What a cache is actually doing
A cache stores a result that can be reused so an expensive operation does not have to run every time. Cached data might come from a database query, computation or external API.
Cache-aside is a common pattern
The application checks the cache first. On a miss it loads the source data, stores the result and returns it. A later request can then use the cached value.
Invalidation is the difficult part
Cached data can become stale when the source changes. Decide how long values may remain stale, when entries are invalidated and what the application should do when the cache is unavailable.
Practice: implement a small TTL-style cache for a slow calculation and test both cache hits and expired entries.
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