Synchronous and asynchronous REST clients

REST clients can be implemented either synchronously or asynchronously. A synchronous client constructs an HTTP structure, sends a request, and waits for a response. An asynchronous client constructs an HTTP structure, sends a request, and moves on. In this case, the client is notified when the response arrives. The original thread, or another thread, can then process the response. Although asynchronous behavior can result in faster overall execution, synchronous behavior might be preferred in certain cases where more simplified code is necessary.

Synchronous REST clients

Synchronous REST clients take advantage of the inherent simplicity of REST architecture by using concise, streamlined code to send clearly defined HTTP requests. If a client is implemented synchronously, you don’t need to worry about managing callbacks, Future, or CompletionStage return types. Synchronous clients might be preferred in situations where service availability is high and low latency is a priority.

However, a synchronous client must wait for an API call to return before code execution can continue. In some cases, this delay might be perceived by users of your app as latency or poor performance. If an application consists of microservices that are making synchronous calls to one another, one failure might set off a chain reaction that results in service denial for the user.

Asynchronous REST clients

Implementing asynchronous REST clients can be a powerful strategy for microservices-based applications. Asynchronous clients don’t need to wait for a response to continue working. Therefore, microservices in an app can continue to process and send data, even when one of their partner services runs into trouble. This capability provides more reliable service to the user and can be especially valuable in cases where service availability is low or overloaded with demand.

For example, asynchronous clients might be implemented within a travel service app that makes REST calls to an airline service, hotel service, and car rental service to make reservations and determine the total cost of a trip. If the hotel service experiences a lag, the airline and car rental service are able to continue working. For an in-depth look at how to implement asynchronous clients in this travel app example, check out Andy McCright’s blog post on Asynchronous REST with JAX-RS and MicroProfile.

Asynchronous REST clients with MicroProfile Rest Client

Both MicroProfile Rest Client and JAX-RS can enable asynchronous clients, though the process is a little different in each case. With MicroProfile Rest Client, asynchronous clients rely on callbacks—functions that are executed after another function completes—to manage the transfer of data between microservices. The return type of the interface method determines whether a RESTful service is invoked synchronously or asynchronously. If the return type is a CompletionStage, the service is invoked asynchronously. A CompletionStage acts as a promise to the service that a particular piece of code will be executed. A wide range of callbacks can be attached to a CompletionStage, enabling different functions after the code executes. In this way, non-blocking systems can be implemented among microservices in an application.

MicroProfile Rest Client is enabled to set up both synchronous and asynchronous REST clients. The following example shows an MicroProfile Rest Client interface that has methods to invoke the same remote service both synchronously and asynchronously.

public interface MyClient {

    Widget getSync();

    CompletionStage<Widget> getAsync();

And here is an example that shows the code to invoke that remote service.

MyClient client = RestClientBuilder.newBuilder().baseUri(someUri).build(MyClient.class);

CompletionStage<Widget> cs = client.getAsync();
// while that request is running, let's invoke it synchronously too:

Widget widgetSync = client.getSync(); // now we have to wait until the request completes
// done.  So now we can see the results of the async request
// that was processing while we did the sync request:

Widget widgetAsync = toCompletableFuture().get();

assertEquals(widgetAsync, widgetSync);

Asynchronous REST clients with JAX-RS

With JAX-RS clients, you need to explicitly build an async client with an API call, as shown in the following example.

Client client = ClientBuilder.newClient();
WebTarget target =;
Invoker.Builder builder = target.request();
AsyncInvoker asyncInvoker = builder.async();

Then, you can choose whether to provide a callback or get a Future, as shown in the following example.

Future<MyResponseObject> future = asyncInvoker.get(MyResponseObject.class);
// or
Future<MyResponseObject> future = asyncInvoker.get( new InvocationCallback<MyResponseObject>() {
    public void completed(MyResponseObject o) {
        // do something with o

    public void failed(Throwable t) {
        // handle the failed request/response

Synchronous and asynchronous REST with MicroProfile

Ready to start building microservices with synchronous and asynchronous REST clients? MicroProfile Rest Client provides a type-safe approach for invoking RESTful services. Although the default implementation is synchronous, you can also make asynchronous calls by using CompletionStage. MicroProfile Rest Client provides an easy-to-build template that gets you up and running with RESTful microservices faster, and without having to worry about the boilerplate code. You can also use MicroProfile’s Fault Tolerance feature to make your asynchronous clients more reliable. MicroProfile Fault Tolerance includes the @Asynchronous annotation, which enables any method within a class to be invoked by a separate thread.