Java Functional Retry with resilience4j-retry

We will go through specific module(Resilience4j-Retry) from Resilience4j which is a fault tolerance library designed for Java8 and functional programming and it is lightweight library with minimal dependencies (mainly vavr)

When you retry , there are many cases for example :

cross micro services communication for remote systems calls which most likely will need circuit breaker logic added as well
if you have business or functional logic that need to get a consistent end state and most likely it is asynchronous Flow

Ok , what you need to do to start using resileience4j retry :

if you are using maven:

	<dependency>
	<groupId>io.github.resilience4j</groupId>
	<artifactId>resilience4j-retry</artifactId>
	<version>0.13.2</version>
	</dependency>
	<dependency>
	<groupId>io.github.resilience4j</groupId>
	<artifactId>resilience4j-core</artifactId>
	<version>0.13.2</version>
	</dependency>

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mavenResilence4j.xml

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if you are using gradle :

	compile "io.github.resilience4j:resilience4j-core:0.13.2"
	compile "io.github.resilience4j:resilience4j-retry:0.13.2"

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resi4j.gradel

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What can be covered by the retry module of resilience4j :

Synchronous retry and Asynchronous retry
Rety on exceptions or response predicate which can be useful if you want to retry on specific response value not just thrown exceptions
Back-off strategy for the retry configuration plus max retry attempts
Ignoring set of exceptions to not retry on
It has support for checked(exception handling added) and unchecked functions executions (ex Function , Supplier , Callable , Runnable..)
it could be integrated with spring if needed.

Now showing examples of the mentioned features up:

How to configure the Asynchronous retry , full code for testing resilience4j retry on Github

	// Given the HelloWorldService returns Hello world
	given(helloWorldService.returnHelloWorld())
	.willReturn(completedFuture("Hello world"));

	final AsyncRetry retryContext = AsyncRetry.of("retryConfig",
	// we set the response type to String
	RetryConfig.<String>custom()
	// max retry attempts
	.maxAttempts(3)
	// what are the ignore exception to no retry on
	.ignoreExceptions(IllegalStateException.class)
	// what are the exceptions to try on
	.retryExceptions(TimeoutException.class)
	// retry if the response contains world
	.retryOnResult(s -> s.contains("world"))
	// retry backoff strategy, IntervalFunction has many built in interface functions you can check it out
	.intervalFunction(IntervalFunction.ofExponentialBackoff())
	.build());

	// Decorate the invocation of the HelloWorldService
	Supplier<CompletionStage<String>> supplier = AsyncRetry.decorateCompletionStage(
	retryContext,
	scheduler,
	() -> helloWorldService.returnHelloWorld());

	// When
	String result = awaitResult(supplier);
	// Then the helloWorldService should be invoked 1 time
	BDDMockito.then(helloWorldService).should(Mockito.times(3)).returnHelloWorld();
	Assertions.assertEquals(result, "Hello world");

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retry.java

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For Synchronous calls , you have many options (Supplier , Callable , Function , plus Checked version of them, please check resilience4j retry APIs for more information) :

Example :

	// Given the HelloWorldService returns Hello world
	BDDMockito.given(helloWorldService.returnHelloWorld()).willReturn("Hello world");
	// Create a Retry with default configuration
	final RetryConfig tryAgain = RetryConfig.<String>custom().retryOnResult(s -> s.contains("Hello world"))
	.maxAttempts(2).build();
	Retry retry = Retry.of("id", tryAgain);
	// Decorate the invocation of the HelloWorldService
	Supplier<String> supplier = Retry.decorateSupplier(retry, helloWorldService::returnHelloWorld);
	// When
	String result = supplier.get();
	// Then the helloWorldService should be invoked 1 time
	BDDMockito.then(helloWorldService).should(Mockito.times(2)).returnHelloWorld();
	assertThat(result).isEqualTo("Hello world");

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SyncRetry.java

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For more information , please check resilience4j Github and the code samples on GitHub as well .

In the next blogs I will cover spring and spring boot integration .

References :

https://github.com/resilience4j/resilience4j

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