0.0.1.BUILD-SNAPSHOT

Introduction

Spring Cloud Circuit breaker provides an abstraction across different circuit breaker implementations. It provides a consistent API to use in your applications allowing you the developer to choose the circuit breaker implementation that best fits your needs for your app.

Core Concepts

To create a circuit breaker in your code you can use the CircuitBreakerFactory API. When you include a Spring Cloud Circuit Breaker starter on your classpath a bean implementing this API will automatically be created for you. A very simple example of using this API is given below

@Service
public static class DemoControllerService {
	private RestTemplate rest;
	private CircuitBreakerFactory cbFactory;

	public DemoControllerService(RestTemplate rest, CircuitBreakerFactory cbFactory) {
		this.rest = rest;
		this.cbFactory = cbFactory;
	}

	public String slow() {
		return cbFactory.create("slow").run(() -> rest.getForObject("/slow", String.class), throwable -> "fallback");
	}

}

The CircuitBreakerFactory.create API will create an instance of a class called CircuitBreaker. The run method takes a Supplier and a Function. The Supplier is the code that you are going to wrap in a circuit breaker. The Function is the fallback that will be executed if the circuit breaker is tripped. The function will be passed the Throwable that caused the fallback to be triggered. You can optionally exclude the fallback if you do not want to provide one.

Circuit Breakers In Reactive Code

If Project Reactor is on the class path then you can also use ReactiveCircuitBreakerFactory for your reactive code.

@Service
public static class DemoControllerService {
	private ReactiveCircuitBreakerFactory cbFactory;
	private WebClient webClient;


	public DemoControllerService(WebClient webClient, ReactiveCircuitBreakerFactory cbFactory) {
		this.webClient = webClient;
		this.cbFactory = cbFactory;
	}

	public Mono<String> slow() {
		return webClient.get().uri("/slow").retrieve().bodyToMono(String.class).transform(
		it -> cbFactory.create("slow").run(it, throwable -> return Mono.just("fallback")));
	}
}

The ReactiveCircuitBreakerFactory.create API will create an instance of a class called ReactiveCircuitBreaker. The run method takes with a Mono or Flux and wraps it in a circuit breaker. You can optionally profile a fallback Function which will be called if the circuit breaker is tripped and will be passed the Throwable that caused the failure.

Configuration

You can configure your circuit breakers using by creating beans of type Customizer. The Customizer interface has a single method called customize that takes in the Object to customize.

Configuring Hystrix Circuit Breakers

Default Configuration

To provide a default configuration for all of your circuit breakers create a Customize bean that is passed a HystrixCircuitBreakerFactory or ReactiveHystrixCircuitBreakerFactory. The configureDefault method can be used to provide a default configuration.

@Bean
public Customizer<HystrixCircuitBreakerFactory> defaultConfig() {
	return factory -> factory.configureDefault(id -> HystrixCommand.Setter
			.withGroupKey(HystrixCommandGroupKey.Factory.asKey(id))
			.andCommandPropertiesDefaults(HystrixCommandProperties.Setter()
			.withExecutionTimeoutInMilliseconds(4000)));
}
Reactive Example
@Bean
public Customizer<ReactiveHystrixCircuitBreakerFactory> defaultConfig() {
	return factory -> factory.configureDefault(id -> HystrixObservableCommand.Setter
			.withGroupKey(HystrixCommandGroupKey.Factory.asKey(id))
			.andCommandPropertiesDefaults(HystrixCommandProperties.Setter()
					.withExecutionTimeoutInMilliseconds(4000)));
}

Specific Circuit Breaker Configuration

Similarly to providing a default configuration, you can create a Customize bean this is passed a HystrixCircuitBreakerFactory

@Bean
public Customizer<HystrixCircuitBreakerFactory> customizer() {
	return factory -> factory.configure(builder -> builder.commandProperties(
					HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(2000)), "foo", "bar");
}
Reactive Example
@Bean
public Customizer<ReactiveHystrixCircuitBreakerFactory> customizer() {
	return factory -> factory.configure(builder -> builder.commandProperties(
					HystrixCommandProperties.Setter().withExecutionTimeoutInMilliseconds(2000)), "foo", "bar");
}

Configuring Resilience4J Circuit Breakers

Default Configuration

To provide a default configuration for all of your circuit breakers create a Customize bean that is passed a Resilience4JCircuitBreakerFactory or ReactiveResilience4JCircuitBreakerFactory. The configureDefault method can be used to provide a default configuration.

