
I'm not experienced with Cassandra and a bit confused about online suggestions that Cassandra will make an entire table scan, which is something that I don't really get realistically. Why would I want to fetch an entire table.
Another suggestion I read about is to create partitioning, e.g. to fetch the latest per day. But this doesn't work for me. You don't know when the latest chat message occurred.
Could be last day, last hour, or last week or month for that matter.
I'm pretty much used to sql or nosql like mongo, but this simple use case seems to be a problem for Cassandra. So what is the recommended approach here?
Edit:
It seems that it is common practise to add a bucket integer.
Let's say I create a bucket per 50 messages, is there a way to auto-increment it when the bucket is full?
I would prefer not having to do a fetch of MAX bucket and calculate when the bucket is full. Seems like bad performance for doing inserts.
Also it seems like a bad idea to manage the buckets in Java. Things like app restarts or load balancing would require extra logic.
(I currently use Java Spring JPA for Cassandra).

Answer 1

It works without bucketing using the following table design:
USE zwoop_chat
CREATE TABLE IF NOT EXISTS public_messages (
chatRoomId text,
date timestamp,
fromUserId text,
fromUserNickName text,
message text,
PRIMARY KEY ((chatRoomId), date)
) WITH CLUSTERING ORDER BY (date DESC);
I had to remove the fromUserId from the partition key, I assume it is required to include it in the where clause to avoid the error.
The jpa query:
publicMessageRepository.findFirst20ByPkChatRoomIdOrderByPkDateDesc(chatRoomId);