[feature] Add support for client-side filter expressions during subscription

[quantum-01]

I'd like to propose adding support for clients to pass an additional filter dictionary when subscribing to a channel/topic.

The core need is this: for each message published to that topic, the Centrifugo server should be able to evaluate a filter expression defined by the client at subscription time. Only if the expression evaluates to true for a given message should that message be delivered to the subscribing client.

This filtering should occur on the server side to efficiently reduce unnecessary message traffic. A suggested implementation approach is to leverage the CEL (Common Expression Language) specification. CEL is well-suited for this type of secure, runtime-evaluated expression logic.

[FZambia]

Hi @quantum-01

Thanks for suggesting. The idea is interesting and I was thinking about it before.

PubNub has sth similar: https://www.pubnub.com/docs/general/channels/subscribe#message-filters

It's not a small addition as should become a part of client protocol, also may be non-trivial to add from publication recovery perspective (requires research how feasible is it given offset constraints). First step here is to find convincing practical use cases where it may be useful. Could you describe what is your use case which requires the feature and why is it hard to solve without this new functionality?

One use case I personally thought about is related to avoid delivering message published by current client, sth Centrifugo does not support now in any form, for a reason, but probably those reasons are not very important for some use cases. CEL filter could look like this (supposing backend attaches tags to publication with publishing client_id in them and client_id is a connection context variable):

publication.tags['client_id'] != client_id

It's still racy, because may collide with reconnect, but if we look at it as a best effort to reduce traffic – may be OK.

[quantum-01]

Indeed, we desire a server-side filter mechanism similar to PubNub's. We have numerous scenarios requiring conditional subscription support. You can refer to this page: https://docs.birdeye.so/docs/websocket – all the APIs there require conditional filtering support.​

[FZambia]

This may be addressed in the alternative way implemented in centrifugal/centrifuge#511

@quantum-01 @thinkwelltwd - please check it out, it seems a much better fit for Centrifugo compared to CEL (reasoning is in the linked PR description).

[FZambia]

Centrifugo v6.4.0 has implementation for filtering publications based on message tags.

Here is a blog post which explains some decisions and why CEL was not used in the implementation - Publication filtering by tags - reducing bandwidth with server-side stream filtering