WSO2 CEP bidirectional REST API - rest

Question

I'm using wso2 cep 4.1
I created receiver to catch some json data from my source. Then I process this data internally and should give the response with additional data. My response should be through the same point as data come in to CEP. It is classical rest API. Is it possible and how can I make it?
Or, I need websocket (websocket-local) for similar purposes?

Answer 1

Hope you are still trying to understand functionalities of WSO2 CEP. Let me explain the basic overview of CEP before addressing your question. If you look at below diagram you will understand what is happening under the hood at a high level. . I will explain what these components suppose to do in the context of event processing.
Event receivers :-
Event receivers receive events that are coming to the CEP. WSO2 CEP supports the most common adapter implementations by default. For specific use cases, you can also plug custom adapters. For more information, see Configuring Event Receivers.
Event streams :-
Event streams contain unique sets of attributes of specific types that provide a structure based on which the events processed by the relevant event flow are selected. Event streams are stored as stream definitions in the file system via the data bridge stream definition store.
Event processors :-
Event processor handles actual event processing. It is the core event processing unit of the CEP. It manages different execution plans and processes events based on logic, with the help of different Siddhi queries. Event Processor gets a set of event streams from the Event Stream Manager, processes them using Siddhi engine, and triggers new events on different event streams back to the Event Stream Manager. For more information, see Creating a Standalone Execution Plan.
Event publishers
Event publishers:- publish events to external systems and store data to databases for future analysis. Like the event receivers, this component also has different adapter implementations. The most common ones are available by default in the CEP. You can implement custom adapters for specific use cases. For more information, see Configuring CEP to Create Alerts.
According to your requirement, you should have HTTP receiver as well as HTTP publisher where the receiver receives a request from a third party API and hand message over to event processors so as to perform some pre-defined tasks.This may compose with several event streams and execution plans. Once processing is done event publishers can be used to publish result to required third-party API as you pointed out.
OOB CEP provides HTTP receiver and HTTP publisher adapters[1-2] which you can try it out.There are some limitations which might not suit for your scenario. You are required to implement your own custom HTTP receiver and publisher[3-4] which does what you intended to do.
Since you need to publish a response to difference endpoints,you can achieve this defining REST API endpoint,user credentials(if required) and HTTP verbs and other information which required to send a message in the event stream[5] as meta information. Then that information you can read from the stream itself and push to desired third-party API as you require.
I need websocket (websocket-local) for similar purposes?
This isn't clear what exactly is to be done. Please raise an another question and ask it again.
https://docs.wso2.com/display/CEP410/HTTP+Event+Receiver
https://docs.wso2.com/display/CEP410/HTTP+Event+Publisher
https://docs.wso2.com/display/CEP410/Building+Custom+Event+Receivers
https://docs.wso2.com/display/CEP410/Building+Custom+Event+Publishers
https://docs.wso2.com/display/CEP410/Understanding+Event+Streams

Answer 2

The feature you are looking for doesn't come OOTB with CEP. However, you can try something similar to below;
Implement a REST API. Probably using Apache CXF since CXF dependencies are present in WSO2 servers by default. You can follow this guide if you are using a swagger based approach to develop the REST API.
Within that custom REST implementation, you need to read the HTTP request, send it to CEP (step 3), wait for an output from CEP (step 4) and then send back that details as HTTP response inside the method which represents your operation.
To send an event to CEP you can use WSO2 Event receiver. Create a receiver at CEP side and then send events to the receiver using DataPublisher client. Make sure you have the same stream definition that you set in CEP receiver in the DataPublisher.publish() method and object array you send adhere to that definition. Also, you might need to set truststore and keystore params here.
After publishing your events successfully you need to block the request thread till you receive a response from CEP. You can use a java object like CountDownLatch for this purpose.
To receive a response you need to consume events though EventStreamService For this you need to implement a WSO2EventConsumer and subscribe to EventStreamService. After successfully subscribing, events coming to stream id mentioned in your event consumer will be forwarded to receive method of your Consumer. From there you can extract the results, unblock the initial request thread and return with those results. To access the EventStreamService from within your web app you can use below code snippet.
EventStreamService eventStreamService = (EventStreamService) PrivilegedCarbonContext.getThreadLocalCarbonContext().getOSGiService(EventStreamService.class, null);
Hope this helped.