Redis

Redis does one thing extremely well: answer in microseconds with data that has already been computed. That is why it fits in three specific places - cache, queues and sessions - and sits awkwardly almost everywhere else.

The decision that matters is not installing it. It is what gets stored and when it stops being valid. A cache with no invalidation policy does not make a site faster; it makes it a site that serves old data unpredictably. The key has to include everything that changes the answer - language, permissions, deployed version - or sooner or later one user sees another user's content.

The second decision is what happens when it is not there. A cache has to be able to fail without taking the site down: the application loses speed and keeps answering. A session store or a job queue does not allow that, and this is where the high-availability topology comes in - a replica and Sentinel, so promotion is automatic and the application never has to know which node is in charge.

In practice that comes down to unglamorous things: the version pinned per environment instead of following the latest tag, the connection string injected as a secret rather than written into the image, and a memory limit with an eviction policy chosen deliberately instead of whatever the default is.

What almost nobody measures is the hit rate. A cache running at 30% hits adds a dependency and a network round trip in exchange for very little, and until it is instrumented it looks like it works because the site has not broken. Measuring hits and misses per key type is what separates a cache that earns its place from one that only occupies memory.

There is also a prior question worth asking before installing anything: why is the query you want to cache slow. Very often the reason is a missing index or a query pulling columns nobody reads, and there a cache does not fix the problem - it hides it, and adds something to maintain. Caching a query that is already fast makes sense when it runs thousands of times a minute; caching a slow one usually just covers up the real fix.