A piece of hardware or software that stores data, typically meant to retrieve that data faster than otherwise.
Caches are often used to store responses to network requests as well as results of computationally-long operations.
Note that data in a cache can become stale if the main source of truth for that data (i.e., the main database behind the cache) gets updated and the cache doesn't.
When requested data is found in a cache.
When requested data could have been found in a cache but isn't. This is typically refers to a negative consequence of a system failure or of a poor design choice.
For example: If a server goes down, our load balancer will have to forward requests to a new server, which will result in cache misses.
Cache Eviction Policy
The policy by which values get evicted or removed from a cache. Popular cache eviction policies include LRU (least-recently-used), FIFO(first-in-out) and LFU(least-frequently-used).
Content Delivery Network
A CDN is a third-party service that acts like a cache for your servers. Sometimes, web applications can be slow for users in a particular region if your servers are located only in another region. A CDN has servers all around the world, meaning that the latency to a CDN's servers will almost always be far better than the latency to your servers. A CDN's servers are often referred to as PoPs (Points of Presence). Two of the most popular CDNs are Cloudflare and Google Cloud CDN.