73. Spring Cloud Zookeeper Dependency Watcher

The Dependency Watcher mechanism allows you to register listeners to your dependencies. The functionality is in fact an implementation of the Observator pattern. When a dependency changes its state (UP or DOWN) then some custom logic can be applied.

73.1 How to activate

Spring Cloud Zookeeper Dependencies functionality needs to be enabled to profit from Dependency Watcher mechanism.

73.2 Registering a listener

In order to register a listener you have to implement an interface 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 then the dependencyName would be the discriminator for your concrete implementation. newState will provide you with information whether your dependency has changed to CONNECTED or DISCONNECTED.

73.3 Presence Checker

Bound with Dependency Watcher is the functionality called Presence Checker. It allows you to provide custom behaviour upon booting of your application to react accordingly 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 required and it’s not in Zookeeper then upon booting your application will throw an exception and shutdown
  • If dependency is not required the org.springframework.cloud.zookeeper.discovery.watcher.presence.LogMissingDependencyChecker will log that application is missing at WARN level

The functionality can be overriden since the DefaultDependencyPresenceOnStartupVerifier is registered only when there is no bean of DependencyPresenceOnStartupVerifier.