@Bean
public Customizer<Resilience4JCircuitBreakerFactory> defaultCustomizer() {
	return factory -> factory.configureDefault(id -> new Resilience4JConfigBuilder(id)
			.timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(4)).build())
			.circuitBreakerConfig(CircuitBreakerConfig.ofDefaults())
			.build());
}
Reactive Example
@Bean
public Customizer<ReactiveResilience4JCircuitBreakerFactory> defaultCustomizer() {
	return factory -> factory.configureDefault(id -> new Resilience4JConfigBuilder(id)
			.circuitBreakerConfig(CircuitBreakerConfig.ofDefaults())
			.timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(4)).build()).build());
}

Specific Circuit Breaker Configuration

Similarly to providing a default configuration, you can create a Customize bean this is passed a Resilience4JCircuitBreakerFactory or ReactiveResilience4JCircuitBreakerFactory.

@Bean
public Customizer<Resilience4JCircuitBreakerFactory> slowCustomizer() {
	return factory -> factory.configure(builder -> builder.circuitBreakerConfig(CircuitBreakerConfig.ofDefaults())
			.timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(2)).build()), "slow");
}

In addition to configuring the circuit breaker that is created you can also customize the circuit breaker after it has been created but before it is returned to the caller. To do this you can use the addCircuitBreakerCustomizer method. This can be useful for adding event handlers to Resilience4J circuit breakers.

@Bean
public Customizer<Resilience4JCircuitBreakerFactory> slowCustomizer() {
	return factory -> factory.addCircuitBreakerCustomizer(circuitBreaker -> circuitBreaker.getEventPublisher()
	.onError(normalFluxErrorConsumer).onSuccess(normalFluxSuccessConsumer), "normalflux");
}
Reactive Example
@Bean
public Customizer<ReactiveResilience4JCircuitBreakerFactory> slowCusomtizer() {
	return factory -> {
		factory.configure(builder -> builder
		.timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(2)).build())
		.circuitBreakerConfig(CircuitBreakerConfig.ofDefaults()), "slow", "slowflux");
		factory.addCircuitBreakerCustomizer(circuitBreaker -> circuitBreaker.getEventPublisher()
        	.onError(normalFluxErrorConsumer).onSuccess(normalFluxSuccessConsumer), "normalflux");
     };
}

Configuring Sentinel Circuit Breakers

Default Configuration

To provide a default configuration for all of your circuit breakers create a Customizer bean that is passed a SentinelCircuitBreakerFactory or ReactiveSentinelCircuitBreakerFactory. The configureDefault method can be used to provide a default configuration.

@Bean
public Customizer<SentinelCircuitBreakerFactory> defaultCustomizer() {
	return factory -> factory.configureDefault(id -> new SentinelConfigBuilder(id)
			.build());
}

You can choose to provide default circuit breaking rules via SentinelConfigBuilder#rules(rules). You can also choose to load circuit breaking rules later elsewhere using DegradeRuleManager.loadRules(rules) API of Sentinel, or via Sentinel dashboard.

Reactive Example
@Bean
public Customizer<ReactiveSentinelCircuitBreakerFactory> defaultCustomizer() {
	return factory -> factory.configureDefault(id -> new SentinelConfigBuilder(id)
			.build());
}

Specific Circuit Breaker Configuration

Similarly to providing a default configuration, you can create a Customizer bean this is passed a SentinelCircuitBreakerFactory.

@Bean
public Customizer<SentinelCircuitBreakerFactory> slowCustomizer() {
	String slowId = "slow";
	List<DegradeRule> rules = Collections.singletonList(
		new DegradeRule(slowId).setGrade(RuleConstant.DEGRADE_GRADE_RT)
			.setCount(100)
			.setTimeWindow(10)
		);
	return factory -> factory.configure(builder -> builder.rules(rules), slowId);
}
Reactive Example
@Bean
public Customizer<ReactiveSentinelCircuitBreakerFactory> customizer() {
	List<DegradeRule> rules = Collections.singletonList(
		new DegradeRule().setGrade(RuleConstant.DEGRADE_GRADE_RT)
			.setCount(100)
			.setTimeWindow(10)
		);
	return factory -> factory.configure(builder -> builder.rules(rules), "foo", "bar");
}

Configuring Spring Retry Circuit Breakers

Spring Retry provides declarative retry support for Spring applications. A subset of the project includes the ability to implement circuit breaker functionality. Spring Retry provides a circuit breaker implementation via a combination of it’s CircuitBreakerRetryPolicy and a stateful retry. All circuit breakers created using Spring Retry will be created using the CircuitBreakerRetryPolicy and a DefaultRetryState. Both of these classes can be configured using SpringRetryConfigBuilder.

Default Configuration

To provide a default configuration for all of your circuit breakers create a Customize bean that is passed a SpringRetryCircuitBreakerFactory. The configureDefault method can be used to provide a default configuration.

@Bean
public Customizer<SpringRetryCircuitBreakerFactory> defaultCustomizer() {
	return factory -> factory.configureDefault(id -> new SpringRetryConfigBuilder(id)
    	.retryPolicy(new TimeoutRetryPolicy()).build());
}

Specific Circuit Breaker Configuration

Similarly to providing a default configuration, you can create a Customize bean this is passed a SpringRetryCircuitBreakerFactory.

