Programmatic API

In addition to the declarative, annotation-based API of MicroProfile Fault Tolerance, SmallRye Fault Tolerance also offers a programmatic API for advanced scenarios. This API is present in the io.smallrye:smallrye-fault-tolerance-api artifact, just like all other additional APIs SmallRye Fault Tolerance provides.

Installation

If you use SmallRye Fault Tolerance as part of a runtime that implements MicroProfile Fault Tolerance, you don’t have to do anything. The programmatic API is ready to use. In this documentation, we’ll call this the CDI implementation of the programmatic API, because it’s integrated with the MicroProfile Fault Tolerance implementation, which is naturally based on CDI.

Quarkus

In Quarkus, the SmallRye Fault Tolerance API is brought in automatically, as a transitive dependency of the Quarkus extension for SmallRye Fault Tolerance. That is, you don’t need to do anything to be able to use the programmatic API.

WildFly

In WildFly, the SmallRye Fault Tolerance API is not readily available to deployments. If you want to use it, you need to add a module dependency to the deployment using jboss-deployment-structure.xml.

Note that at the time of this writing, the SmallRye Fault Tolerance module in WildFly is considered private. If you do add a module dependency on it, to be able to use the SmallRye Fault Tolerance API, you may be stepping out of the WildFly support scope.

In addition to the CDI implementation, SmallRye Fault Tolerance also offers a standalone implementation that is meant to be used outside of any runtime. This implementation does not need CDI or anything else. If you want to use SmallRye Fault Tolerance in a standalone fashion, just add a dependency on io.smallrye:smallrye-fault-tolerance-standalone. The API is brought in transitively.

Usage

The entrypoint to the programmatic API is the FaultTolerance interface.

This interface represents a configured set of fault tolerance strategies. Their configuration, order of application and behavior in general corresponds to the declarative API, so if you know that, you’ll feel right at home. If not, the javadoc has all the information you need (though it often points to the annotation-based API for more information).

To create an instance of FaultTolerance, you can use the create and createAsync static methods. They return a builder which has to be used to add and configure all the fault tolerance strategies that should apply. There is no external configuration, so all configuration properties have to be set explicitly, using the builder methods. If you don’t set a configuration property, it will default to the same value the annotation-based API uses.

Disabling Fault Tolerance

There’s one exception to the "no external configuration" rule.

The CDI implementation looks for the well-known MP_Fault_Tolerance_NonFallback_Enabled configuration property in MicroProfile Config. The standalone implementation looks for a system property with the same name.

If such property exists and is set to false, only the fallback and thread offload fault tolerance strategies will be applied. Everything else will be ignored.

Note that this is somewhat different to the declarative, annotation-based API, where only fallback is retained and the @Asynchronous strategy is skipped as well. Since this significantly changes execution semantics, the programmatic API will apply thread offload even if fault tolerance is disabled.

Similarly to the declarative API, implementations of the programmatic API also read this property only once, when the FaultTolerance API is first used. It is not read again later.

Let’s take a look at a simple example:

public class MyService {
    private static final FaultTolerance<String> guarded = FaultTolerance.<String>create() (1)
        .withFallback().handler(() -> "fallback").done() (2)
        .build(); (3)

    public String hello() throws Exception {
        return guarded.call(() -> externalService.hello()); (4)
    }
}
1 The create invocation typically has to include an explicit type argument (<String>). The FaultTolerance interface has a type parameter which should be set to the return type of the guarded actions. This is required to be able to guarantee that a fallback value is of the same type.
2 The fallback handler may be a simple supplier of the fallback value, or a function that takes the exception and transforms it to the fallback value. Here, we use the simpler option. Note that we don’t set any other configuration options. This means that the default set of exceptions is used to determine when fallback should apply.
3 The build method returns a FaultTolerance<String> instance, which can later be used to guard arbitrary String-returning actions.
4 Here, we call externalService.hello() and guard the call with the previously configured set of fault tolerance strategies. (In this case, just fallback.) The call method uses the Callable type to represent the called action. Similar methods exist that accept a Supplier (get) or Runnable (run).

The previous example shows how to apply fault tolerance to synchronous actions. SmallRye Fault Tolerance naturally also supports guarding asynchronous actions, using the CompletionStage type. Unlike the declarative API, the programmatic API doesn’t support asynchronous actions that return the Future type.

public class MyService {
    private static final FaultTolerance<CompletionStage<String>> guarded = FaultTolerance.<String>createAsync() (1)
        .withBulkhead().done() (2)
        .withThreadOffload(true) (3)
        .build();

    public CompletionStage<String> hello() throws Exception {
        return guarded.call(() -> externalService.hello()); (4)
    }
}
1 The createAsync method takes a type parameter of String and returns a builder for fault tolerance of type CompletionStage<String>.
2 Here, we add a bulkhead. Since we don’t configure any property, default values are used. That is, at most 10 concurrent executions are permitted, and 10 more executions may be waiting in a queue.
3 And here, we add a thread offload. This is only possible for asynchronous actions, and corresponds to the @Asynchronous annotation from MicroProfile Fault Tolerance.
4 Note that here, unlike the previous example, the externalService.hello() method is assumed to return CompletionStage<String>.

