Circuit breaker is a pattern used for fault tolerance and the term was first introduced by Michael Nygard in his famous book titled "Release It!". The idea is, rather than wasting valuable resources trying to invoke an operation that keeps failing, the system backs off for a period of time, and later tries to see whether the operation that was originally failing works.
A good example would be, a service receiving a request, which in turn leads to a database call. At some point in time, the connectivity to the database could fail. After a series of failed calls, the circuit trips, and there will be no further attempts to connect to the database for a period of time. We call this the "open" state of the circuit breaker. During this period, the callers of the service will be served from a cache. After this period has elapsed, the next call to the service will result in a call to the database. This stage of the circuit breaker is called the "half-open" stage. If this call succeeds, then the circuit breaker goes back to the closed stage and all subsequent calls will result in calls to the database. However, if the database call during the half-open state fails, the circuit breaker goes back to the open state and will remain there for a period of time, before transitioning to the half-open state again.
Other typical examples of the circuit breaker pattern being useful would be a service making a call to another service, and a client making a call to a service. In both cases, the calls could fail, and instead of indefinitely trying to call the relevant service, the circuit breaker would introduce some back-off period, before attempting to call the service which was failing.
This sample will demonstrate how a circuit breaker can be implemented using the WSO2 Microservices Framework for Java (MSF4J) & Netflix Hystrix. We take the stockquote service sample, and enable circuit breaker. Assume that the stock quotes are loaded from a database. We wrap the calls to this database in a Hystrix command. If database calls fail, the circuit trips and stock quotes are served from cache.
NOTE: To keep things simple and focus on the implementation of the circuit breaker pattern, rather than making actual database calls, we have a class called org.example.service.StockQuoteDatabase and calls to its getStock method could result in timeouts or failures. To see an MSF4J example on how to make actual database calls, see the JPA sample.
The complete call sequence is shown below. StockQuoteService is an MSF4J microservice.
The circuit breaker is configured as shown below.
CircuitBreakerCommand(StockQuoteDatabase db, String symbol) {
super(HystrixCommand.Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("MyGroup"))
.andCommandPropertiesDefaults(
HystrixCommandProperties.Setter()
.withCircuitBreakerEnabled(true)
.withCircuitBreakerRequestVolumeThreshold(50)
.withExecutionTimeoutEnabled(true)
.withExecutionTimeoutInMilliseconds(10)
));
this.db = db;
this.symbol = symbol;
}We are enabling circuit breaker & timeout, and then setting the threshold of failures which will trigger circuit tripping to 50, and also timeout to 10ms. So any database call that takes more than 10ms will also be registered as a failure. For other configuration parameters, please see https://github.com/Netflix/Hystrix/wiki/Configuration
Use Maven to build the sample.
mvn clean package
Next run the MSF4J service.
java -jar target/circuitbreaker-*.jar
Now let's use cURL to repeatedly invoke the service. Run the following command;
while true; do curl -v http://localhost:8080/stockquote/IBM ; done
The above command will keep invoking the service. Observe the output of the service in the terminal. You will see that some of the calls will fail on the service side and you will be able to see the circuit breaker fallback in action and also the circuit breaker tripping, then going into the half-open state, and then closing.