[Issue apache#533] Adding design doc for EventMesh Workflow

docs/en/features/eventmesh-workflow-design.md

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+# EventMesh Workflow
+
+## Business Problem
+
+Imaging you are building a simple Order Management System for an E-Commerce Store. The system is able to taking new orders from store website,
+and process the orders, handle the payments, and finally process the shipment.
+
+For the high availability and high performance, you architect the system using event-driven architecture (EDA), and build microservice apps to handle
+store frontend, order management, payment processing, and shipment management.
+You deploy the whole system in a cloud environment. To handle the high workload, you leverage the messaging system to buffer the loads,
+and scale up multiple instances of microservices. The architecture looks similar to the followings:
+
+![eventmesh-workflow-uc](../../images/features/eventmesh-workflow-usecase.jpg?raw=true)
+
+While each microservice is acting on its own event channels, EventMesh plays a crucial role of doing Event Orchestration.
+
+We use CNCF Serverless Workflow to describe this Event Workflow Orchestration.
+
+## About CNCF Serverless Workflow
+
+CNCF Serverless Workflow defines a vendor-neutral, open-source, and fully community-driven ecosystem
+for defining and running DSL-based workflows that target the Serverless technology domain.
+
+Serverless workflow has Domain Specific Language (DSL) to describe a workflow orchestration of serverless functions and microservices.
+
+More about this can be found in its [official github site](https://github.com/serverlessworkflow/specification)
+
+## EventMesh Workflow
+
+We leverage Serverless Workflow DSL to describe the EventMesh workflow. Based on its spec, the workflow is consists of a serials of states.
+At this moment we only support event related states. See the supported states in [Workflow DSL Design](#workflow-dsl-design-wip).
+
+In the workflow `state`, it has the applicable `Action` to perform. The `Action` has reference to the `Function` definition.
+In EDA solution, we usually defined the event-driven microservice using AsyncAPI.
+Serverless workflow supports AsyncAPI as the `Function` definition.
+See [Using Funtions for AsyncAPI Service](https://github.com/serverlessworkflow/specification/blob/main/specification.md#using-functions-for-async-api-service-invocations)
+
+### AsyncAPI
+
+AsyncAPI is an open source initiative that seeks to improve the current state of Event-Driven Architectures (EDA).
+Our long-term goal is to make working with EDAs as easy as it is to work with REST APIs.
+That goes from documentation to code generation, from discovery to event management.
+Most of the processes you apply to your REST APIs nowadays would be applicable to your event-driven/asynchronous APIs too.
+
+See AsyncAPI detail in the [official site](https://www.asyncapi.com/docs/getting-started)
+
+### Workflow Example
+
+In this example, we build the event-driven workflow of the Order management system above.
+
+First, we need the AsyncAPI definitions of the microservice apps. 
+
+- Online Store App
+
+```yaml
+asyncapi: 2.2.0
+info:
+  title: Online Store application
+  version: '0.1.0'
+channels:
+  store/order:
+    subscribe:
+      operationId: receivedStoreOrder
+      message:
+        name: StoreOrder
+```
+
+- Order Service
+
+```yaml
+asyncapi: 2.2.0
+info:
+  title: Order Service
+  version: '0.1.0'
+channels:
+  order/inbound:
+    publish:
+      operationId: sendOrder
+      message:
+        name: Order
+  order/outbound:
+    subscribe:
+      operationId: receivedPurchaseOrder
+      message:
+        name: PurchaseOrder
+```
+
+- Payment Service
+
+```yaml
+asyncapi: 2.2.0
+info:
+  title: Payment Service
+  version: '0.1.0'
+channels:
+  payment/inbound:
+    publish:
+      operationId: sendPayment
+      message:
+        name: Payment
+  payment/outbound:
+    subscribe:
+      operationId: receivedPaymentReceipt
+      message:
+        name: PaymentReceipt
+```
+
+- Shipment Service
+
+```yaml
+asyncapi: 2.2.0
+info:
+  title: Shipment Service
+  version: '0.1.0'
+channels:
+  shipmet/inbound:
+    publish:
+      operationId: sendShipment
+      message:
+        name: Shipment
+```
+
+Then we write the workflow using Serverless Workflow DSL.
