7.2.7 Client Fallbacks

In distributed systems, failure happens and it is best to be prepared for it and handle it in as graceful a manner possible.

In addition, when developing Microservices it is quite common to work on a single Microservice without other Microservices the project requires being available.

With that in mind Micronaut features a native fallback mechanism that is integrated into Retry Advice that allows falling back to another implementation in the case of failure.

Using the @Fallback annotation you can declare a fallback implementation of a client that will be picked up and used once all possible retries have been exhausted.

In fact the mechanism is not strictly linked to Retry, you can declare any class as @Recoverable and if a method call fails (or, in the case of reactive types, an error is emitted) a class annotated with @Fallback will be searched for.

To illustrate this consider again the PetOperations interface declared earlier. You can define a PetFallback class that will be called in the case of failure:

Defining a Fallback

@Fallback
public class PetFallback implements PetOperations {
    @Override
    public Single<Pet> save(String name, int age) {
        Pet pet = new Pet();
        pet.setAge(age);
        pet.setName(name);
        return Single.just(pet);
    }
}

Defining a Fallback

@Fallback
class PetFallback implements PetOperations {
    @Override
    public Single<Pet> save(String name, int age) {
        Pet pet = new Pet()
        pet.setAge(age)
        pet.setName(name)
        return Single.just(pet)
    }
}

Defining a Fallback

@Fallback
open class PetFallback : PetOperations {
    override fun save(name: String, age: Int): Single<Pet> {
        val pet = Pet()
        pet.age = age
        pet.name = name
        return Single.just(pet)
    }
}

As you can see the fallback does not perform any network operations and is quite simple, hence will provide a successful result in the case of an external system being down.

Of course, the actual behaviour of the fallback is down to you. You could for example implement a fallback that pulls data from a local cache when the real data is not available, and sends alert emails to operations about downtime or whatever.