Redis

“Redis is single-threaded.” You’ve heard this in every system design interview, every blog post, every tech talk. It was true in 2009. It’s not true anymore - and hasn’t been since 2020. But the way Redis adopted multithreading is more interesting than just flipping a switch. It’s a story about knowing exactly where your bottleneck is and only parallelizing that part. Why Single-Threaded Was the Right Call When Salvatore Sanfilippi built Redis in 2009, he made what seemed like a strange choice - a single-threaded event loop for a high-performance database. But it wasn’t strange at all. It was the smartest possible design for an in-memory data store. ...

You spin up a single Redis instance, throw your session data in it, and everything works great. Then your app grows. One day Redis goes down for 30 seconds during a deploy, and every user gets logged out. Your manager asks: “Why don’t we have high availability?” You Google “Redis HA” and find Sentinel, Cluster, sharding, replication, pipelining - and suddenly a key-value store feels as complex as a distributed database. ...

When Redis changed its license in 2024, the community forked it into Valkey. Two years later, there is a clear answer on which one most teams should use.