HostProvider.java

/**
 * 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.zookeeper.client;

import java.net.InetSocketAddress;

/**
 * A set of hosts a ZooKeeper client should connect to.
 * 
 * Classes implementing this interface must guarantee the following:
 * 
 * * Every call to next() returns an InetSocketAddress. So the iterator never
 * ends.
 * 
 * * The size() of a HostProvider may never be zero.
 * 
 * A HostProvider must return resolved InetSocketAddress instances on next(),
 * but it's up to the HostProvider, when it wants to do the resolving.
 * 
 * Different HostProvider could be imagined:
 * 
 * * A HostProvider that loads the list of Hosts from an URL or from DNS 
 * * A HostProvider that re-resolves the InetSocketAddress after a timeout. 
 * * A HostProvider that prefers nearby hosts.
 */
public interface HostProvider {
    public int size();

    /**
     * The next host to try to connect to.
     * 
     * For a spinDelay of 0 there should be no wait.
     * 
     * @param spinDelay
     *            Milliseconds to wait if all hosts have been tried once.
     */
    public InetSocketAddress next(long spinDelay);

    /**
     * Notify the HostProvider of a successful connection.
     * 
     * The HostProvider may use this notification to reset it's inner state.
     */
    public void onConnected();
}