Envoy routes for Subscribing, Topic Creation and Event Publishing

[BLasan]

In the current implementation, there are three listeners introduced for Async APIs.

1. listener port 8080 - This serves all routes created per channel to accept requests from event publishers and subscribers. Subscription requests (subscribing to topics) are also sent via these routes. The routes exposed via this listener are exposed to event sources(Ex: github) and API developers(or any other partition) to subscribe and unsubscribe topics.
2. listener port 8083 - Internal communication for gateway-controller and websubhub. This route is used to perform topic registration and deregistration that is initiated by the gateway controller. This route can be further used, if we need to perform any websubhub specific analytics. (ex: get topics, get subscribers for topics, status of the subscriptions
3. listener port 8082 - Dynamic forward proxy to delivery all events received to a topic in WebSubHub to callbackURLs via envoy proxy. (Accept OUTBOUND requests from WebSubHub)

Concerns

1. If we go with above routing plan, there will be a conflict when identifying the request type in listener port 8080 without a custom logic written in the policy. (whether it is a subscribing or publishing). A policy logic should be written to read the request body and identify the hub.mode. In a subscription request it contains the following fields in the request body.

hub.mode=subscribe
hub.topic=issues
hub.callbackurl="xxx"

However, a publish request contains the following query parameters and payload holds only the event data.
hub.mode=publish&hub.topic=issues

As event sources do not know what parameters are requested by WebSubHub, envoy proxy should intercept and add these query parameters(via a policy) to make the request WebSubHub understandable. However, as mentioned same route is used for both Subscribing and Publishing, a separate policy might be needed to process the request, extract data and check for the request type.