+
+```yaml
+id: StoreOrderWorkflow
+version: '1.0'
+specVersion: '0.8'
+name: Store Order Management Workflow
+start: Received Store Order
+functions:
+  - name: receivedStoreOrder
+    operation: file://onlineStoreApp.yaml#receivedStoreOrder
+  - name: dispatchOrder
+    operation: file://orderService.yaml#sendOrder
+  - name: recievedPO
+    operation: file://orderService.yaml#receivedPurchaseOrder
+  - name: dispatchPayment
+    operation: file://paymentService.yaml#sendPayment
+  - name: receivedReceipt
+    operation: file://paymentService.yaml#receivedPaymentReceipt
+  - name: dispatchShipment
+    operation: file://shipmentService.yaml#sendShipment
+events:
+  - name: StoreOrder
+    type: StoreOrderType
+    source: OnlineStoreApp
+  - name: PurchaseOrder
+    type: PurchaseOrderType
+    source: OrderService
+  - name: PaymentReceipt
+    type: PaymentReceiptType
+    source: PaymentService
+  - name: PaymentDenial
+    type: PaymentDenialType
+    source: PaymentService
+states:
+- name: Received Store Order
+  type: operation
+  actions:
+    - functionRef:
+        refName: receivedStoreOrder
+  transition: Process Store Order
+- name: Process Store Order
+  type: event
+  onEvents:
+    - eventRefs:
+        - StoreOrder
+      actionMode: sequential
+      actions:
+        - functionRef:
+            refName: dispatchOrder
+        - functionRef:
+            refName: recievedPO
+  transition: Process Order Payment
+- name: Process Order Payment
+  type: event
+  onEvents:
+    - eventRefs:
+        - PurchaseOrder
+      actionMode: sequential
+      actions:
+        - functionRef:
+            refName: dispatchPayment
+        - functionRef:
+            refName: receivedReceipt
+  transition: Check Payment Receipt
+- name: Check Payment Receipt
+  type: switch
+  eventConditions:
+    - eventRef: PaymentReceipt
+      transition: Process Order Shipment
+    - eventRef: PaymentDenial
+      end: true
+  defaultCondition
+    end: true
+- name: Process Order Shipment
+  type: operation
+  action:
+    - functionRef:
+        refName: dispatchShipment    
+  end: true
+```
+
+## EventMesh Workflow Engine
+
+![eventmesh-workflow-arch](../../images/features/eventmesh-workflow-arch.jpg?raw=true)
+
+## EventMesh Workflow Engine Design
+
+EventMesh Workflow Engine consists of the following modules:
+
+- Workflow Parser
+  
+  Using the SDK provided by Serverless Workflow community (see supported [SDKs](https://github.com/serverlessworkflow/specification#sdks)),
+  parse and validated the workflow DSL inputs, and generate workflow definition.
+  
+
+- AsyncAPI Parser
+ 
+  Using the SDK provided by AsyncAPI community (see [tool list](https://www.asyncapi.com/docs/community/tooling)),
+  parse and validated the AsyncAPI yaml inputs, and generate the AsyncAPI definition.
+
+
+- Workflow Module
+  
+  It manages a workflow instance life cycle, from create, start, stop to destroy.
+
+
+- State Module
+
+  It manages workflow state life cycle. We support the event-related states, and the supported state list below is Work-in-Progress.
+
+  | Workflow State | Description | 
+  | --- | --- |
+  | Operation | Execute the AsyncAPI functions defined in the Actions |
+  | Event | Check if the defined Event matched, if so execute the defined AsyncAPI functions |
+  | Switch | Check the event is matched with the event-conditions, and execute teh defined AsyncAPI functions |
+  | Parallel | Execute the defined AsyncAPI functions in parallel |
+  | ForEach | Iterate the inputCollection and execute the defined AsyncAPI functions |
+  
+- Action Module
+
+  It managed the functions inside the action. 
+
+
+- Function Module
+ 
+  It manages the AsyncAPI functions by creating the publisher and/or subscriber in EventMesh Runtime, and manage the publisher/subscriber life cycle.
+
+    | AsyncAPI Operation | EventMesh Runtime | 
+    | --- | --- | 
+    |  Publish | Publisher | 
+    | Subscribe | Subscriber |
+
+
+- Event Module
+
+  It manages the CloudEvents data model, including event filter, correlation and transformation using the rules defined in the workflow DSL.
+
+
+- Retry Module
+
+  It manages the retry logic of the event publishing into EventMesh Runtime.