Asynchronous actions may be blocking or non-blocking. In the example above, we assume the externalService.hello() call is blocking, so we set thread offload to true. SmallRye Fault Tolerance will automatically move the actual execution of the action to another thread.

If we didn’t configure withThreadOffload, however, the execution would continue on the original thread. This is often desired for non-blocking actions, which are very common in modern reactive architectures.

Synchronous vs. Asynchronous

What’s the difference between FaultTolerance.<CompletionStage<String>>create() and FaultTolerance.<String>createAsync()? Both may be used to guard an action that returns CompletionStage<String>, correct?

Well, yes and no.

The synchronous variant (created using create()) will only guard the synchronous part of the action — the part that ends by returning the CompletionStage instance. It will not guard the asynchronous behavior.

For example, if an action returns a CompletionStage object, synchronous fault tolerance will consider that action successfully finished. If that CompletionStage later completes with an exception, synchronous fault tolerance will never know. What’s more, the fact that this action has already "finished" means that the action will also leave the bulkhead, so concurrency limiting will not work properly.

The asynchronous variant (created using createAsync()), on the other hand, will not treat the action as finished until the CompletionStage actually completes. That is, the asynchronous action will only leave the bulkhead when it’s complete, so concurrency limiting works as expected. And if the CompletionStage completes exceptionally, asynchronous fault tolerance will treat that as a failure and react accordingly.

To summarize, if you need to guard asynchronous actions, blocking or non-blocking, always use createAsync.

Single-Action Usage

The FaultTolerance API is general and permits guarding multiple different actions using the same set of fault tolerance strategies. Often, that isn’t necessary and we need to guard just a single action, altough possibly several times.

For such use case, the FaultTolerance API provides shortcuts that work with the Callable<T>, Supplier<T> and Runnable types.

First off, a FaultTolerance<T> instance may be adapted to a Callable<T>, Supplier<T> or Runnable using the adapt* methods. For example:

public class MyService {
    private static final FaultTolerance<String> guard = FaultTolerance.<String>create()
        .withTimeout().duration(5, ChronoUnit.SECONDS).done()
        .build(); (1)

    public String hello() throws Exception {
        Callable<String> callable = guard.adaptCallable(() -> externalService.hello()); (2)

        return callable.call(); (3)
    }

}
1 Create a FaultTolerance<String> object that can guard arbitrary String-returning actions.
2 Adapt the general FaultTolerance instance to a Callable that guards the externalService.hello() invocation. Similar methods exist that accept and return a Supplier (adaptSupplier) and Runnable (adaptRunnable).
3 You can do whatever you wish with the adapted Callable. Here, we just call it once, which isn’t very interesting, but it could possibly be called multiple times, passed to other methods etc.

This style of usage still creates a FaultTolerance instance first. If that is not necessary, you can create a Callable, Supplier or Runnable directly:

public class MyService {
    private static final Callable<String> guard = FaultTolerance.createCallable(() -> externalService.hello()) (1)
        .withTimeout().duration(5, ChronoUnit.SECONDS).done()
        .build();

    public String hello() throws Exception {
        return guard.call(); (2)
    }
}
1 The createCallable method returns a fault tolerance builder that provides the same configuration options, but in the end, returns a Callable. In this case, a Callable<String>. These methods typically don’t require an explicit type argument, because it can be inferred from the type of action passed in. Similar methods exist that return a builder which, in the end, returns a Supplier (createSupplier) or Runnable (createRunnable).
2 Here, we don’t have to do anything special, just call the existing Callable. Again, it could possibly be called multiple times, passed to other methods etc.

Stateful Fault Tolerance Strategies

The circuit breaker and bulkhead strategies are stateful. That is, they hold some state required for their correct functioning, such as the number of current executions for bulkhead, or the rolling window of successes/failures for circuit breaker. If you use these strategies, you have to consider their lifecycle.

The SmallRye Fault Tolerance programmatic API makes such reasoning pretty straightforward. Each FaultTolerance object has its own instance of each fault tolerance strategy, including the stateful strategies. If you use a single FaultTolerance object for guarding multiple different actions, all those actions will be guarded by the same bulkhead and/or circuit breaker. If, on the other hand, you use different FaultTolerance objects for guarding different actions, each action will be guarded by its own bulkhead and/or circuit breaker.

If you use the adapt* methods, the resulting Callable, Supplier or Runnable objects will guard the underlying action using the original FaultTolerance instance, so stateful strategies will be shared.

If you use the create* methods that directly return Callable, Supplier or Runnable, each such creation will have its own FaultTolerance instance under the hood, so stateful strategies will not be shared.

