Given a client and a supplier service, circuit breaker intent is:
- to avoid cascading failures
- to avoid waiting for supplier timeouts while supplier is down
- to provide a default response while supplier is down
A Client, a Supplier and a mediator between them called Circuit Breaker.
The Circuit Breaker monitors Supplier availability.
When the Client requests the Supplier, the Circuit Breaker delegates to Supplier if and only if commnunication with supplier is up (circuit closed), otherwise the circuit breaker returns a fallback response to client (circuit open).
Circuit breaker keeps returning the fallback until the communitation with supplier is re-established.
- Bookstore: Supplier, provides recommended books: http://localhost:8090/recommended
- To-Read: Client, depends on Bookstore for retrieving recommended books: http://localhost:8080/to-read
- Dashboard: Circuit breaker monitor Dashboard: http://localhost:7979
- Start Supplier service
- Start Client service
- Request Client service with Supplier up, receive response from Supplier (circuit closed)
- Stop Supplier service
- Request Client service with Supplier down, receive fallback response (circuit open)
If you use gnome-terminal, open a terminal and execute: ./run-demo
The script will open 3 gnome-terminal tabs and execute Supplier, Client and Dashboard applications.
If you don't use gnome-terminal you will have to execute the following commands, each on a different terminal:
Tab 1: cd supplier && mvn clean spring-boot:run
Tab 2: cd client && mvn clean spring-boot:run
Tab 3: cd hystrix-dashboard && mvn clean spring-boot:run
Get http://localhost:8090/recommended
You should get: Spring in Action (Manning), Cloud Native Java (O'Reilly), Learning Spring Boot (Packt)
Get http://localhost:8080/to-read
You should get: Spring in Action (Manning), Cloud Native Java (O'Reilly), Learning Spring Boot (Packt)
Kill Supplier Service: hit CTRL+C in Supplier terminal
Get http://localhost:8080/to-read
You should get fallback response from circuit breaker: Cloud Native Java (O'Reilly)
The Circuit Breaker continues returning the fallback response while the circuit is open (pinging the Supplier service doesn't get a response).
We are going to use spring-cloud-starter-hystrix as the dependency for adding Circuit Breaker support to our Client app.
We add fault tolerance to a service method. The service class must be annotated with @Service:
@Service
public class BookService {
...Then the service method is annotated with @HystrixCommand.
@HystrixCommand(fallbackMethod = "reliable")
public String readingList() {
...The fallbackMethod attribute of @HystrixCommand indicates the name of the method that provides the fallback, in this example reliable().
public String reliable() {
return "Cloud Native Java (O'Reilly)";
}NOTE: the fallback method reliable() must be declared in the same class than the intercepted method readingList().
For the complete example see BookService.java.
In this demo, Hystrix Dashboards runs on http://localhost:7979
In the home page of the Dashboard, connect to the event stream http://localhost:8080/hystrix.stream
Short-Circuited represents the requests that were sent to the fallback.
Shows a summary of the client-supplier collaboration.
Shows metrics of the thread pool defined by Hystrix on the Client.
https://spring.io/blog/2016/10/27/spring-tips-circuit-breakers
https://spring.io/guides/gs/circuit-breaker/
https://github.com/Netflix/Hystrix/tree/master/hystrix-contrib/hystrix-javanica
https://github.com/spring-cloud-samples/hystrix-dashboard