What is zset in redis?

A zset, or sorted set, is one of Redis's most powerful data structures. The simplest way to think of it is as a game leaderboard. It's the perfect analogy for what it does.

A zset holds a collection of unique items, which we call 'members'. In this respect, it’s just like a regular Redis Set. The crucial difference, however, is that every member is associated with a numerical 'score'. Redis uses this score to keep the entire collection sorted from the lowest score to the highest.

This constant sorting makes zsets incredibly efficient for specific tasks. For example, you can ask Redis for the 'top 10' members instantly, without needing to sort the data yourself. You can also fetch all members within a certain score range, say between 50 and 100. You can even ask for a specific member's rank within the set.

Under the bonnet, Redis cleverly combines a hash table and a data structure called a skip list. This gives you the best of both worlds: fast lookups of individual members and extremely quick retrieval of ranges.

So, while leaderboards are the classic example, they are also brilliant for priority queues, rate-limiting systems, or secondary indexing. In essence, it’s a unique collection of items, kept sorted by an associated number. A wonderfully useful tool.