Shape of Phoenix Channel socket messages


I’m wondering what the first 2 indices of the arrays sent/received over Phoenix socket connections are. Here are some examples from my Network tab in the browser dev tools:

(send)    ["3","3","room:36","phx_join",{}]
(receive) ["3","3","room:36","phx_reply",{"response":{"some_join_response_key": "some_join_response_value"},"status":"ok"}]
(send)    [null,"4","phoenix","heartbeat",{}]
(receive) [null,"4","phoenix","phx_reply",{"response":{},"status":"ok"}]
(send)    ["3","5","room:36","event_initiated_from_this_window", <data...>]
(receive) ["3",null,"room:36","event_initiated_from_other_window", <data...>]
(receive) [null,null,"room:36","presence_diff",{"joins":{"2": <presence info>},"leaves":{}}]

The second looks like the heartbeat counter. What I’m confused about is what the "3" means in the first index, and under what conditions do each of them get set to null.

I was trying to find some answers in the source code, but I couldn’t find anywhere parsing a message as an array. This line in socket.js seems most relevant, but not sure how it ties in since it expects an object.

FWIW, I ask because I’m starting to write front-end tests for a React app which gets its data from a Phoenix channel. To do this, I’m mocking a WebSocket connection, sending the client data, and asserting what gets rendered. So mostly what I’m wondering is whether it makes any difference to pass bogus values for those fields in a mock setting. Also, if there is any advice on that use case as well, I’d love to hear :slight_smile:


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First “3” (0th position) Channel Id or topic Id which is maintained internal to the socket.

Second “3” (1st position) is the message id - this will be used in match request and response when a message is received. This is called refId

room:36 (2nd position) is the topic name or channel name

"phx_join" (3rd position) is the event name

"{}" is the payload or message received or sent.

Heartbeat won’t not have topic Id, but will have refId.

Push message from server on a channel wont have message id or refId. Below message is mostly server push.

I am recollecting from a C# implementation of channels I made.

I had written a event recorder and event player socket which extends base socket. Event recorder records time and message sent - all this is persisted in a json file. Event player socket will replay the messages with exact timing when it is initialised. We use this for testing, debugging and replaying player sessions.


Those are ref and join_ref as described here: phoenix/socket.ex at b225c772648a678fe18d9515292eff47b159e2b1 · phoenixframework/phoenix · GitHub


@kartheek @moogle19 Thank you both!! I should be able to figure out what I need from this.

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