77. Spring Cloud Zookeeper Dependency Watcher
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
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.
If the dependency is marked us
requiredand is not in Zookeeper, when your application boots, it throws an exception and shuts down.
If the dependency is not
org.springframework.cloud.zookeeper.discovery.watcher.presence.LogMissingDependencyCheckerlogs that the dependency is missing at the
DefaultDependencyPresenceOnStartupVerifier is registered only when there is no bean of type
DependencyPresenceOnStartupVerifier , this functionality can be overridden.