Coherence manages cluster membership, automatically adding new servers to the cluster when they start up and automatically detecting their departure when they are shut down or fail. Applications have full access to this information, and can sign up to receive event notifications when members join and leave the cluster. Coherence also tracks all the services that each member is providing and consuming, and uses this information to plan for service resiliency in case of server failure, and to load-balance data management and other responsibilities across all members of the cluster.
Cluster and Service objects
From any cache, the application can obtain a reference to the local representation of a cache's service. From any service, the application can obtain a reference to the local representation of the cluster.
From the Cluster object, the application can determine the set of services that are running in the cluster:
The ServiceInfo object provides information about the service, including its name, type, version and membership.
For more information on this feature, see the API documentation for NamedCache, CacheService, Service, ServiceInfo and Cluster.
The primary information that an application can determine about each member in the cluster is:
- The Member's IP address
- What date/time the Member joined the cluster
As an example, if there are four servers in the cluster with each server running one copy ("instance") of the application and all four instances of the application are clustered together, then the cluster is composed of four Members. From the Cluster object, the application can determine what the local Member is:
From the Cluster object, the application can also determine the entire set of cluster members:
From the ServiceInfo object, the application can determine the set of cluster members that are participating in that service:
For more information on this feature, see the [API documentation for Member.
Listener interface and Event object
To listen to cluster and/or service membership changes, the application places a listener on the desired Service. As discussed before, the Service can come from a cache:
The Service can also be looked up by its name:
To receive membership events, the application implements a MemberListener. For example, the following listener example prints out all the membership events that it receives:
The MemberEvent object carries information about the event type (joined / leaving / left), the member that generated the event, and the service that acts as the source of the event. Additionally, the event provides a method, isLocal(), that indicates to the application that it is this member that is joining or leaving the cluster. This is useful for recognizing soft restarts in which an application automatically rejoins a cluster after a failure occurs. For example:
For more information on these feature, see the [API documentation for Service, MemberListener and MemberEvent.