Tag: resilience

fault tolerance for jax-ws with resilience4j

In a recent project, we still have to use SOAP webservices and I wanted to apply some resilience pattern such as retry to my project.
Also a collegue just presented a java library called resilience4j so I wanted to use that one.

Of course, there are a lot of other possibilites like using other libraries (like Hystrix) or applying the sidecar pattern outside of my application in a cluster.

As you can see in the documentation, resilience4j is build for functional programming style and it supports some functional interfaces
which can be decorated to apply the retry mechanism to the function invocation. In the examples, you can always find a simple setup to pass the supplier and decorate it only for the particular method.

In my use case, I do not want to write the decoration code for each and every method (or function). I have a third party WSDL, generated the webservice interface and port via wsimport and now I want to generically apply the retry mechanism to the webservice client. But my solution is not very complicated, it is using a reflection proxy and invocation handler to direcly decorate and execute the method via the Retry classes.

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Proxy;
import io.github.resilience4j.retry.Retry;
public final class WebserviceFactory{
static <T> T decorateWithRetryer(final T service, Retry retry) {
InvocationHandler invocationHandler = (proxy, method, args) -> retry.executeCheckedSupplier(() -> method.invoke(service, args));
return (T) Proxy.newProxyInstance(service.getClass().getClassLoader(),
service.getClass().getInterfaces(), invocationHandler);
}
}
view raw WebserviceFactory.java hosted with ❤ by GitHub

This way I am able to decorate my whole service interface.

One note I can make regarding the exception handling: In case of an exception, an InvocationTargetException is thrown. If you want to have the target one, you have to unwrap it.

The source code of my example is available at https://github.com/mwiede/jaxws-resilience4j.

Exploring Feign – Retrying

Feign is a library, which makes it easier to implement a http client. Recently more and more people start writing http clients, because they are creating microservices which communicate with http protocol. So there are all sorts of libraries supporting this task like Jersey, Resteasy and others – and there is Feign.

Today I do not want to explain the basic functionality, this is all done on the Readme page itself. Today I want to get into the details of a feature, which becomes more and more important, because in modern distributed systems, you want to have resilient behaviour, which means that you want to design your service in the way, that it can handle unexpected situations without noticing on user’s site. For example an API you are calling is not reachable at the moment, the request times out or the requested resource is not yet available. To solve this issue, you need to apply a retry pattern, so that you increase the chance that the service request is successfull after the first, the second or the nth attempt.

What most developers don’t know, Feign has a default retryer built-in.

Now I show a few code examples, what you can expect from this feature. What I am showing are junit tests with a client mock, so that we are able to stub certain errors and verify, how many retries have been made.

Case 1) Success

no retry needed.

@Test
public void testSuccess() throws IOException {
when(clientMock.execute(any(Request.class), any(Options.class))).thenReturn(
Response.builder().status(200).headers(Collections.<String, Collection<String>>emptyMap())
.build());
final GitHub github =
Feign.builder().client(clientMock).decoder(new GsonDecoder())
.target(GitHub.class, "https://api.github.com");
github.contributors("OpenFeign", "feign");
verify(clientMock, times(1)).execute(any(Request.class), any(Options.class));
}
view raw FeignTest.java hosted with ❤ by GitHub

Case 2) Destination never reachable.

In this case, we can see the Default Retryer working, which ends up doing 5 attempts, but finally the client invocation throws an exception.

@Test
public void testDefaultRetryerGivingUp() throws IOException {
when(clientMock.execute(any(Request.class), any(Options.class))).thenThrow(
new UnknownHostException());
final GitHub github =
Feign.builder().client(clientMock).decoder(new GsonDecoder())
.target(GitHub.class, "https://api.github.com");
try {
github.contributors("OpenFeign", "feign");
fail("not failing");
} catch (final Exception e) {
} finally {
verify(clientMock, times(5)).execute(any(Request.class), any(Options.class));
}
}
view raw FeignTest.java hosted with ❤ by GitHub

Case 3) Configure maximal number of attempts

Taking the same error scenario from case 2, this example shows how to configure the retryer to stop trying after the 3rd attempt.

