Support for GraphQL over WebSocket Ping and Pong message types

[ExidCuter]

This PR implements the missing PING and PONG GraphQL WS messages for the client-side "heartbeat / keep alive" function.

Now the system does not close the connection when it receives a PING message, but sends a corresponding (PONG) message back, as specified in the graphql-ws protocol.

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[rstoyanchev]

I'm not sure WebGraphQlHandler should be involved. It's more of an internal API for invoking the WebInterceptor chain, it's not meant to be implemented by an application. I think for now we would simply respond to the the ping with a pong. The payload on the pong is optional.

[ExidCuter]

Hi, I'm not quite sure I understand your comment. Do you mean I should remove handleWebSocketPing and handleWebSocketPong from the WebGraphQlHandler interface and just respond directly with Mono?

I added methods to WebGraphQlHandler because the PING and PONG messages are GraphQL-WS message types and I thought they should be implemented the same way as the others.

[rstoyanchev]

Yes, I meant that those methods should be removed from WebGraphQlHandler. They don't serve any purpose and they don't help to handle PING messages. For now we would automatically handle those.

[rstoyanchev]

The code has evolved since, so I've applied the equivalent change manually, also on the client side since these are bidrectional messages. Thanks for the PR in any case!

[andrey-nakin]

@rstoyanchev

• Does server automatically reply to PING messages sent from the client?
• Can server send PING messages to the client periodically? Is there any configuration property for this?

[rstoyanchev]

Yes the server replies automatically. There isn't a way to send pings.