/*
 * The contents of this file are subject to the terms of the Common Development and
 * Distribution License (the License). You may not use this file except in compliance with the
 * License.
 *
 * You can obtain a copy of the License at legal/CDDLv1.0.txt. See the License for the
 * specific language governing permission and limitations under the License.
 *
 * When distributing Covered Software, include this CDDL Header Notice in each file and include
 * the License file at legal/CDDLv1.0.txt. If applicable, add the following below the CDDL
 * Header, with the fields enclosed by brackets [] replaced by your own identifying
 * information: "Portions Copyright [year] [name of copyright owner]".
 *
 * Copyright 2007-2009 Sun Microsystems, Inc.
 * Portions Copyright 2013-2016 ForgeRock AS.
 * Portions Copyright 2026 3A Systems, LLC.
 */
package org.opends.server.replication.plugin;

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.SortedMap;
import java.util.SortedSet;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

import net.jcip.annotations.GuardedBy;

import org.forgerock.opendj.ldap.DN;
import org.opends.server.core.AddOperation;
import org.opends.server.core.DeleteOperation;
import org.opends.server.core.ModifyDNOperationBasis;
import org.opends.server.core.ModifyOperation;
import org.opends.server.replication.common.CSN;
import org.opends.server.replication.common.ServerState;
import org.opends.server.replication.protocol.AddMsg;
import org.opends.server.replication.protocol.DeleteMsg;
import org.opends.server.replication.protocol.LDAPUpdateMsg;
import org.opends.server.replication.protocol.ModifyDNMsg;
import org.opends.server.replication.protocol.ModifyMsg;
import org.opends.server.replication.protocol.OperationContext;
import org.opends.server.types.Operation;

/**
 * This class is used to store the list of remote changes received
 * from a replication server and that are either currently being replayed
 * or that are waiting for being replayed.
 *
 * It is used to know when the ServerState must be updated and to compute
 * the dependencies between operations.
 *
 * One of this object is instantiated for each ReplicationDomain.
 */
final class RemotePendingChanges
{
  /** A map used to store the pending changes. */
  @GuardedBy("pendingChangesLock")
  private final SortedMap<CSN, PendingChange> pendingChanges = new TreeMap<>();

  /**
   * A sorted set containing the list of PendingChanges that have
   * not been replayed correctly because they are dependent on
   * another change to be completed.
   */
  @GuardedBy("dependentChangesLock")
  private final SortedSet<PendingChange> dependentChanges = new TreeSet<>();
  /**
   * {@code activeAndDependentChanges} also contains changes discovered to be dependent
   * on currently in progress changes.
   */
  private final ConcurrentSkipListSet<PendingChange> activeAndDependentChanges = new ConcurrentSkipListSet<>();

  private final ReentrantReadWriteLock pendingChangesLock = new ReentrantReadWriteLock(true);
  private final ReentrantReadWriteLock.ReadLock pendingChangesReadLock = pendingChangesLock.readLock();
  private final ReentrantReadWriteLock.WriteLock pendingChangesWriteLock = pendingChangesLock.writeLock();
  private final ReentrantLock dependentChangesLock = new ReentrantLock();

  /** The ServerState that will be updated when LDAPUpdateMsg are fully replayed. */
  private final ServerState state;

  /**
   * How many of the changes listed here have a replay failure recorded against them.
   * <p>
   * A change is counted from its first failed replay until it leaves this map, and it is
   * what tells a change which can not be applied from a backend which is serving again:
   * an outage fails everything in flight, while a change which can never be applied here
   * fails alone, among changes which replay perfectly well. The session restart backoff
   * reads it, so that the successful replays which surround a failing change do not keep
   * resetting the wait this domain has reached on it (issue #889).
   */
  @GuardedBy("pendingChangesLock")
  private int failingChanges;

  /**
   * Creates a new RemotePendingChanges using the provided ServerState.
   *
   * @param state   The ServerState that will be updated when LDAPUpdateMsg
   *                have been fully replayed.
   */
  public RemotePendingChanges(ServerState state)
  {
    this.state = state;
  }

  /**
   * Returns the number of changes waiting to be replayed.
   *
   * @return The number of changes waiting to be replayed
   */
  public int getQueueSize()
  {
    pendingChangesReadLock.lock();
    try
    {
      return pendingChanges.size();
    }
    finally
    {
      pendingChangesReadLock.unlock();
    }
  }

