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Comment by robmccoll | Hacker News Reader

Cassandra is essentially an append-mostly distributed fault-tolerant hash table. If you need specifically that with high write throughput, it's a good choice. I don't understand why people use it as a database. You run into it's limitations immediately and the pain of trying to use it like a database only gets worse with scale.

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FTA:

> In Cassandra, reads are more expensive than writes.

This makes it insane as a message store for a chat server to me. It seems appropriate for a logging destination for a distributed system, one where you want lots of clients to dump data but most of the time you don't even need to audit the logs, so the number of reads for a given item is less than one. This is obviously not true for Discord messages.

Not too sure - I would have guessed that most of the messages are written once, read by the constant number of participants (say 1-100 or so) and then they disappear off the screen and are never accessed again, ever. Maybe a few people will scroll or search, or use some custom extension to load and export the history, but very rarely.

The sentence makes it sounds like Cassandra and Scylla are slow for writes, which isn't the case at all. It's just that writes require a bit less I/O. Reads are still very fast. If reads were slow, nobody would use Cassandra and Scylla for the purposes that they're being used for.

Actually read performance is one of the main challenges in LSM based storages.

All the Casandra documentation and web site say it is a database. You can't blame anyone from getting confused. In my experience, I have never seen a project that started to use it, continue to use it after a year or so it may take a year to run into its limitations before having to replace it, with a database, like Postgres.

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