Circuit Breaker Maintenance

The CircuitBreakerMaintenance API, accessed through FaultTolerance.circuitBreakerMaintenance() or by injection in the CDI implementation, can be used to manipulate all named circuit breakers. A circuit breaker is given a name by calling withCircuitBreaker().name("...") on the fault tolerance builder, or using the @CircuitBreakerName annotation in the declarative API.

Additionally, CircuitBreakerMaintenance.resetAll() will also reset all unnamed circuit breakers declared using the @CicruitBreaker annotation. For this to work, all unnamed circuit breakers have to be remembered. This is safe in case of the declarative, annotation-based API, because the number of such declared circuit breakers is fixed. At the same time, this would not be safe to do for all unnamed circuit breakers created using the programmatic API, as their number is potentially unbounded. (In other words, remembering all unnamed circuit breakers created using the programmatic API would easily lead to a memory leak.)

Therefore, all circuit breakers created using the programmatic API must be given a name when CircuitBreakerMaintenance is supposed to affect them. Note that duplicate names are not permitted and lead to an error, so lifecycle of the circuit breaker must be carefully considered.

Event Listeners

The programmatic API has one feature that the declarative API doesn’t have: ability to observe certain events. For example, when configuring a circuit breaker, it is possible to register a callback for circuit breaker state changes or for a situation when an open circuit breaker prevents an invocation. When configuring a timeout, it is possible to register a callback for when the invocation times out, etc. etc. For example:

private static final FaultTolerance<String> guard = FaultTolerance.<String>create()
    .withTimeout().duration(5, ChronoUnit.SECONDS).onTimeout(() -> ...).done() (1)
    .build();
1 The onTimeout method takes a Runnable that will later be executed whenever an invocation guarded by guard times out.

Summary of FaultTolerance Methods

There’s a number of static create* methods on the FaultTolerance interface. Which one do you want to call depends on the result type of the builder and whether the guarded actions are synchronous or asynchronous.

The builder result type Synchronous actions Asynchronous actions

FaultTolerance

create() → FaultTolerance<T>

createAsync() → FaultTolerance<CompletionStage<T>>

Callable

createCallable(Callable<T>) → Callable<T>

createAsyncCallable(Callable<CompletionStage<T>>) → Callable<CompletionStage<T>>

Supplier

createSupplier(Supplier<T>) → Supplier<T>

createAsyncSupplier(Supplier<CompletionStage<T>>) → Supplier<CompletionStage<T>>

Runnable

createRunnable(Runnable) → Runnable

createAsyncRunnable(Runnable) → Runnable

When you have an instance of FaultTolerance, there’s also a number of instance methods that either execute an action, or adapt an unguarded action to a guarded one. Which one do you want to call depends on the type used to represent the action.

The action type Execute Adapt

Callable<T>

call(Callable<T>) → T

adaptCallable(Callable<T>) → Callable<T>

Supplier<T>

get(Supplier<T>) → T

adaptSupplier(Supplier<T>) → Supplier<T>

Runnable

run(Runnable) → void

adaptRunnable(Runnable) → Runnable

Mutiny Support

In addition to the FaultTolerance interface, which provides support for guarding synchronous actions and asynchronous actions using CompletionStage, there’s a special programmatic API entrypoint for asynchronous actions using the Mutiny library. It is enough to include the Mutiny support library io.smallrye:smallrye-fault-tolerance-mutiny, as described in Additional Asynchronous Types.

This entrypoint is called MutinyFaultTolerance and it includes static factory methods for creating a Callable<Uni<T>>, Supplier<Uni<T> and FaultTolerance<Uni<T>>. Guarding a Multi is not supported.

These factory methods return the common fault tolerance builder, which is supposed to be used just like the builder used when guarding an async action of type CompletionStage<T>. For example:

public class MyService {
    private final Supplier<Uni<String>> guard = MutinyFaultTolerance.createSupplier(() -> externalService.hello()) (1)
        .withTimeout().duration(5, ChronoUnit.SECONDS).done()
        .withFallback().handler(() -> Uni.createFrom().item("fallback")).done()
        .build();

    public Uni<String> hello() {
        return guard.get();
    }
}
1 The call to externalService.hello() is supposed to return Uni<String>.

Note that the Uni type is lazy, so the action itself won’t execute until the guarded Uni is subscribed to.

Quarkus

In Quarkus, the Mutiny support library is present by default. You can use MutinyFaultTolerance out of the box.

Configuration and Metrics

As mentioned above, with the single exception of MP_Fault_Tolerance_NonFallback_Enabled, there is no external configuration support. This may change in the future, though possibly only in the CDI implementation.

At the moment, the programmatic API of SmallRye Fault Tolerance is not integrated with metrics. This will change in the future, though possibly only in the CDI implementation.

Integration Concerns

Integration concerns, which are particularly interesting for users of the standalone implementation, are described in the integration section.