Spring boot Liveness State vs Readiness State

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Application Availability

When deployed on platforms, applications can provide information about their availability to the platform using infrastructure such as Kubernetes Probes. Spring Boot includes out-of-the box support for the commonly used “liveness” and “readiness” availability states. If you are using Spring Boot’s “actuator” support then these states are exposed as health endpoint groups.

In addition, you can also obtain availability states by injecting the ApplicationAvailability interface into your own beans.

Liveness State

The “Liveness” state of an application tells whether its internal state allows it to work correctly, or recover by itself if it’s currently failing. A broken “Liveness” state means that the application is in a state that it cannot recover from, and the infrastructure should restart the application.

In general, the "Liveness" state should not be based on external checks, such as Health checks. If it did, a failing external system (a database, a Web API, an external cache) would trigger massive restarts and cascading failures across the platform.
The internal state of Spring Boot applications is mostly represented by the Spring ApplicationContext. If the application context has started successfully, Spring Boot assumes that the application is in a valid state. An application is considered live as soon as the context has been refreshed, see Spring Boot application lifecycle and related Application Events.

Readiness State

The “Readiness” state of an application tells whether the application is ready to handle traffic. A failing “Readiness” state tells the platform that it should not route traffic to the application for now. This typically happens during startup, while CommandLineRunner and ApplicationRunner components are being processed, or at any time if the application decides that it’s too busy for additional traffic.

An application is considered ready as soon as application and command-line runners have been called, see Spring Boot application lifecycle and related Application Events.

Tasks expected to run during startup should be executed by CommandLineRunner and ApplicationRunner components instead of using Spring component lifecycle callbacks such as @PostConstruct.

Managing the Application Availability State

Application components can retrieve the current availability state at any time, by injecting the ApplicationAvailability interface and calling methods on it. More often, applications will want to listen to state updates or update the state of the application.

For example, we can export the "Readiness" state of the application to a file so that a Kubernetes "exec Probe" can look at this file:

@Component
public class ReadinessStateExporter {

    @EventListener
    public void onStateChange(AvailabilityChangeEvent<ReadinessState> event) {
        switch (event.getState()) {
        case ACCEPTING_TRAFFIC:
            // create file /tmp/healthy
        break;
        case REFUSING_TRAFFIC:
            // remove file /tmp/healthy
        break;
        }
    }

}
We can also update the state of the application, when the application breaks and cannot recover:

@Component
public class LocalCacheVerifier {

    private final ApplicationEventPublisher eventPublisher;

    public LocalCacheVerifier(ApplicationEventPublisher eventPublisher) {
        this.eventPublisher = eventPublisher;
    }

    public void checkLocalCache() {
        try {
            //...
        }
        catch (CacheCompletelyBrokenException ex) {
            AvailabilityChangeEvent.publish(this.eventPublisher, ex, LivenessState.BROKEN);
        }
    }

}

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