Circuit Breaker Pattern in Apache Camel

Camel is very often used in distributed environments for accessing remote resources. Remote services may fail for various reasons and periods. For services that are temporarily unavailable and recoverable after short period of time, a retry strategy may help. But some services can fail or hang for longer period of time making the calling application unresponsive and slow. A good strategy to prevent from cascading failures and exhaustion of critical resources is the Circuit Breaker pattern described by Michael Nygard in the Release It! book.
Circuit Breaker is a stateful pattern that wraps the failure-prone resource and monitors for errors. Initially the Circuit Breaker is in closed state and passes all calls to the wrapped resource. When the failures reaches a certain threshold, the circuit moves to open state where it returns error to the caller without actually calling the wrapped resource. This prevents from overloading the already failing resource. While at this state, we need a mechanism to detect whether the failures are over and start calling the protected resource. This is where the third state called half-open comes into play. This state is reached after a certain time following the last failure. At this state, the calls are passed through to the protected resource, but the result of the call is important. If the call is successful, it is assumed that the protected resource has recovered and the circuit is moved into closed state, and if the call fails, the timeout is reset, and the circuit is moved back to open state where all calls are rejected. Here is the state diagram of Circuit Breaker from Martin Fowler's post:

How Circuit Breaker is implemented in Camel?

Circuit Breaker is available in the latest 2.14 version of Camel as a Load balancer policy. Camel Load Balancer already has policies for Round Robin, Random, Failover, etc. and now also CircuiBreaker policy.
Here is an example load balancer that uses Circuit Breaker policy with threshold of 2 errors and halfOpenAfter timeout of 1 second. Notice also that this policy applies only to errors caused by MyCustomException
And here is the same example using Spring XML DSL:

A Docker Maven Plugin for Integration Testing

What is Docker?
Docker is the buzzword that is taking the DevOps world. If you don't know yet what is Docker, be warned, you will find yourself using it one way or another very soon. The rest of this post assumes some basic understanding of Docker, but if you are not familiar with it now, I'm sure you will come back later and read this.
Docker is ideal for integration testing, complex demos of distributed systems or even running production systems. It is an open source software container. You can imagine it as a very lightweight and ultra fast virtual machine.
An example
Inspired by the "Integration testing with Maven and Docker" article and using Docker Java API I've created a simple Docker Maven Plugin that can manage Docker containers. Given a Docker image, the plugin will create a container from it and start it as part of maven build process and stop and remove the container when the build process is finished. If the image is not available locally it will pull it down from the public Docker registry before creating a container from it.

The following integration test in Apache Camel is ignored because it requires a running Redis instance:

To make it pass, we can use docker-maven-plugin with a Redis image and 6379 port accessible for the test:

The plugin will start a Docker container at compile phase that has a running Redis instance and shut it down at post-integration-test phase.
This is a very simple example, but the plugin can support more advanced scenarios with multiple images configured differently and started/stopped at different phases. Enjoy.

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