What is the purpose of colons within Redis keys

The colons have been in earlier redis versions as a concept for storing namespaced data. In early versions redis supported only strings, if you wanted to store the email and the age of 'bob' you had to store it all as a string, so colons were used:

SET user:bob:email [email protected]
SET user:bob:age 31

They had no special handling or performance characteristics in redis, the only purpose was namespacing the data to find it again. Nowadays you can use hashes to store most of the coloned keys:

 HSET user:bob email [email protected]
 HSET user:bob age 31

You don't have to name the hash "user:bob" we could name it "bob", but namespacing it with the user-prefix we instantly know which information this hash should/could have.

Colons are a way to structure the keys. They are not interpreted by redis in any way. You can also use any other delimiter you like or none at all. I personally prefer /, which makes my keys look like file system paths. They have no influence on performance but you should not make them excessively long since redis has to keep all keys in memory.

A good key structure is important to leverage the power of the sort command, which is redis' answers to SQL's join.

GET user:bob:color   -> 'blue'
GET user:alice:color -> 'red'

SMEMBERS user:peter:friends -> alice, bob

SORT user:peter:friends BY NOSORT GET user:*:color   -> 'blue', 'red'

You can see that the key structure enables SORT to lookup the user's colors by referencing the structured keys.