Fault Tolerance 5.0

Roughly a year ago, we announced the release of SmallRye Fault Tolerance 4.0, the new implementation of Eclipse MicroProfile Fault Tolerance. Today, we bring you version 5.0, an evolution of the 4.x stream that fully implements MicroProfile Fault Tolerance 3.0 and brings a lot of improvements and even new features. These new features are not present in the MicroProfile Fault Tolerance specification, but were requested by users, especially in the Quarkus community. In addition, the API for integrators has been simplified significantly, and the first version of documentation is also ready. Let’s take a look.

MicroProfile Fault Tolerance 3.0

From user’s perspective, MicroProfile Fault Tolerance 3.0 is a relatively small release, including just two important changes: metrics overhaul, and lifecycle specification for circuit breakers and bulkheads. It is a major release because the metrics changes are very much backwards incompatible, so pay attention to that.

Metrics overhaul

The fault tolerance metrics now take advantage of tags. Most importantly, the full method name used to be a part of the metric name, but it’s now a metric tag. For example, suppose we have a method guarded with the @Timeout strategy:

public class MyService {
    public String hello() {

Previously, if you wanted to know how many times this method timed out, you’d have to look at the metric called ft.com.example.MyService.hello.timeout.callsTimedOut.total. With MicroProfile Fault Tolerance 3.0, you need to look at ft.timeout.calls.total{method="com.example.MyService.hello", timedOut="true"} instead.

You can find all the new metric names and possible tags in the MicroProfile Fault Tolerance 3.0 specification.

Also, fault tolerance metrics moved from the application scope to base.

Lifecycle specification for circuit breakers and bulkheads

Previously, MicroProfile Fault Tolerance didn’t specify lifecycle for stateful fault tolerance strategies, that is, circuit breakers and bulkheads. This becomes important when you have @RequestScoped beans, such as:

public class MyService {
    public String hello() {

You get a new instance of the bean for each request, and the MicroProfile Fault Tolerance specification didn’t say whether all such instances share the same circuit breaker for the hello method, or if each instance has its own. The specification now mandates that all instances share the same circuit breaker. Bulkheads behave identically.

This behavior was actually implemented by SmallRye Fault Tolerance since the beginning, so no behavioral change for you.

SmallRye Fault Tolerance improvements

The SmallRye Fault Tolerance 5.0 release brings a lof of additional improvements, both internal and user-facing. Changes that concern SmallRye Fault Tolerance integrators are described in more detail in a subsequent section.

Circuit breaker maintenance

We have previously introduced a way to observe circuit breaker state changes, using the CircuitBreakerStateChanged CDI event. This was subsequently deprecated, because we found a better way.

In this release, the CircuitBreakerStateChanged CDI event is removed, and a new API for circuit breaker maintenance is introduced.

Circuit breakers can now be given a name using the @CircuitBreakerName annotation. Afterwards, you can @Inject a CircuitBreakerMaintenance and use its currentState method to obtain current status of the circuit breaker.

In addition, CircuitBreakerMaintenance also allows you to reset any given named circuit breaker (reset), or all circuit breakers (resetAll), to the initial state.

Read the documentation for more information and examples.

@Blocking and @NonBlocking

In addition to the MicroProfile Fault Tolerance @Asynchronous annotation, we also introduce support for the @Blocking and @NonBlocking annotations from SmallRye Common.

@Blocking means that the annotated method blocks and hence its execution must be offloaded to another thread. This is basically equivalent to @Asynchronous.

On the other hand, @NonBlocking means that the annotated method doesn’t block and hence execution doesn’t have to be moved to another thread, yet all asynchronous fault tolerance behaviors are still supported.

Note that these annotations are only taken into account if the annotated method also applies some other fault tolerance strategy, such as @Fallback or @Retry, and if the method returns CompletionStage (or some of the additional asynchronous types as described below). If there’s no fault tolerance annotation, or if the method doesn’t return CompletionStage, SmallRye Fault Tolerance will ignore the @Blocking and @NonBlocking annotations completely.

Read the documentation for more information and examples.

Additional asynchronous types

In addition to CompletionStage, SmallRye Fault Tolerance now supports additional asynchronous types from libraries such as RxJava, Mutiny or Reactor. Note that only single-valued types, such as Single or Uni, are supported; stream-like types are not, because their semantics can’t be easily expressed in terms of CompletionStage.

This support is based on the SmallRye Reactive Converters project, so you have to make sure that the corresponding converter library is present. Integrators may include some converters by default if they choose so.

Read the documentation for more information and examples.

Other improvements

Exception handling in half-open circuit breakers was fixed. Previously, half-open circuit breakers would treat all exceptions as failures, which is wrong. Even in the half-open state, circuit breakers have to consider the failOn and skipOn configuration. This bug was not caught, because the MicroProfile Fault Tolerance TCK doesn’t have a test. That is something we are also going to fix.

Circuit breakers were also improved to reject excess attempts in the half-open state. Previously, half-open circuit breakers would allow all invocations to go through, which kinda defeats the purpose of the half-open state. This was improved to only allow the first successThreshold invocations (also called "probe invocations"); the excess ones are outright rejected. It is only after moving to the closed state when all invocations are allowed.

All CompletionStage fault tolerance strategies were examined and improved to avoid premature thread offload. This was required for the @NonBlocking support described above, but it’s also an important optimization.

Thread pool usage was radically improved. Previously, SmallRye Fault Tolerance would use 1 thread pool for executing @Asynchronous methods, 1 thread pool for watching for timeouts, and 1 thread pool for each thread pool style bulkhead. Now, SmallRye Fault Tolerance uses a single thread pool for everything. Thread pool style bulkheads were also significantly optimized.

The metrics subsystem was completely rewritten.

Trace logging has been added for all core implementations of fault tolerance strategies. We also started using JBoss Logging Tools to generate logger implementations.

Integration changes

How SmallRye Fault Tolerance can be integrated into a runtime has changed in two important ways: the thread pool integration is now based on CDI, and some additional dependencies are required.

Previously, the thread pool integration was based on ServiceLoader. Integrators had to provide implementation of the ExecutorFactory interface, which allowed customizing how all the different thread pools are created. With the 5.0 release, SmallRye Fault Tolerance no longer insists on creating its own thread pools. Instead, it works with a single thread pool and expects integrator to provide it via CDI.

To do that, integrators should provide a CDI bean implementing the AsyncExecutorProvider interface. This implementation should be @Singleton, must be marked as alternative and selected globally for the application. The interface has one method that returns the thread pool which the integrator desires to use for fault tolerance.

If the integrator doesn’t want to manage their own thread pool for fault tolerance, they can subclass DefaultAsyncExecutorProvider. This at least allows customizing the ThreadFactory.

As described above, the SmallRye Reactive Converters project is used to add support for additional asynchronous types. This means that SmallRye Fault Tolerance requires the io.smallrye.reactive:smallrye-reactive-converter-api artifact to be present. Presence of Reactive Converter API implementations is optional, but the API itself is mandatory.

Read the integration documentation for more information.


Looking back at the 4.0 announcement, we have since implemented almost all the planned improvements in SmallRye Fault Tolerance internals. On the other hand, the user-facing improvements we planned had to be postponed — but we implemented other user-facing improvements instead. We still intend to work on @FailFast or @AdaptiveBulkhead, but feature requests or bug reports by actual users will always have priority.

With that in mind, please don’t hesitate to get in touch. You can use the SmallRye Fault Tolerance issue tracker, the Zulip Chat, the SmallRye Fault Tolerance Gitter, or the SmallRye mailing list.