JSON.DEL emits no del event when removing the entire value

[martin-cowie]

The Observation

When listening to the keyspace events for built-in complex data structures (LIST, SET, HASH, etc.) we observe that when a complex value is left empty because of the actions of a command, that the value is removed and a del keyspace event emitted. We can interpret this as evidence that the value has been removed.

Using a Redis list for example: open two CLIs and call them "Listener" and "Actor"

Listener

1. CONFIG SET notify-keyspace-events AKE
2. PSUBSCRIBE __keyspace@0__:*

Actor

1. create a list value LPUSH my:list:value 1 2 3
2. empty it: 
LPOP my:list:value 3

Listener

1) "pmessage"
2) "__keyspace@0__:*"
3) "__keyspace@0__:my:list:value"
4) "lpush"
1) "pmessage"
2) "__keyspace@0__:*"
3) "__keyspace@0__:my:list:value"
4) "lpop"
1) "pmessage"
2) "__keyspace@0__:*"
3) "__keyspace@0__:my:list:value"
4) "del"

Observe the del event at the tail of the output. This pattern is true across all built in datatypes.

Compared to a RedisJSON value, using the same two Redis CLIs

Listener

1. CONFIG SET notify-keyspace-events AKE
2. PSUBSCRIBE __keyspace@0__:*

Actor

1. create a JSON value 
JSON.SET my:json:value $ '{"life":42}'
2. delete it: 
JSON.DEL my:json:value

Listener

1) "pmessage"
2) "__keyspace@0__:*"
3) "__keyspace@0__:my:json:value"
4) "json.set"
1) "pmessage"
2) "__keyspace@0__:*"
3) "__keyspace@0__:my:json:value"
4) "json.del"

We observe no del event. As JSON.DEL can also remove subsets of the the RedisJSON value we cannot interpret json.del as proof of the values removal.

Proposal

If a RedisJSON value removed from the database the RedisJSON module should also emit a del event for consistency with built-in datatypes.