Package org.apache.sling.discovery

Interface ClusterView

All Known Implementing Classes:
DefaultClusterView, LocalClusterView
@ProviderType public interface ClusterView
A ClusterView represents the instances of a cluster that are up and running and that all can see each other at a certain point in time.

'Seeing each other'/A cluster refers to being hooked to the same underlying persistence layer/jcr repository.

A ClusterView can also consist of just one single instance.

All instances in a cluster must and do have a unique sling.id.

See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    String
    getId()
    Returns a stable id of the cluster represented by this cluster view.
    List<InstanceDescription>
    getInstances()
    Provides the list of InstanceDescriptions with a stable ordering.
    InstanceDescription
    getLeader()
    Provides the InstanceDescription belonging to the leader instance.

  • Method Details

    • getId

      String getId()
      Returns a stable id of the cluster represented by this cluster view.

      Similar to the SlingID, this id is persisted and thus remains stable accross instance/cluster restarts.

      When an instance joins a cluster, it will inherit the id from the joined cluster (thus incurs a cluster id change).

      Note: When instances part from a cluster (eg due to a network partitioning) the cluster id is retained. This results in potentially multiple clusters with the same id. This fact could be used to detect such partitioning/split brain situations. It also implies though that the cluster id is not guaranteed to be unique in a topology!

      Addition in 1.0.4: this id must consist only of alphanumeric characters plus dash '-' and underscore '_'.

      Returns:
      an id of this cluster view
      Since:
      exists since 1.0.0 - stable since 1.0.2 - alphanumeric + '_' and'-' since 1.0.4
      See Also:

    • getInstances

      List<InstanceDescription> getInstances()
      Provides the list of InstanceDescriptions with a stable ordering.

      Stable ordering implies that unless an instance leaves the cluster (due to shutdown/crash/network problems) the instance keeps the relative position in the list.

      Returns:
      the list of InstanceDescriptions (with a stable ordering)

    • getLeader

      InstanceDescription getLeader()
      Provides the InstanceDescription belonging to the leader instance.

      Every ClusterView is guaranteed to have one and only one leader.

      The leader is stable: once a leader is elected it stays leader unless it leaves the cluster (due to shutdown/crash/network problems)

      Returns:
      the InstanceDescription belonging to the leader instance