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MicroProfile Fault Tolerance - Fallback

This is an example of how to use Microprofile @Fallback in TomEE.

Fallback Feature

Fault Tolerance Fallback provides an alternative execution in case of failure. This alternative will be called when Retry or CircuitBreaker has failed.

To use this feature you need to annotate the method with @Fallback.

The Fallback policy allows to configure :

  • value: A class which implements FallbackHandler

  • fallbackMethod: a method which will be executed.

The parameters value and fallbackMethod cannot be specified at the same time.

Check the specification for more details.

Examples

Run the application

mvn clean install tomee:run

Example 1

The method statusOfDay will always fail throwing a WeatherException and as the @CircuitBreaker annotation is configured to failOn in case of that exception, the fallback, WeatherDayStatusFallbackHandler#handle will be invoked.

@RequestScoped
public class WeatherService {
   ...
   @GET
   @Path("/day/status")
   @CircuitBreaker(failOn = WeatherException.class)
   @Fallback(WeatherDayStatusFallbackHandler.class)
   public String dayStatus() {
       throw new WeatherException();
   }
   ...
 }

public class WeatherDayStatusFallbackHandler implements FallbackHandler<String> {

   @Override
   public String handle(ExecutionContext executionContext) {
       return "Hi, today is a sunny day!";
   }
}

Day status call

GET http://localhost:8080/mp-faulttolerance-fallback/weather/day/status

Server log

SEVERE [http-nio-8080-exec-2] org.superbiz.rest.WeatherDayStatusFallbackHandler.handle WeatherDayStatusFallbackHandler was triggered due a fail

Response

Hi, today is a sunny day!

Example 2

The method statusOfDay will always fail throwing a WeatherException and as the @Retry annotation is configured to maxRetries = 1 in case of fail, the fallback method, fallbackForWeekStatus will be invoked after retrying once.

@RequestScoped
public class WeatherService {
  ...
  @GET
  @Path("/week/status")
  @Retry(maxRetries = 1)
  @Fallback(fallbackMethod = "fallbackForWeekStatus")
  public String weekStatus() {
      throw new WeatherException();
  }

  public String fallbackForWeekStatus() {
      return "Hi, week will be mostly sunny!";
  }
  ...
}

Week status call

GET http://localhost:8080/mp-faulttolerance-fallback/weather/week/status

Server log

SEVERE [http-nio-8080-exec-2] org.superbiz.rest.WeatherService.fallbackForWeekStatus Fallback was triggered due a fail

Response

Hi, week will be mostly sunny!

Run the tests

You can also try it out using the WeatherServiceTest.java available in the project.

mvn clean test
[INFO] Results:
[INFO]
[INFO] Tests run: 2, Failures: 0, Errors: 0, Skipped: 0

APIs Used