@Test
public void testRetryerAttempts() throws IOException {
when(clientMock.execute(any(Request.class), any(Options.class))).thenThrow(
new UnknownHostException());
final int maxAttempts = 3;
final GitHub github =
Feign.builder().client(clientMock).decoder(new GsonDecoder())
.retryer(new Retryer.Default(1, 100, maxAttempts))
.target(GitHub.class, "https://api.github.com");
try {
github.contributors("OpenFeign", "feign");
fail("not failing");
} catch (final Exception e) {
} finally {
verify(clientMock, times(maxAttempts)).execute(any(Request.class), any(Options.class));
}
}
view raw FeignTest.java hosted with ❤ by GitHub

Case 4) trigger retrying by error code decoding

For some (restful) services, http status code 409 (conflict) is used to express a wrong state of the target resource, that might change after resubmitting the request. We simulate, that the first retry will lead to a successfull response.

@Test
public void testCustomRetryConfigByErrorDecoder() throws IOException {
when(clientMock.execute(any(Request.class), any(Options.class))).thenReturn(
Response.builder().status(409).headers(Collections.<String, Collection<String>>emptyMap())
.build(),
Response.builder().status(200).headers(Collections.<String, Collection<String>>emptyMap())
.build());
class RetryOn409ConflictStatus extends ErrorDecoder.Default {
@Override
public Exception decode(final String methodKey, final Response response) {
if (409 == response.status()) {
return new RetryableException("getting conflict and retry", null);
} else
return super.decode(methodKey, response);
}
}
final GitHub github =
Feign.builder().client(clientMock).decoder(new GsonDecoder())
.errorDecoder(new RetryOn409ConflictStatus())
.target(GitHub.class, "https://api.github.com");
github.contributors("OpenFeign", "feign");
verify(clientMock, times(2)).execute(any(Request.class), any(Options.class));
}
view raw FeignTest.java hosted with ❤ by GitHub

Case 4a) Behavior without error decoder

If no error decoder is configured, no retry is executed by Feign.

@Test
public void test409Error() throws IOException {
when(clientMock.execute(any(Request.class), any(Options.class))).thenReturn(
Response.builder().status(409).headers(Collections.<String, Collection<String>>emptyMap())
.build(),
Response.builder().status(200).headers(Collections.<String, Collection<String>>emptyMap())
.build());
final GitHub github =
Feign.builder().client(clientMock).decoder(new GsonDecoder())
.target(GitHub.class, "https://api.github.com");
try {
github.contributors("OpenFeign", "feign");
fail("not failing");
} catch (final Exception e) {
} finally {
verify(clientMock, times(1)).execute(any(Request.class), any(Options.class));
}
}
view raw FeignTest.java hosted with ❤ by GitHub

Case 5) Evaluation of Retry-After header

In contrast to the cases 4 and 4a, any response having a Retry-After header, which is a standard header defined in http protocol, the default Feign behavior is to honor this and trigger a retry at the date given.

@Test
public void test400ErrorWithRetryAfterHeader() throws IOException {
when(clientMock.execute(any(Request.class), any(Options.class))).thenReturn(
Response
.builder()
.status(400)
.headers(
Collections.singletonMap(Util.RETRY_AFTER,
(Collection<String>) Collections.singletonList("1"))).build(),
Response.builder().status(200).headers(Collections.<String, Collection<String>>emptyMap())
.build());
final GitHub github =
Feign.builder().client(clientMock).decoder(new GsonDecoder())
.target(GitHub.class, "https://api.github.com");
github.contributors("OpenFeign", "feign");
verify(clientMock, times(2)).execute(any(Request.class), any(Options.class));
}
view raw FeignTest.java hosted with ❤ by GitHub

You can download my example on Github.