Automatically removing deleted keys from a set

[Luveh Keraph]

Consider the following sequence:

127.0.0.1:6379> SADD My:Set SomeKey

(integer) 1

127.0.01:6379> SISMEMBER My:Set SomeKey

(integer) 1

127.0.0.1:6379> DEL SomeKey

(integer) 1

127.0.0.1:6379> KEYS SomeKey

(empty array)

127.0.01:6379> SISMEMBER My:Set SomeKey
(integer) 1

127.0.01:6379> SMEMBERS My:Set
1) "SomeKey"

In spite of the fact that SomeKey has been deleted from the database, it still appears as a member of the My:Set set. 

Is it possible to do things so that when a particular key is deleted it is also automatically removed from all the sets that it may belong to? If not, what is the strategy recommended to adopt in order to make sure that any key deleted from the database is removed from the sets it belongs to, bearing in mind that we may be talking about thousands of sets with thousands of elements each?

[Matt Palmer]

On Thu, Oct 26, 2023 at 08:32:25AM -0600, Luveh Keraph wrote:
> Consider the following sequence:
>
> 127.0.0.1:6379> SADD My:Set SomeKey
> (integer) 1
> 127.0.01:6379> SISMEMBER My:Set SomeKey
> (integer) 1
> 127.0.0.1:6379> DEL SomeKey
> (integer) 1
> 127.0.0.1:6379> KEYS SomeKey
> (empty array)
> 127.0.01:6379> SISMEMBER My:Set SomeKey
> (integer) 1
> 127.0.01:6379> SMEMBERS My:Set
> 1) "SomeKey"
>
> In spite of the fact that SomeKey has been deleted from the database, it
> still appears as a member of the My:Set set.

*Technically*, it's not the *key* that appears as a member of the set, it's
a string that just happens to have the same name as a key. Redis is not a
relational data store, and attempts to use it as one often end in sadness.

> Is it possible to do things so that when a particular key is deleted it is
> also automatically removed from all the sets that it may belong to?

Not in open source Redis, as far as I'm aware, but there appears to be a
keyspace trigger feature in the Redis Cloud commercial product:
https://redis.io/docs/interact/programmability/triggers-and-functions/concepts/triggers/keyspace_triggers/

> If not, what is the strategy recommended to adopt in order to make sure
> that any key deleted from the database is removed from the sets it belongs
> to, bearing in mind that we may be talking about thousands of sets with
> thousands of elements each?

I can think of two ways:

1) Do it in your application, and ensure atomicity with a MULTI/EXEC block;
or

2) Write lua procedures that do all the necessary work of managing the
sets alongside manipulation of the data in the keys themselves, and call
those instead of the low-level data manipulation operations directly.

- Matt