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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.fs.Path;
import org.apache.hadoop.hbase.Coprocessor;
import org.apache.hadoop.hbase.HBaseInterfaceAudience;
import org.apache.hadoop.hbase.HRegionInfo;
import org.apache.yetus.audience.InterfaceAudience;
import org.apache.yetus.audience.InterfaceStability;
import org.apache.hadoop.hbase.wal.WALEdit;
import org.apache.hadoop.hbase.wal.WALKey;

/**
 * It's provided to have a way for coprocessors to observe, rewrite,
 * or skip WALEdits as they are being written to the WAL.
 *
 * Note that implementers of WALObserver will not see WALEdits that report themselves
 * as empty via {@link WALEdit#isEmpty()}.
 *
 * {@link org.apache.hadoop.hbase.coprocessor.RegionObserver} provides
 * hooks for adding logic for WALEdits in the region context during reconstruction.
 *
 * Defines coprocessor hooks for interacting with operations on the
 * {@link org.apache.hadoop.hbase.wal.WAL}.
 *
 * 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 WALObserver extends Coprocessor {
  /**
   * Called before a {@link WALEdit}
   * is writen to WAL.
   *
   * @return true if default behavior should be bypassed, false otherwise
   */
  // TODO: return value is not used
  default boolean preWALWrite(ObserverContext ctx,
      HRegionInfo info, WALKey logKey, WALEdit logEdit) throws IOException {
    return false;
  }

  /**
   * Called after a {@link WALEdit}
   * is writen to WAL.
   */
  default void postWALWrite(ObserverContext ctx,
      HRegionInfo info, WALKey logKey, WALEdit logEdit) throws IOException {}

  /**
   * Called before rolling the current WAL
   * @param oldPath the path of the current wal that we are replacing
   * @param newPath the path of the wal we are going to create
   */
  default void preWALRoll(ObserverContext ctx,
      Path oldPath, Path newPath) throws IOException {}

  /**
   * Called after rolling the current WAL
   * @param oldPath the path of the wal that we replaced
   * @param newPath the path of the wal we have created and now is the current
   */
  default void postWALRoll(ObserverContext ctx,
      Path oldPath, Path newPath) throws IOException {}
}