Redis - Automatic creation and removal of keys

Automatic creation and removal of keys

So far in our examples we never had to create empty lists before pushing elements, or removing empty lists when they no longer have elements inside. It is Redis' responsibility to delete keys when lists are left empty, or to create an empty list if the key does not exist and we are trying to add elements to it, for example, with LPUSH.

This is not specific to lists, it applies to all the Redis data types composed of multiple elements -- Streams, Sets, Sorted Sets and Hashes.

Basically we can summarize the behavior with three rules:

1. When we add an element to an aggregate data type, if the target key does not exist, an empty aggregate data type is created before adding the element.
2. When we remove elements from an aggregate data type, if the value remains empty, the key is automatically destroyed. The Stream data type is the only exception to this rule.
3. Calling a read-only command such as LLEN (which returns the length of the list), or a write command removing elements, with an empty key, always produces the same result as if the key is holding an empty aggregate type of the type the command expects to find.

Examples of rule 1:

>_ Redis CLI

> DEL new_bikes
(integer) 0
> LPUSH new_bikes bike:1 bike:2 bike:3
(integer) 3

 

However we can't perform operations against the wrong type if the key exists:

>_ Redis CLI

> SET new_bikes bike:1
OK
> TYPE new_bikes
string
> LPUSH new_bikes bike:2 bike:3
(error) WRONGTYPE Operation against a key holding the wrong kind of value

 

Example of rule 2:

>_ Redis CLI

> RPUSH bikes:repairs bike:1 bike:2 bike:3
(integer) 3
> EXISTS bikes:repairs
(integer) 1
> LPOP bikes:repairs
"bike:3"
> LPOP bikes:repairs
"bike:2"
> LPOP bikes:repairs
"bike:1"
> EXISTS bikes:repairs
(integer) 0

 

The key no longer exists after all the elements are popped.

Example of rule 3:

>_ Redis CLI

> DEL bikes:repairs
(integer) 0
> LLEN bikes:repairs
(integer) 0
> LPOP bikes:repairs
(nil)

 

 

 

Copied from: https://redis.io/docs/data-types/lists/