@Bean
public Customizer<SpringRetryCircuitBreakerFactory> slowCustomizer() {
	return factory -> factory.configure(builder -> builder.retryPolicy(new SimpleRetryPolicy(1)).build(), "slow");
}

In addition to configuring the circuit breaker that is created you can also customize the circuit breaker after it has been created but before it is returned to the caller. To do this you can use the addRetryTemplateCustomizers method. This can be useful for adding event handlers to the RetryTemplate.

@Bean
public Customizer<SpringRetryCircuitBreakerFactory> slowCustomizer() {
	return factory -> factory.addRetryTemplateCustomizers(retryTemplate -> retryTemplate.registerListener(new RetryListener() {

		@Override
		public <T, E extends Throwable> boolean open(RetryContext context, RetryCallback<T, E> callback) {
			return false;
		}

		@Override
		public <T, E extends Throwable> void close(RetryContext context, RetryCallback<T, E> callback, Throwable throwable) {

		}

		@Override
		public <T, E extends Throwable> void onError(RetryContext context, RetryCallback<T, E> callback, Throwable throwable) {

		}
	}));
}

Building

Basic Compile and Test

To build the source you will need to install JDK 1.8.

Spring Cloud uses Maven for most build-related activities, and you should be able to get off the ground quite quickly by cloning the project you are interested in and typing

$ ./mvnw install
You can also install Maven (>=3.3.3) yourself and run the mvn command in place of ./mvnw in the examples below. If you do that you also might need to add -P spring if your local Maven settings do not contain repository declarations for spring pre-release artifacts.
Be aware that you might need to increase the amount of memory available to Maven by setting a MAVEN_OPTS environment variable with a value like -Xmx512m -XX:MaxPermSize=128m. We try to cover this in the .mvn configuration, so if you find you have to do it to make a build succeed, please raise a ticket to get the settings added to source control.

For hints on how to build the project look in .travis.yml if there is one. There should be a "script" and maybe "install" command. Also look at the "services" section to see if any services need to be running locally (e.g. mongo or rabbit). Ignore the git-related bits that you might find in "before_install" since they’re related to setting git credentials and you already have those.

The projects that require middleware generally include a docker-compose.yml, so consider using Docker Compose to run the middeware servers in Docker containers. See the README in the scripts demo repository for specific instructions about the common cases of mongo, rabbit and redis.

If all else fails, build with the command from .travis.yml (usually ./mvnw install).

Documentation

The spring-cloud-build module has a "docs" profile, and if you switch that on it will try to build asciidoc sources from src/main/asciidoc. As part of that process it will look for a README.adoc and process it by loading all the includes, but not parsing or rendering it, just copying it to ${main.basedir} (defaults to ${basedir}, i.e. the root of the project). If there are any changes in the README it will then show up after a Maven build as a modified file in the correct place. Just commit it and push the change.

Working with the code

If you don’t have an IDE preference we would recommend that you use Spring Tools Suite or Eclipse when working with the code. We use the m2eclipse eclipse plugin for maven support. Other IDEs and tools should also work without issue as long as they use Maven 3.3.3 or better.

Importing into eclipse with m2eclipse

We recommend the m2eclipse eclipse plugin when working with eclipse. If you don’t already have m2eclipse installed it is available from the "eclipse marketplace".

Older versions of m2e do not support Maven 3.3, so once the projects are imported into Eclipse you will also need to tell m2eclipse to use the right profile for the projects. If you see many different errors related to the POMs in the projects, check that you have an up to date installation. If you can’t upgrade m2e, add the "spring" profile to your settings.xml. Alternatively you can copy the repository settings from the "spring" profile of the parent pom into your settings.xml.

Importing into eclipse without m2eclipse

If you prefer not to use m2eclipse you can generate eclipse project metadata using the following command:

$ ./mvnw eclipse:eclipse

The generated eclipse projects can be imported by selecting import existing projects from the file menu.

Contributing

Spring Cloud is released under the non-restrictive Apache 2.0 license, and follows a very standard Github development process, using Github tracker for issues and merging pull requests into master. If you want to contribute even something trivial please do not hesitate, but follow the guidelines below.

Sign the Contributor License Agreement

Before we accept a non-trivial patch or pull request we will need you to sign the Contributor License Agreement. Signing the contributor’s agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team, and given the ability to merge pull requests.

Code of Conduct

This project adheres to the Contributor Covenant code of conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to spring-code-of-conduct@pivotal.io.

Code Conventions and Housekeeping

None of these is essential for a pull request, but they will all help. They can also be added after the original pull request but before a merge.

