subscription interface and event streaming for low latency push events

[andorsk]

TP here: https://hackmd.io/vrC1iCudTVuM8nryo3QQKA#Subscribing-to-your-own-Node

A few things I want to clarify here:

1. Subscriptions are not limited to WRITE events. Subscriptions are generalized to an event stream, which is allows you to stream EventMessages into the stream. The EventStream will emit events. You can build EventStreams with filters to build a dynamic pipeline for listeners.
2. Scopes are not implemented yet ( for filters and event scope ), but will be implemented soon.
3. There's some conversation about how grant authorization works on a persistent stream.

SubscriptionRequest returns an EventEmitter, which is what a dwn will listen to for streaming events.

sequenceDiagram
  participant Bob
  participant Alice 
  participant EventStream 

  loop Permission Grant Flow
    Bob -->> Alice : Permission Grant of type SubscriptionRequest
    note left of Alice: authorizes or rejects
    Alice -->> Bob : Permission Grant sent back with SubscriptionRequest scope
  end
  loop Subscription Flow 
    Bob -->> Alice : Subscription Request to Alice
    note left of Alice : validates

    EventStream --> Alice : subscriptionRequestHandler returns an Event Stream to Alice
    Alice -->> Alice : action such as writes to Dwn. processMessage sends Event to EventStream
    Alice -->> EventStream : event stream receives message and emits it over "event" channel
    EventStream -->> EventStream : subscriptions chains events over eventstream. 
    EventStream -->> Alice : emits filtered event to Alice if found. 
    Alice -->> Bob: sends to Bob
  end

Loading

Note: the dwn-sdk-js is not in charge of actually managing the connection between Alice and Bob. Alice is simply getting an "event hook" to listen into that is scoped to a subscription request. She then can choose to send it or not.

I have an update to dwn-server to handle the web socket connections coming.

State: Initial feedback would be helpful, but not ready for merging yet.

[LiranCohen]

Just left a few small nits, and a comment about signing the event log messages.

[LiranCohen]

Sanity: I don't think we're not able to sign here because EventLog.append() is called from within the handler after the message has already been signed and received by the DWN.

But I think you're right, not sure we need to sign this at all.

[andorsk]

yea...i fixed this by just forwarding the message itself for now.

[LiranCohen]

Suggested change

	messageId?: string;
	messageCid?: string;

[LiranCohen]

Suggested change

	messageId : options.messageId,
	messageCid : options. messageCid,

[LiranCohen]

Suggested change

	messageId?: string; // attached message
	messageCid?: string; // attached message

[LiranCohen]

Closing, this work has been migrated to: #607