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ECF/Asynchronous Remote Services

ECF's Remote Services API, which is used to implement OSGi remote services and remote service admin specifications, has the ability for consumers/clients to use asynchronous method calls.

Synchronous OSGi (Remote) Services

When an OSGi service is actually invoked today, this is done by the caller calling a method on the service. For example, consider a simple 'hello' service:

package org.eclipse.ecf.examples.remoteservices.hello;
public interface IHello {
	public String hello(String from);

Once a consumer of this service gets a valid service reference (i.e. through ServiceTracker, injection via declarative services (DS) or other injection framework, BundleContext.getServiceReference or others), it can actually invoke/use the service:

String response = helloService.hello("slewis");

With OSGi remote services, this 'helloService' will typically be a proxy. When invoked, the proxy will marshall the arguments (i.e. 'slewis' String in this case), communicate with the remote service host via some protocol. If there is a result, then it will then be unmarshalled and returned to the caller thread on the consumer. The thread that calls 'hello' will block until this entire process is complete and the return value is available.

Asynchronous OSGi Remote Services

ECF has added the ability to declare asynchronous access to a remote method, so that the consumer can ge guaranteed that calling the service will not block. For example, it's now possible to make a non-blocking call to the hello service like this

Future<String> future = helloAsyncService.helloAsync("slewis");

With an ECF created asynchronous proxy, the consumer thread that calls helloAsyncService.helloAsync is guaranteed not to block, and the returned Future>String< can be used to subsequently access the String result.

With ECF's asynchronous proxy support, neither the consumer nor the host have to actually implement the asynchronous proxy. The proxy is automatically constructed by the ECF remote services implementation.

At this point, you might ask: How is the asynchronous proxy defined? i.e. where does the helloAsyncService come from?

The answer to this is that it is defined in a new/second service interface...called the asynchronous service interface that is related to the IHello service interface:

public interface IHelloAsync {
	public Future<String> helloAsync(String from);

Notice that this asynchronous service interface declaration resembles the IHello service interface declaration, but differs from it in several specific ways:

  1. The name is IHelloAsync rather than IHello
  2. The method name is helloAsync rather than hello
  3. The return value is Future<String> (or with Java8, CompletableFuture, see below)

With ECF remote services, when a IHello proxy is created, the proxy will also implement the IHelloAsync interface. So, for example

IHello helloService = ...get service via DS/injections, ServiceTracker or otherServiceTracker, or other...
if (helloService instanceof IHelloAsync) {
   IHelloAsync helloServiceAsync = (IHelloAsync) helloService;
   // call it asynchronously
   Future<String> future = helloAsync("slewis");
   // do other things
   String result = future.get();
   // do something with result...


IHelloAsync helloAsyncService = ...get service via DS/injections, ServiceTracker or other
// call it blocking
Future<String> future = helloAsync("slewis");
// do other things
String result = future.get();
// do something with result...

Having both the synchronous (IHello) and asynchronous (IHelloAsync) service types gives the consumer flexibility in determining how a given remote invocation will occur (i.e. synchronously or asynchronously). Synchronous, asynchronous, or both invocation methods may be used.

Java 8 CompletableFuture

With Java8's introduction of the class java.util.concurrent.CompletableFuture and ECF 3.8.1/Juno remote services, it's now possible to return a CompletableFuture rather than a Future in the asynchronous service interface:

package my.package;
import java.util.concurrent.CompletableFuture;
public interface IHelloAsync {
    public CompletableFuture<String> hello(String from);

with CompletableFuture it's not necessary for the consumer to call Future.get (and possibly block) directly...but rather you can write succinct, simple, and guaranteed to be non-blocking calls such as:

IHelloAsync helloAsync = ...get remote service via DS/injections, ServiceTracker or other
CompletableFuture<String> cf = helloAsync("slewis");
cf.thenAccept((response) -&gt; System.out.println("response to hello: " + response));

The use of CompletableFuture has some very nice properties for remote service design, as well as some nice guarantees for the remote service consumer. For remote service designers, they may declare and implement normal OSGi remote services (IHello) without having to concern themselves with the consumer's invocation style (synchronous or asynchronous).

For remote service consumers, with CompletableFuture, remote service callers can be assured that there will be no blocking in their code (via the guarantees of CompletableFuture), use lambda to simply and succinctly express arbitrary code execution...guaranteed to be done asynchronously when the remote response becomes available.

Further, for both the service designer and consumer, there are no extra class dependencies in the service types (IHello and IHelloAsync). That is, no dependencies on OSGi, OSGi Remote Services, or ECF classes...meaning that if they wish to use/reuse these services in other runtime contexts (e.g. outside of OSGi, OSGi Remote Services, and/or ECF), the only dependencies necessary are to their own types and java.util.concurrent.CompletableFuture.

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