Get failure exception in @HystrixCommand fallback method

Question

Is there a way to get the reason a HystrixCommand failed when using the @HystrixCommand annotation within a Spring Boot application? It looks like if you implement your own HystrixCommand, you have access to the getFailedExecutionException but how can you get access to this when using the annotation? I would like to be able to do different things in the fallback method based on the type of exception that occurred. Is this possible?

I saw a note about HystrixRequestContext.initializeContext() but the HystrixRequestContext doesn't give you access to anything, is there a different way to use that context to get access to the exceptions?

Answer 1

Simply add a Throwable parameter to the fallback method and it will receive the exception which the original command produced.

From https://github.com/Netflix/Hystrix/tree/master/hystrix-contrib/hystrix-javanica

    @HystrixCommand(fallbackMethod = "fallback1")
    User getUserById(String id) {
        throw new RuntimeException("getUserById command failed");
    }

    @HystrixCommand(fallbackMethod = "fallback2")
    User fallback1(String id, Throwable e) {
        assert "getUserById command failed".equals(e.getMessage());
        throw new RuntimeException("fallback1 failed");
    }

Answer 2

I haven't found a way to get the exception with Annotations either, but creating my own Command worked for me like so:

public static class DemoCommand extends HystrixCommand<String> {

    protected DemoCommand() {
        super(HystrixCommandGroupKey.Factory.asKey("Demo"));
    }

    @Override
    protected String run() throws Exception {
        throw new RuntimeException("failed!");
    }

    @Override
    protected String getFallback() {
        System.out.println("Events (so far) in Fallback: " + getExecutionEvents());
        return getFailedExecutionException().getMessage();
    }

}

Hopefully this helps someone else as well.

Answer 3

I couldn't find a way to obtain the exception with the annotations, but i found HystrixPlugins , with that you can register a HystrixCommandExecutionHook and you can get the exact exception in that like this :

HystrixPlugins.getInstance().registerCommandExecutionHook(new HystrixCommandExecutionHook() {
            @Override
            public <T> void onFallbackStart(final HystrixInvokable<T> commandInstance) {

            }
        });

The command instance is a GenericCommand.

Answer 4

Most of the time just using getFailedExecutionException().getMessage() gave me null values.

   Exception errorFromThrowable = getExceptionFromThrowable(getExecutionException());
   String errMessage = (errorFromThrowable != null) ? errorFromThrowable.getMessage()

this gives me better results all the time.

Answer 5

As said in the documentation Hystrix-documentation getFallback() method will be thrown when:

1. Whenever a command execution fails: when an exception is thrown by construct() or run()
2. When the command is short-circuited because the circuit is open
3. When the command’s thread pool and queue or semaphore are at capacity
4. When the command has exceeded its timeout length.

So you can easily get what raised your fallback method called by assigning the the execution exception to a Throwable object.

Assuming your HystrixCommand returns a String

public class ExampleTask extends HystrixCommand<String> {
   //Your class body
}

do as follows:

@Override
    protected ErrorCodes getFallback() {
        Throwable t = getExecutionException();
        if (circuitBreaker.isOpen()) {
            // Log or something
        } else if (t instanceof RejectedExecutionException) {
            // Log and get the threadpool name, could be useful
        } else {
            // Maybe something else happened
        }
        return "A default String"; // Avoid using any HTTP request or ypu will need to wrap it also in HystrixCommand
    }

More info here