How Discord Stores Billions of Messages (via) Fascinating article from 2017 describing how Discord migrated their primary message store to Cassandra (from MongoDB, but I could easily see them making the same decision if they had started with PostgreSQL or MySQL).
The trick with scalable NoSQL databases like Cassandra is that you need to have a very deep understanding of the kinds of queries you will need to answer - and Discord had exactly that.
In the article they talk about their desire to eventually migrate to Scylla (a compatible Cassandra alternative written in C++) - in the Hacker News comments they confirm that in 2021 they are using Scylla for a few things but they still have their core messages in Cassandra.
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