How to get all 'key namespaces' (object types) in Redis?

Question

As we may know, good naming convention for Redis keys is "object-type:id"

How to get all key namespaces (symbols before :)?

E.g.

127.0.0.1:6379> KEYS *
1) "bar:1"
2) "baz:1"
3) "baz:3"
4) "foo:1"
5) "foo:2"
6) "baz:2"

I want a command like

127.0.0.1:6379> SCAN 0 MATCH "(^\w+):" COUNT 10
1) "6"
2) "foo"
3) "bar"
4) "baz"

But seems like MATCH option of SCAN command does not support PCRE pattern syntax.

In real, I got

127.0.0.1:6379> SCAN 0 MATCH "(^\w+):" COUNT 10
1) "6"
2) (empty list or set)

Any ideas ?

Answer 1

You could create an index key and add each namespace to it every time you add a new key: SADD "namespaces" "foo". But there would be problem with deleting them, as you'd have to check if the deleted object was the last of it's type.

The second option would be creating a module (if you're using Redis >= 4.0) with a command that you could use to add and remove said keys, that would have it's own type in which it would store object-type counters (so it wouldn't have to call KEYS * which is quite slow). The problem here would be if you had key expiration, or oom key deletion configured. The counter would be incorrect once a key gets deleted by Redis itself.

The third option is to create a module with one command that would call KEYS *, then get the first part of the key and add it to a hash map/dictionary (using RedisModuleDict) and then return it using RedisModule_ReplyWithString(ctx, str) in a loop

I think that you could also write it in Lua instead of creating a module, but a module written in C is probably going to be faster... and I don't know Lua so I won't be able to help you there.

You could always write a simple script in bash that would get the object-type from each key and SADD it to a temporary key then read it, print and delete afterwards, but that's the same as the third option but slower and has to use a temporary key which might be used by some other script or something.