This commit moves the core Liveness and Readiness support to its own
`availability` package. We've made this a core concept independent of
Kubernetes.
Spring Boot now produces `LivenessStateChanged` and
`ReadinessStateChanged` events as part of the typical application
lifecycle.
Liveness and Readiness Probes (`HealthIndicator` components and health
groups) are still configured only when deployed on Kubernetes.
This commit also improves the documentation around Probes best practices
and container lifecycle considerations.
See gh-19593
Prior to this commit and as of Spring Boot 2.2.0, we would advise
developers to use the Actuator health groups to define custom "liveness"
and "readiness" groups and configure them with subsets of existing
health indicators.
This commit addresses several limitations with that approach.
First, `LivenessState` and `ReadinessState` are promoted to first class
concepts in Spring Boot applications. These states should not only based
on periodic health checks. Applications should be able to track changes
(and adapt their behavior) or update states (when an error happens).
The `ApplicationStateProvider` can be injected and used by applications
components to get the current application state. Components can also
track specific `ApplicationEvent` to be notified of changes, like
`ReadinessStateChangedEvent` and `LivenessStateChangedEvent`.
Components can also publish such events with an
`ApplicationEventPublisher`. Spring Boot will track startup event and
application context state to update the liveness and readiness state of
the application. This infrastructure is available in the
main spring-boot module.
If Spring Boot Actuator is on the classpath, additional
`HealthIndicator` will be contributed to the application:
`"LivenessProveHealthIndicator"` and `"ReadinessProbeHealthIndicator"`.
Also, "liveness" and "readiness" Health groups will be defined if
they're not configured already.
Closes gh-19593
Brian Clozel
Andy Wilkinson