How does client handles deferred result in long running rest calls

Question

I am learning rest with spring mvc/boot. I understand basic rest calls but i am having trouble understanding long running/non-blocking rest calls.

I understand that with long running rest calls we start a separate thread and the server returns a DeferredResult object to client but how does a client is notified when the processing thread is completed ?

Can someone provide me an example on how to handle this at clients end ?

Also how the non-java clients can handle such request ?

Answer 1

I am giving this answer related to Spring and not related to any non-java clients.

There is AsyncRestTemplate that can be utilized for retrieving from long running Rest Webservice

http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/client/AsyncRestTemplate.html

//WAY1

    @Test
    public void testGetClusterAsyncWay1() 
    {
        Future<ResponseEntity<ClusterDTO>> future =  asyncRestTemplate.getForEntity(BASE_URI+"/async/{clusterId}", ClusterDTO.class,1);

        //Do some other work in your method and after that ping the service to retrieve the result

        //Waits for the computation to get complete and then get the result , you can use different version of get which has timeout option
        ResponseEntity<ClusterDTO> response = future.get();

        ClusterDTO cluster = response.getBody();

        System.out.println("testGetClusterAsync()"+cluster.getClusterName());

}

//WAY2

    @Test
    public void testGetClusterAsyncWay2() 
    {

          ListenableFuture<ResponseEntity<ClusterDTO>> future = 
                    asyncRestTemplate.getForEntity(BASE_URI+"/async/{clusterId}", ClusterDTO.class,17);

       future.addCallback(new MyCallbackHandler());

        //do stuff
        for(int i=0;i<10000;i++)
        {

            System.out.println(i);
        }

    }
    class MyCallbackHandler implements  ListenableFutureCallback<ResponseEntity<ClusterDTO>>{

    @Override
     public void onSuccess(ResponseEntity<ClusterDTO> response) {
      ClusterDTO cluster = response.getBody();
      System.out.println(cluster);
      System.out.println("Success******************************");
     }

    @Override
    public void onFailure(Throwable ex) {
    System.out.println("onFailure******************************");
    ex.printStackTrace();

    }

}

Answer 2

Web was not built for long running tasks, it was purely request/response. However, you can use Server Sent Events for notification back to the client or use Websockets. Spring supports both.

https://github.com/cedricziel/demo-sse-spring-boot

https://spring.io/guides/gs/messaging-stomp-websocket/