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The Adobe Experience Manager SDK
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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.sling.discovery;

import java.util.List;

import aQute.bnd.annotation.ProviderType;

/**
 * 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 InstanceDescription#getSlingId()
 */
@ProviderType
public interface ClusterView {

    /**
     * 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 '_'.
     * @return an id of this cluster view
     * @see SLING-3164
     * @since exists since 1.0.0 - stable since 1.0.2 - alphanumeric + '_' and'-' since 1.0.4
     */
    String getId();

    /**
     * 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.
     * @return the list of InstanceDescriptions (with a stable ordering)
     */
    List getInstances();

    /**
     * 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)
     * @return the InstanceDescription belonging to the leader instance
     */
    InstanceDescription getLeader();
}