Package org.apache.sling.discovery

Interface ClusterView

  • All Known Implementing Classes:
    DefaultClusterView, LocalClusterView
    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:
    InstanceDescription.getSlingId()

    • Method Detail

      • getId

        java.lang.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:
        SLING-3164

      • getInstances

        java.util.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