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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.
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package org.apache.hadoop.hbase.coprocessor;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.CoprocessorEnvironment;
import org.apache.hadoop.hbase.HBaseInterfaceAudience;
import org.apache.hadoop.hbase.ServerName;
import org.apache.hadoop.hbase.client.Connection;
import org.apache.hadoop.hbase.metrics.MetricRegistry;
import org.apache.yetus.audience.InterfaceAudience;
import org.apache.yetus.audience.InterfaceStability;

@InterfaceAudience.LimitedPrivate(HBaseInterfaceAudience.COPROC)
@InterfaceStability.Evolving
public interface MasterCoprocessorEnvironment extends CoprocessorEnvironment<MasterCoprocessor> {
  /**
   * @return Hosting Server's ServerName
   */
  ServerName getServerName();

  /**
   * Returns the hosts' Connection to the Cluster. <b>Do not close! This is a shared connection
   * with the hosting server. Throws {@link UnsupportedOperationException} if you try to close
   * or abort it</b>.
   *
   * For light-weight usage only. Heavy-duty usage will pull down
   * the hosting RegionServer responsiveness as well as that of other Coprocessors making use of
   * this Connection. Use to create table on start or to do administrative operations. Coprocessors
   * should create their own Connections if heavy usage to avoid impinging on hosting Server
   * operation. To create a Connection or if a Coprocessor requires a region with a particular
   * Configuration, use {@link org.apache.hadoop.hbase.client.ConnectionFactory} or
   * {@link #createConnection(Configuration)}}.
   *
   * <p>Be aware that operations that make use of this Connection are executed as the RegionServer
   * User, the hbase super user that started this server process. Exercise caution running
   * operations as this User (See {@link #createConnection(Configuration)}} to run as other than
   * the RegionServer User).
   *
   * <p>Be careful RPC'ing from a Coprocessor context. RPC's will fail, stall, retry, and/or crawl
   * because the remote side is not online, is struggling or it is on the other side of a network
   * partition. Any use of Connection from inside a Coprocessor must be able to handle all such
   * hiccups.
   *
   * @see #createConnection(Configuration)
   * @return The host's Connection to the Cluster.
   */
  Connection getConnection();

  /**
   * Creates a cluster connection using the passed Configuration.
   *
   * Creating a Connection is a heavy-weight operation. The resultant Connection's cache of
   * region locations will be empty. Therefore you should cache and reuse Connections rather than
   * create a Connection on demand. Create on start of your Coprocessor. You will have to cast
   * the CoprocessorEnvironment appropriately to get at this API at start time because
   * Coprocessor start method is passed a subclass of this CoprocessorEnvironment or fetch
   * Connection using a synchronized accessor initializing the Connection on first access. Close
   * the returned Connection when done to free resources. Using this API rather
   * than {@link org.apache.hadoop.hbase.client.ConnectionFactory#createConnection(Configuration)}
   * returns a Connection that will short-circuit RPC if the target is a local resource. Use
   * ConnectionFactory if you don't need this ability.
   *
   * <p>Be careful RPC'ing from a Coprocessor context. RPC's will fail, stall, retry, and/or crawl
   * because the remote side is not online, is struggling or it is on the other side of a network
   * partition. Any use of Connection from inside a Coprocessor must be able to handle all such
   * hiccups.
   * @return Connection created using the passed conf.
   */
  Connection createConnection(Configuration conf) throws IOException;

  /**
   * Returns a MetricRegistry that can be used to track metrics at the master level.
   *
   * <p>See ExampleMasterObserverWithMetrics class in the hbase-examples modules for examples
   * of how metrics can be instantiated and used.</p>
   * @return A MetricRegistry for the coprocessor class to track and export metrics.
   */
  MetricRegistry getMetricRegistryForMaster();
}
