[justnoise]

I'd guess that Discord's storage systems lean towards processing a lot more writes than reads. Postgres and other databases that use B-tree indexing are ideally suited for read heavy workloads. LSM based databases like Cassandra/Scylla are designed for write intensive workloads and have very good horizontal scaling properties built into the system.

[Aeolun]

Would you actually have more writes than reads? Are messages read by fewer people than post them?

[sadeshmukh]

When you send a message, afaik it sends to all people looking at it at the time. So there is no read when in a conversation, and maybe the reads are batched when reading multiple.

[jhgg]

Read traffic is much higher than write traffic due to mobile clients needing to sync chat history more often as their sessions are much shorter lived. Also search queries execute 1 query per result. And don't forget people doing GDPR data dump requests. It adds up.