Experiment: Websockets

[RobinTail]

Exploring the possibilities based on the discussion: #991

In the end I moved it to a separate repo: https://github.com/RobinTail/zod-sockets

Concept

• Using socket.io library as on optional peer dependency for the best quality service including acknowledgements (I believe it's an important feature).
• Sockets is an opt-in feature by applying the new attachSockets() method on the httpServer or the httpsServer returned by the createServer() method;
• Incoming events (from a client) are handled by the entities called Actions having validation schemas for the incoming payload and the outgoing acknowledgement (optional), similar to Endpoint.
• Those schemas, in opposite to the Endpoint, are not z.object(), but z.tuple().
• The returns of the Action's handler is the outgoing acknowledgement (optional), which must be the last argument received in a raw payload (a callback function, optional).
• Actions are combined into an ActionsMap, a dictionary associating the incoming event names with their handlers, similar to Routing.
• Actions are produced on an ActionsFactory (similar to the EndpointsFactory).
• That factory is a class, the constructor of which accepts the EmissionMap — a dictionary associating the outgoing event names with their payload validation schemas (including acknowledgements, optional).
• That makes each Action produced on the ActionsFactory aware of the available events, the payload constraints of which is ensured by the method emit() provided for the Action's handler.
• The main attachRouting method also requires the emission for ensuring constraints on the offered features: onConnection, onDisconnect and onAnyEvent which are having the same type as Action handlers and equipped with the emit() method, allowing customizations.

Next

• In case of success, the event maps can be documented according to AsyncAPI specification.

[coveralls]

coverage: 94.857% (-5.1%) from 100.0%
when pulling b5a5ef7 on exp-websockets
into 9240b34 on master.

[RobinTail]

At this moment it seems that the feature should probably be a separate package.

[RobinTail]

Another problem may come from assumption that acknowledgement should be emitted in the end of the incoming event processing. But it may not be generic enough.

Actually, it's kinda OK, because otherwise there is no way to ensure that ack callback would be called.
Immediate acknowledgement can be achieved by using setTimeout(..., 0); return ...;

[RobinTail]

closing this PR in order to continue in a separate repo:
https://github.com/RobinTail/zod-sockets