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Apache Geode provides a database-like consistency model, reliable transaction processing and a shared-nothing architecture to maintain very low latency performance with high concurrency processing

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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.geode.internal.cache;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * A lock used for the backup process. This is a reentrant lock that provides a "backup" mode, where
 * the lock is held by a "backup thread" which can be assigned later than the time we lock.
 * 
 * We need this because our backup process is two phase. In the first phase we aquire the lock and
 * in the second phase we actually do the backup. During the second phase we need to reenter the
 * lock and release it with a different thread.
 *
 */
public class BackupLock extends ReentrantLock {

  private Thread backupThread;
  boolean isBackingUp;
  Condition backupDone = super.newCondition();

  public void lockForBackup() {
    super.lock();
    isBackingUp = true;
    super.unlock();
  }

  public void setBackupThread(Thread thread) {
    super.lock();
    backupThread = thread;
    super.unlock();
  }

  public void unlockForBackup() {
    super.lock();
    isBackingUp = false;
    backupThread = null;
    backupDone.signalAll();
    super.unlock();
  }

  /**
   * Acquire this lock, waiting for an in progress backup if one is in progress.
   */
  @Override
  public void lock() {
    lock(true);
  }

  /**
   * Acquire this lock, Optionally waiting for a backup to finish the first phase. Any operations
   * that update metadata related to the distributed system state should pass true for this flag,
   * because we need to make sure we get a point in time snapshot of the init files across members
   * to for metadata consistentency.
   * 
   * Updates which update only record changes to the local state on this member(eg, switching
   * oplogs), do not need to wait for the backup.
   * 
   * @param waitForBackup if true, we will wait for an in progress backup before acquiring this
   *        lock.
   */
  public void lock(boolean waitForBackup) {
    super.lock();
    while (isBackingUp && waitForBackup && !(Thread.currentThread() == backupThread)) {
      backupDone.awaitUninterruptibly();
    }
  }
}