Caching

Redis Basics

Redis is a key-value store that lives in memory, and executes one command at a time. In-memory nature makes it very fast (a single node can handle 100K writes per second), while the sequential model makes it easy to reason about operations.

Redis has two persistence modes. RDB takes periodic snapshots, and AOF logs every write but only fsyncs once per second by default – therefore, Redis is not durable. While you can configure AOF to fsync on every write, you’d lose the speed that you came to Redis for.

Cache Basics

For read-heavy applications, storing frequently accessed data in fast memory (e.g., Redis) allows you to skip the DB entirely for some reads. A cache hit on Redis takes ~1ms compared to 20-50ms for a typical DB query, and this speedup is impactful in the order of millions of requests. In a web application, user sessions are typically stored in a distributed cache, allowing the system to quickly retrieve the data when a user makes a request.