Get Notification only if the value of a key has been changed ?

[Josef Meier]

Hi,

I have a few publishers to my redis database and many subscribers.

I'm trying to find a way to notify the subscribers by the redis-server if a key's value(!)  has been changed (!). Currently I use the keyspace notification mechanism of redis like this:

psubscribe __key*__

The reason for doing that is to minimize the network traffic in my system while publishers are writing into the database.

Example of what I try to gain:

If I send this in redis-cli:

set LULU 1234

set LULU 1234

I only want to get a notification for the first "set" (cause it has changed the value of LULU) and NOT for the second "set" (cause it doesn't change the value of LULU).

How is this possible?

Do I have to patch redis for that :-( ?

Best regards,

Josef

[Itamar Haber]

Hi Josef,

> Do I have to patch redis for that :-( ?

It appears so - Redis doesn't differentiate between an update that actually changes the value and one that doesn't. Alternatively, you could perform the `SET` using a Lua script that also performs the equality check and emit a regualr PubSub (not keyspace notification) message from it to the relevant subscribers.

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[Josef Meier]

Dear Itamar,

thanks for your response.

I'm surprised that this performance gain (reduced network load if key value doesn't change) is not supported out of the box by redis.

A global flag in redis which allows to do this verify in the core c code would be easy to implement and should be useful for others to in my opinion. Any chance to open a feature request for that?

Best regards,

Josef

[Itamar Haber]

Inline

On Wed, Dec 23, 2015 at 4:51 PM, Josef Meier <jo.m...@gmx.de> wrote:

Dear Itamar,

thanks for your response.

My pleasure always.

 

I'm surprised that this performance gain (reduced network load if key value doesn't change) is not supported out of the box by redis.

The gain is offset by the loss - an additional check (compare old and new value) has a computational price on the server :) One of the reasons, perhaps, why this isn't available OOtB is because it wasn't requested.

 

A global flag in redis which allows to do this verify in the core c code would be easy to implement and should be useful for others to in my opinion. Any chance to open a feature request for that?

I don't know how easy/hard it will be to implement, but feature requests can and should be made by anyone. You can discuss them here - usually a good idea - to get feedback from the community. If you feel apt/up to it, you can even make a pull request to the Redis repository with the proposed changes to merge.