[Feature apache#564] Support CloudEvents protocols for pub/sub in Eve…

…ntMesh-feature design (apache#574)

docs/en/features/eventmesh-cloudevents-sdk-binding.md

@@ -1,30 +1,25 @@
-# Lightweight EventMesh SDK (CloudEvents)
+# EventMesh pluggable protocols (CloudEvents)
 
 ## Introduction
 
-[EventMesh(incubating)](https://github.com/apache/incubator-eventmesh) is a dynamic
-cloud-native eventing infrastructure.
+[EventMesh(incubating)](https://github.com/apache/incubator-eventmesh) is a dynamic cloud-native eventing infrastructure.
 
-[CloudEvents](https://github.com/cloudevents/spec) is a specification for describing
-event data in common formats to provide interoperability across services, platforms and systems.
+[CloudEvents](https://github.com/cloudevents/spec) is a specification for describing event data in common formats to provide interoperability across services, platforms and systems.
 
-As of May 2021, EventMesh contains the following
-major components: `eventmesh-runtime`, `eventmesh-sdk-java` and `eventmesh-connector-rocketmq`.
+As of May 2021, EventMesh contains the following major components: `eventmesh-runtime`, `eventmesh-sdk-java` and `eventmesh-connector-rocketmq`.
 For a customer to use EventMesh, `eventmesh-runtime` can be deployed as microservices to transmit
 customer's events between event producers and consumers. Customer's applications can then interact
 with `eventmesh-runtime` using `eventmesh-sdk-java` to publish/subscribe for events on given topics.
 
 CloudEvents support has been a highly desired feature by EventMesh users. There are many reasons
 for users to prefer using a SDK with CloudEvents support:
+
 - CloudEvents is a more widely accepted and supported way to describe events. `eventmesh-sdk-java`
   currently uses the `LiteMessage` structure to describe events, which is less standardized.
 - CloudEvents's Java SDK has a wider range of distribution methods. For example, EventMesh users
   currently need to use the SDK tarball or build from source for every EventMesh release. With
-  CloudEvents support, it's easier for users to take a dependency on EventMesh's SDK using CloudEvents's
-  public distributions (e.g. through a Maven configuration).
-- CloudEvents's SDK supports multiple languages. Although EventMesh currently only supports a Java SDK,
-  in future if more languages need to be supported, the extensions can be easier with experience on
-  binding Java SDK with CloudEvents.
+  CloudEvents support, it's easier for users to take a dependency on EventMesh's SDK using CloudEvents's public distributions (e.g. through a Maven configuration).
+- CloudEvents's SDK supports multiple languages. Although EventMesh currently only supports a Java SDK, in future if more languages need to be supported, the extensions can be easier with experience on binding Java SDK with CloudEvents.
 
 ## Requirements
 
@@ -35,6 +30,8 @@ for users to prefer using a SDK with CloudEvents support:
 | F-1            | EventMesh users should be able to depend on a public SDK to publish/subscribe events in CloudEvents format | Functionality |
 | F-2            | EventMesh users should continue to have access to existing EventMesh client features (e.g. load balancing) with an SDK that supports CloudEvent | Feature Parity |
 | F-3            | EventMesh developers should be able to sync `eventmesh-sdk-java` and an SDK with CloudEvents support without much effort/pain | Maintainability |
+| F-4 | EventMesh support pluggable protocols for developers integrate other protocols (e.g. CloudEvents\EventMesh Message\OpenMessage\MQTT ...) | Functionality |
+| F-5 | EventMesh support the unified api for publish/subscribe events to/from event store | Functionality |
 
 ### Performance Requirements
 
@@ -47,9 +44,64 @@ for users to prefer using a SDK with CloudEvents support:
 Binding with the CloudEvents Java SDK (similar to what Kafka already did, see Reference for more details)
 should be an easy way to achieve the requirements.
 
-Design details TBD.
+### Pluggable Protocols
+
+![pluggable-protocols](../../images/features/pluggable-protocols.png)
+
+### Process of CloudEvents under EventMesh
+
+#### For TCP
+
+##### SDK side for publish
+
+- add the CloudEvents identifier in `package` header
+- use `CloudEventBuilder` build the CloudEvent and put it into the `package` body 
+
+##### SDK side for subscribe
+
+- add `convert` function under the `ReceiveMsgHook` interface, for converting the `package` body to the specific protocol with the identifier in `package` header
+- different protocols should implement the `ReceiveMsgHook`  interface 
+
+##### Server side for publish
+
+- design the protocol convert api contains `decodeMessage` interface which convert the package's body to CloudEvent 
+- update `Session.upstreamMsg()` in `MessageTransferTask` change the input parameter Message to CloudEvent, the CloudEvent use the last step `decodeMessage` api convert
+- update `SessionSender.send()`  change the input parameter `Message` to `CloudEvent`
+- update `MeshMQProducer` api support send `CloudEvents` in runtime
+- support the implementation in `connector-plugin` for send `CloudEvents` to EventStore
+
+##### Server side for subscribe
+
+- support change the `RocketMessage` to `CloudEvent` in connector-plugin
+
+- overwrite the `AsyncMessageListener.consume()` function, change the input parameter `Message` to `CloudEvent`
+
+- update the `MeshMQPushConsumer.updateOffset()` implementation change the the input parameter `Message` to `CloudEvent`
+
+- update `DownStreamMsgContext` , change the input parameter `Message` to `CloudEvent`, update the `DownStreamMsgContext.ackMsg` 
+
+#### For HTTP
+
+##### SDK side for publish
+
+- support `LiteProducer.publish(cloudEvent)`
+- add the CloudEvents identifier in http request header
+
+##### SDK side for subscribe
+
+##### Server side for publish
+
+- support build the `HttpCommand.body` by pluggable protocol plugins according the protocol type in `HttpCommand` header
+- support publish the CloudEvent in message processors
+
+##### Server side for subscribe
+
+- update the `EventMeshConsumer.subscribe()`
+
+- update `HandleMsgContext` , change the input parameter `Message` to `CloudEvent`
+- update `AsyncHttpPushRequest.tryHTTPRequest()`
 
 ## Appendix
 
 ### References
-- https://cloudevents.github.io/sdk-java/kafka
+- https://cloudevents.github.io/sdk-java/kafka