Circuit Breaker Pattern in Apache Camel
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Join For FreeCamel 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:
Circuit Breaker is available in the latest snapshot 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.
new RouteBuilder() { public void configure() { from("direct:start").loadBalance() .circuitBreaker(2, 1000L, MyCustomException.class) .to("mock:result"); } };And here is the same example using Spring XML DSL:
<route> <from uri="direct:start"/> <loadBalance> <circuitBreaker threshold="2" halfOpenAfter="1000"> <exception>MyCustomException</exception> </circuitBreaker> <to uri="mock:result"/> </loadBalance> </route>
Published at DZone with permission of Bilgin Ibryam, DZone MVB. See the original article here.
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