The Dependency Watcher mechanism lets you register listeners to your dependencies. The
functionality is, in fact, an implementation of the Observator pattern. When a
dependency changes, its state (to either UP or DOWN), some custom logic can be applied.
Spring Cloud Zookeeper Dependencies functionality needs to be enabled for you to use the Dependency Watcher mechanism.
To register a listener, you must implement an interface called
org.springframework.cloud.zookeeper.discovery.watcher.DependencyWatcherListener and
register it as a bean. The interface gives you one method:
void stateChanged(String dependencyName, DependencyState newState);If you want to register a listener for a particular dependency, the dependencyName would
be the discriminator for your concrete implementation. newState provides you with
information about whether your dependency has changed to CONNECTED or DISCONNECTED.
Bound with the Dependency Watcher is the functionality called Presence Checker. It lets you provide custom behavior when your application boots, to react according to the state of your dependencies.
The default implementation of the abstract
org.springframework.cloud.zookeeper.discovery.watcher.presence.DependencyPresenceOnStartupVerifier
class is the
org.springframework.cloud.zookeeper.discovery.watcher.presence.DefaultDependencyPresenceOnStartupVerifier,
which works in the following way.
required and is not in Zookeeper, when your application
boots, it throws an exception and shuts down.required, the
org.springframework.cloud.zookeeper.discovery.watcher.presence.LogMissingDependencyChecker
logs that the dependency is missing at the WARN level.Because the DefaultDependencyPresenceOnStartupVerifier is registered only when there is
no bean of type DependencyPresenceOnStartupVerifier, this functionality can be
overridden.