Add connection_error to protocol

[andyrichardson]

Story

Currently there is no way to send an error if a semantically invalid message is sent on connection_init.

It would be great if the protocol supported a connection_error message similar to this.

Note: I'm aware of the existing error message - it requires a subscription ID which would not be present at connection_init time

Acceptance criteria

• The protocol accommodates for errors on connection_init
• These errors do not have an id attribute

[michelepra]

what about #75 and #78 ???

[andyrichardson]

I was looking through the server side code and it looks like the current implementation closes on failure following connection_init - I'll go with that 👍

[enisdenjo]

Exactly as @michelepra suggested. The WebSocket close event is meant to communicate the connection close reason. You're free to pick any user-land close code number within 4000-4999 and whatever close reason string you like (or even a stringified JSON if you want to transfer more meaning). Use the client-side error or on.disconnect to act accordingly.

[lovasoa]

Hasura GraphQL uses a connection_error message: #499

[enisdenjo]

This connection_error message type is OK in the deprecated subscriptions-transport-ws, but does not exist in the GraphQL over WebSocket spec and should not be used.

Also read #112 (comment).

[lovasoa]

But is is actually used in real applications that are deployed out there... Maybe it should be added to the PROTOCOL.md in this repo ?

[enisdenjo]

Mainly because deprecated subscriptions-transport-ws is still out there.

[lovasoa]

I'm struggling to see any upside to not adding it to PROTOCOL.md, but I clearly see the downside: people connecting to existing applications not being able to debug connection errors. Maybe the upside is avoiding to encourage new implementers to use it instead of the connection close reason ? But then, the spec could just say that. The spec in this repo is not of much use if it does not describe what's actually out there.

[enisdenjo]

As explained in #112 (comment), the connection_error message is superfluous and therefore completely unnecessary.