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Server functionality for HBase
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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.hadoop.hbase.coprocessor;

import java.io.IOException;

import org.apache.hadoop.hbase.Coprocessor;
import org.apache.hadoop.hbase.HBaseInterfaceAudience;
import org.apache.hadoop.hbase.replication.ReplicationEndpoint;
import org.apache.yetus.audience.InterfaceAudience;
import org.apache.yetus.audience.InterfaceStability;

/**
 * Defines coprocessor hooks for interacting with operations on the
 * {@link org.apache.hadoop.hbase.regionserver.HRegionServer} process.
 *
 * Since most implementations will be interested in only a subset of hooks, this class uses
 * 'default' functions to avoid having to add unnecessary overrides. When the functions are
 * non-empty, it's simply to satisfy the compiler by returning value of expected (non-void) type.
 * It is done in a way that these default definitions act as no-op. So our suggestion to
 * implementation would be to not call these 'default' methods from overrides.
 * 


 *
 * Exception Handling
 * For all functions, exception handling is done as follows:
 * 
 *   Exceptions of type {@link IOException} are reported back to client.
 *   For any other kind of exception:
 *     
 *       If the configuration {@link CoprocessorHost#ABORT_ON_ERROR_KEY} is set to true, then
 *         the server aborts.
 *       Otherwise, coprocessor is removed from the server and
 *         {@link org.apache.hadoop.hbase.DoNotRetryIOException} is returned to the client.
 *     
 *   
 * 
 */
@InterfaceAudience.LimitedPrivate(HBaseInterfaceAudience.COPROC)
@InterfaceStability.Evolving
public interface RegionServerObserver {
  /**
   * Called before stopping region server.
   * @param ctx the environment to interact with the framework and region server.
   */
  default void preStopRegionServer(
    final ObserverContext ctx) throws IOException {}

  /**
   * This will be called before executing user request to roll a region server WAL.
   * @param ctx the environment to interact with the framework and region server.
   */
  default void preRollWALWriterRequest(
      final ObserverContext ctx)
      throws IOException {}

  /**
   * This will be called after executing user request to roll a region server WAL.
   * @param ctx the environment to interact with the framework and region server.
   */
  default void postRollWALWriterRequest(
      final ObserverContext ctx)
      throws IOException {}

  /**
   * This will be called after the replication endpoint is instantiated.
   * @param ctx the environment to interact with the framework and region server.
   * @param endpoint - the base endpoint for replication
   * @return the endpoint to use during replication.
   */
  default ReplicationEndpoint postCreateReplicationEndPoint(
      ObserverContext ctx, ReplicationEndpoint endpoint) {
    return endpoint;
  }

  // TODO remove below 2 hooks when we implement AC as a core impl than a CP impl.
  /**
   * This will be called before executing replication request to shipping log entries.
   * @param ctx the environment to interact with the framework and region server.
   * @deprecated As of release 2.0.0 with out any replacement. This is maintained for internal
   * usage by AccessController. Do not use these hooks in custom co-processors.
   */
  @Deprecated
  default void preReplicateLogEntries(final ObserverContext ctx)
      throws IOException {
  }

  /**
   * This will be called after executing replication request to shipping log entries.
   * @param ctx the environment to interact with the framework and region server.
   * @deprecated As of release 2.0.0 with out any replacement. This is maintained for internal
   * usage by AccessController. Do not use these hooks in custom co-processors.
   */
  @Deprecated
  default void postReplicateLogEntries(
      final ObserverContext ctx) throws IOException {
  }

  /**
   * This will be called before clearing compaction queues
   * @param ctx the environment to interact with the framework and region server.
   */
  default void preClearCompactionQueues(
      final ObserverContext ctx)
      throws IOException {}

  /**
   * This will be called after clearing compaction queues
   * @param ctx the environment to interact with the framework and region server.
   */
  default void postClearCompactionQueues(
      final ObserverContext ctx)
      throws IOException {}
}