Resilience4j EIP

This component supports the Circuit Breaker EIP with the Resilience4j library.

Camel provides the Circuit Breaker EIP in the route model, which allows to plugin different implementations. Resilience4j is one such implementation.

Maven users will need to add the following dependency to their pom.xml to use this EIP:

<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-resilience4j</artifactId>
    <version>x.x.x</version><!-- use the same version as your Camel core version -->
</dependency>

Configuration options

The Hystrix EIP supports 2 options which are listed below:

Name Description Default Type

resilienceConfiguration

Configures the Resilience EIP Use end when configuration is complete, to return back to the Resilience EIP.

ResilienceConfigurationDefinition

resilienceConfigurationRef

Refers to a Resilience configuration to use for configuring the Resilience EIP.

String

See Resilience4j Configuration for all the configuration options on Resilience Circuit Breaker.

Samples

Below is an example route showing a Resilience endpoint that protects against a downstream HTTP operation by falling back to the in-lined fallback route.

from("direct:start")
    .circuitBreaker()
        .to("http://fooservice.com/faulty")
    .onFallback()
        .transform().constant("Fallback message")
    .end()
    .to("mock:result");

And in XML DSL:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <circuitBreaker>
      <to uri="http://fooservice.com/faulty"/>
      <onFallback>
        <transform>
          <constant>Fallback message</constant>
        </transform>
      </onFallback>
    </circuitBreaker>
    <to uri="mock:result"/>
  </route>
</camelContext>

Configuring Resilience4j

You can fine-tune Resilience4j by the many Resilience4j Configuration options.

For example to use a 2 second execution timeout, you can do as follows:

from("direct:start")
    .circuitBreaker()
        // use 2 second timeout
        .resilience4jConfiguration().timeoutEnabled(true).timeoutDuration(2000).end()
        .log("Resilience processing start: ${threadName}")
        .toD("direct:${body}")
        .log("Resilience processing end: ${threadName}")
    .end()
    .log("After Resilience ${body}");

And in XML:

<route>
  <from uri="direct:start"/>
  <circuitBreaker>
    <resilience4jConfiguration timeoutEnabled="true" timeoutDuration="2000"/>
    <log message="Resilience processing start: ${threadName}"/>
    <toD uri="direct:${body}"/>
    <log message="Resilience processing end: ${threadName}"/>
  </circuitBreaker>
  <log message="After Resilience: ${body}"/>
</route>

Fallback

See onFallback.

Other examples

You can find an example with the source code: camel-example-spring-boot-resilience4j.

Using Resilience4j with Spring Boot

See the Resilience4j Component.

Camel’s Error Handler and Circuit Breaker EIP

By default the Circuit Breaker EIP handles errors by itself. This means if the circuit breaker is open and the message fails, then Camel’s error handler is not reacting also. However, you can enable Camels error handler with circuit breaker by enabling the inheritErrorHandler option, as shown:

// Camel's error handler that will attempt to redeliver the message 3 times
errorHandler(deadLetterChannel("mock:dead").maximumRedeliveries(3).redeliveryDelay(0));

from("direct:start")
    .to("log:start")
    // turn on Camel's error handler on circuit breaker so Camel can do redeliveries
    .circuitBreaker().inheritErrorHandler(true)
        .to("mock:a")
        .throwException(new IllegalArgumentException("Forced"))
    .end()
    .to("log:result")
    .to("mock:result");

This example is from an unit test, where you can see the Circuit Breaker EIP block has been hardcoded to always fail by throwing an exception. Because the inheritErrorHandler has been enabled, then Camel’s error handler will attempt to call the Circuit Breaker EIP block again.

That means the mock:a endpoint will receive the message again, and a total of 1 + 3 = 4 message (first time + 3 redeliveries).

If we turn off the inheritErrorHandler option (default) then the Circuit Breaker EIP will only be executed once because it handled the error itself.