WildFly allows any user MSC service to be installed as a singleton MSC service via a public API. Once installed, the service will only ever start on 1 node in the cluster at a time. If the node providing the service is shutdown, or fails, another node on which the service was installed will start automatically.
Installing an MSC service using an existing singleton policy
While singleton MSC services have been around since AS7, WildFly 10 adds the ability to leverage the singleton subsystem to create singleton MSC services from existing singleton policies.
The singleton subsystem exposes capabilities for each singleton policy it defines. These policies, represented via the org.wildfly.clustering.singleton.SingletonPolicy interface, can be referenced via the following name: “org.wildfly.clustering.singleton.policy”
e.g.
public class MyServiceActivator implements ServiceActivator {
@Override
public void activate(ServiceActivatorContext context) {
ServiceName name = ServiceName.parse(“my.service.name”);
Service<?> service = new MyService();
try {
SingletonPolicy policy = (SingletonPolicy) context.getServiceRegistry().getRequiredService(ServiceName.parse(SingletonPolicy.CAPABILITY_NAME)).awaitValue();
policy.createSingletonServiceBuilder(name, service).build(context.getServiceTarget()).install();
} catch (InterruptedException e) {
throw new ServiceRegistryException(e);
}
}
}
Installing an MSC service using dynamic singleton policy
Alternatively, you can build singleton policy dynamically, which is particularly useful if you want to use a custom singleton election policy. Specifically, SingletonPolicy is a generalization of the org.wildfly.clustering.singleton.SingletonServiceBuilderFactory interface, which includes support for specifying an election policy and, optionally, a quorum.
e.g.
public class MyServiceActivator implements ServiceActivator {
@Override
public void activate(ServiceActivatorContext context) {
String containerName = “server”;
ElectionPolicy policy = new MySingletonElectionPolicy();
int quorum = 3;
ServiceName name = ServiceName.parse(“my.service.name”);
Service<?> service = new MyService();
try {
SingletonServiceBuilderFactory factory = (SingletonServiceBuilderFactory) context.getServiceRegistry().getRequiredService(SingletonServiceName.BUILDER.getServiceName(containerName))).awaitValue();
factory.createSingletonServiceBuilder(name, service)
.electionPolicy(policy)
.quorum(quorum)
.build(context.getServiceTarget()).install();
} catch (InterruptedException e) {
throw new ServiceRegistryException(e);
}
}
}