  • Use the Spring Framework code format conventions. If you use Eclipse you can import formatter settings using the eclipse-code-formatter.xml file from the Spring Cloud Build project. If using IntelliJ, you can use the Eclipse Code Formatter Plugin to import the same file.

  • Make sure all new .java files to have a simple Javadoc class comment with at least an @author tag identifying you, and preferably at least a paragraph on what the class is for.

  • Add the ASF license header comment to all new .java files (copy from existing files in the project)

  • Add yourself as an @author to the .java files that you modify substantially (more than cosmetic changes).

  • Add some Javadocs and, if you change the namespace, some XSD doc elements.

  • A few unit tests would help a lot as well — someone has to do it.

  • If no-one else is using your branch, please rebase it against the current master (or other target branch in the main project).

  • When writing a commit message please follow these conventions, if you are fixing an existing issue please add Fixes gh-XXXX at the end of the commit message (where XXXX is the issue number).

Checkstyle

Spring Cloud Build comes with a set of checkstyle rules. You can find them in the spring-cloud-build-tools module. The most notable files under the module are:

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            └── checkstyle.xml (1)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules

Checkstyle configuration

Checkstyle rules are disabled by default. To add checkstyle to your project just define the following properties and plugins.

pom.xml
<properties>
<maven-checkstyle-plugin.failsOnError>true</maven-checkstyle-plugin.failsOnError> (1)
        <maven-checkstyle-plugin.failsOnViolation>true
        </maven-checkstyle-plugin.failsOnViolation> (2)
        <maven-checkstyle-plugin.includeTestSourceDirectory>true
        </maven-checkstyle-plugin.includeTestSourceDirectory> (3)
</properties>

<build>
        <plugins>
            <plugin> (4)
                <groupId>io.spring.javaformat</groupId>
                <artifactId>spring-javaformat-maven-plugin</artifactId>
            </plugin>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>

    <reporting>
        <plugins>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>
    </reporting>
</build>
1 Fails the build upon Checkstyle errors
2 Fails the build upon Checkstyle violations
3 Checkstyle analyzes also the test sources
4 Add the Spring Java Format plugin that will reformat your code to pass most of the Checkstyle formatting rules
5 Add checkstyle plugin to your build and reporting phases

If you need to suppress some rules (e.g. line length needs to be longer), then it’s enough for you to define a file under ${project.root}/src/checkstyle/checkstyle-suppressions.xml with your suppressions. Example:

projectRoot/src/checkstyle/checkstyle-suppresions.xml
<?xml version="1.0"?>
<!DOCTYPE suppressions PUBLIC
		"-//Puppy Crawl//DTD Suppressions 1.1//EN"
		"https://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
<suppressions>
	<suppress files=".*ConfigServerApplication\.java" checks="HideUtilityClassConstructor"/>
	<suppress files=".*ConfigClientWatch\.java" checks="LineLengthCheck"/>
</suppressions>

It’s advisable to copy the ${spring-cloud-build.rootFolder}/.editorconfig and ${spring-cloud-build.rootFolder}/.springformat to your project. That way, some default formatting rules will be applied. You can do so by running this script:

$ curl https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/.editorconfig -o .editorconfig
$ touch .springformat

IDE setup

Intellij IDEA

In order to setup Intellij you should import our coding conventions, inspection profiles and set up the checkstyle plugin. The following files can be found in the Spring Cloud Build project.

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            ├── checkstyle.xml (1)
            └── intellij
                ├── Intellij_Project_Defaults.xml (4)
                └── Intellij_Spring_Boot_Java_Conventions.xml (5)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules
4 Project defaults for Intellij that apply most of Checkstyle rules
5 Project style conventions for Intellij that apply most of Checkstyle rules
Code style
Figure 1. Code style

Go to FileSettingsEditorCode style. There click on the icon next to the Scheme section. There, click on the Import Scheme value and pick the Intellij IDEA code style XML option. Import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Spring_Boot_Java_Conventions.xml file.

Code style
Figure 2. Inspection profiles

Go to FileSettingsEditorInspections. There click on the icon next to the Profile section. There, click on the Import Profile and import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Project_Defaults.xml file.

Checkstyle

To have Intellij work with Checkstyle, you have to install the Checkstyle plugin. It’s advisable to also install the Assertions2Assertj to automatically convert the JUnit assertions

Checkstyle

Go to FileSettingsOther settingsCheckstyle. There click on the + icon in the Configuration file section. There, you’ll have to define where the checkstyle rules should be picked from. In the image above, we’ve picked the rules from the cloned Spring Cloud Build repository. However, you can point to the Spring Cloud Build’s GitHub repository (e.g. for the checkstyle.xml : https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/main/resources/checkstyle.xml). We need to provide the following variables:

Remember to set the Scan Scope to All sources since we apply checkstyle rules for production and test sources.