Resilience4j EIP

The Resilience4j EIP provides integration with Resilience4j Resilience4j to be used as Circuit Breaker in the Camel routes.

Configuration options

The Resilience4j 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.

Resilience4jConfigurationDefinition

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.

Using Resilienc4j EIP

Below is an example route showing a Resilience4j circuit breaker that protects against a downstream HTTP operation with fallback.

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

And in XML DSL:

<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>

In case the calling the downstream HTTP service is failing, and an exception is thrown then the circuit breaker will react and execute the fallback route instead.

If there was no fallback, then the circuit breaker will throw an exception.

For more information about fallback see onFallback.

Configuring Resilience4j

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

For example to use 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}")
        .to("http://fooservice.com/faulty")
        .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}"/>
    <to uri="http://fooservice.com/faulty"/>
    <log message="Resilience processing end: ${threadName}"/>
  </circuitBreaker>
  <log message="After Resilience: ${body}"/>
</route>

In this example if calling the downstream service does not return a response within 2 seconds, a timeout is triggered, and the exchange will fail with a TimeoutException.

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 a 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.

Dependencies

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>
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