# Asynchronous HTTP Request Processing

Asynchronous HTTP Request Processing is a relatively new technique that allows you to process a single HTTP request using non-blocking I/O and, if desired in separate threads. Some refer to it as COMET capabilities. The primary use case for Asynchronous HTTP is in the case where the client is polling the server for a delayed response. The usual example is an AJAX chat client where you want to push/pull from both the client and the server. These scenarios have the client blocking a long time on the server’s socket waiting for a new message. What happens in synchronous HTTP where the server is blocking on incoming and outgoing I/O is that you end up having a thread consumed per client connection. This eats up memory and valuable thread resources. Not such a big deal in 90% of applications (in fact using asynchronous processing may actually hurt your performance in most common scenarios), but when you start getting a lot of concurrent clients that are blocking like this, there’s a lot of wasted resources and your server does not scale that well.

## Using the `@Suspended` annotation

The JAX-RS 2.0 specification has added asynchronous HTTP support via two classes. The `@Suspended` annotation, and AsyncResponse interface.

Injecting an AsynchronousResponse as a parameter to your jax-rs methods tells RESTEasy that the HTTP request/response should be detached from the currently executing thread and that the current thread should not try to automatically process the response.

The AsyncResponse is the callback object. The act of calling one of the resume() methods will cause a response to be sent back to the client and will also terminate the HTTP request. Here is an example of asynchronous processing:

    import javax.ws.rs.Suspend;
    import javax.ws.rs.core.AsynchronousResponse;

    @Path("/")
    public class SimpleResource
    {

       @GET
       @Path("basic")
       @Produces("text/plain")
       public void getBasic(@Suspended final AsyncResponse response) throws Exception
       {
          Thread t = new Thread()
          {
             @Override
             public void run()
             {
                try
                {
                   Response jaxrs = Response.ok("basic").type(MediaType.TEXT_PLAIN).build();
                   response.resume(jaxrs);
                }
                catch (Exception e)
                {
                   response.resume(e);
                }
             }
          };
          t.start();
       }
    }

AsyncResponse also has other methods to cancel the execution. See javadoc for more details.

NOTE: The old RESTEasy proprietary API for async http has been deprecated and may be removed as soon as RESTEasy 3.1. In particular, the RESTEasy @Suspend annotation is replaced by `javax.ws.rs.container.Suspended`, and `org.jboss.resteasy.spi.AsynchronousResponse` is replaced by `javax.ws.rs.container.AsyncResponse`. Note that @Suspended does not have a value field, which represented a timeout limit. Instead, `AsyncResponse.setTimeout()` may be called.

## Using Reactive return types

The JAX-RS 2.1 specification adds support for declaring asynchronous resource methods by returning a `CompletionStage` instead of using the `@Suspended` annotation.

Whenever a resource method returns a `CompletionStage`, it will be subscribed to, the request will be suspended, and only resumed when the `CompletionStage` is resolved either to a value (which is then treated as the return value for the method), or as an error case, in which case the exception will be processed as if it were thrown by the resource method.

Here is an example of asynchronous processing using `CompletionStage`:

    import javax.ws.rs.Suspend;
    import javax.ws.rs.core.AsynchronousResponse;

    @Path("/")
    public class SimpleResource
    {

       @GET
       @Path("basic")
       @Produces("text/plain")
       public CompletionStage<Response> getBasic() throws Exception
       {
          final CompletableFuture<Response> response = new CompletableFuture<>();
          Thread t = new Thread()
          {
             @Override
             public void run()
             {
                try
                {
                   Response jaxrs = Response.ok("basic").type(MediaType.TEXT_PLAIN).build();
                   response.complete(jaxrs);
                }
                catch (Exception e)
                {
                   response.completeExceptionally(e);
                }
             }
          };
          t.start();
          return response;
       }
    }

RESTEasy supports more reactive types for asynchronous programming.

## Asynchronous filters

It is possible to write filters that also turn the request asynchronous. Whether or not filters turned the request asynchronous before execution of your method makes absolutely no difference to your method: it does not need to be declared asynchronous in order to function as specified. Synchronous methods and asynchronous methods will work as specified by the spec.