  /**
   * Returns the number of changes actively being replayed.
   * <p>
   * A change whose replay failed counts here until it is applied or given up on: it stays
   * a dependency of the changes which follow it, since a change which is not in the data
   * is exactly what they must wait for, and the delivery which comes next takes it over.
   *
   * @return the number of changes actively being replayed.
   */
  public int changesInProgressSize()
  {
    return activeAndDependentChanges.size();
  }

  /**
   * Returns the number of changes depending on other changes.
   *
   * @return the number of changes depending on other changes.
   */
  public int getDependentChangesSize()
  {
    dependentChangesLock.lock();
    try
    {
      return dependentChanges.size();
    }
    finally
    {
      dependentChangesLock.unlock();
    }
  }

  /**
   * Add a new LDAPUpdateMsg that was received from the replication server
   * to the pendingList.
   * <p>
   * A change which is already listed and which a replay thread owns - it is being
   * replayed, it waits for the change it depends on, or it has been replayed and waits
   * for the changes before it - is left alone: that copy knows what this one does not,
   * and replaying it again is exactly what the duplicate check is there to prevent
   * (OPENDJ-1115).
   * <p>
   * A change which is listed but which no replay thread owns is taken over by this
   * delivery: it is the one to replay, and the copy which may still wait in the replay
   * queue is dropped by {@link #markInProgress(LDAPUpdateMsg)}. That is a change whose
   * replay failed - it is deliberately left out of the ServerState, so the replication
   * server sends it again over the session which was restarted (issue #889) - and it is
   * also, harmlessly, a change which was listed a moment ago by a delivery no replay
   * thread has picked up yet: the two deliveries carry the same change, and the last one
   * listed is the one replayed.
   * <p>
   * The failures the change went through are kept, whichever delivery replays it: they
   * are what has this replica eventually give up on a change it can not apply.
   *
   * @param update The LDAPUpdateMsg that was received from the replication
   *               server and that will be added to the pending list.
   * @return {@code false} if the update was already registered in the pending
   *         changes and is owned by a replay thread.
   */
  public boolean putRemoteUpdate(LDAPUpdateMsg update)
  {
    pendingChangesWriteLock.lock();
    try
    {
      final CSN csn = update.getCSN();
      final PendingChange listed = pendingChanges.get(csn);
      if (listed == null)
      {
        pendingChanges.put(csn, new PendingChange(csn, null, update));
        return true;
      }
      if (listed.isCommitted() || listed.isOwned())
      {
        return false;
      }
      listed.setMsg(update);
      return true;
    }
    finally
    {
      pendingChangesWriteLock.unlock();
    }
  }

  /**
   * Mark an update message as committed.
   *
   * @param csn
   *          The CSN of the update message that must be set as committed.
   */
  public void commit(CSN csn)
  {
    pendingChangesWriteLock.lock();
    try
    {
      PendingChange curChange = pendingChanges.get(csn);
      if (curChange == null)
      {
        throw new NoSuchElementException();
      }
      curChange.setCommitted(true);
      curChange.setOwned(false);
      activeAndDependentChanges.remove(curChange);

      final Iterator<PendingChange> it = pendingChanges.values().iterator();
      while (it.hasNext())
      {
        PendingChange pendingChange = it.next();
        if (!pendingChange.isCommitted())
        {
          break;
        }
        if (pendingChange.getMsg().contributesToDomainState())
        {
          state.update(pendingChange.getCSN());
        }
        if (pendingChange.getReplayFailures() > 0)
        {
          // The change is leaving this map, so it is not one of the failing ones anymore.
          failingChanges--;
        }
        it.remove();
      }
    }
    finally
    {
      pendingChangesWriteLock.unlock();
    }
  }

  /**
   * Gives up the ownership a replay thread had on the change with the provided CSN,
   * whose replay failed.
   * <p>
   * The change stays listed here and stays uncommitted: it is the barrier which keeps
   * the ServerState - and every change which follows it - from moving past a change
   * which is not in the data (issue #889), and it is what has the replication server
   * send it again over the restarted session. Only the mark which says a replay thread
   * owns it is dropped, so that {@link #putRemoteUpdate(LDAPUpdateMsg)} takes the next
   * delivery instead of discarding it as a duplicate.
   * <p>
   * It also stays listed among the changes the newer ones are checked against: a change
   * which is not in the data yet is exactly what the changes which follow it must depend
   * on, whether or not a replay thread owns it right now.
   * <p>
   * The changes another replay thread is applying right now are left alone: they are
   * about to commit, and forgetting them would have their {@code commit()} fail, the
   * ServerState stay behind them and the replication server replay them a second time.
   *
   * @param csn the CSN of the change whose replay failed
   */
  public void replayFailed(CSN csn)
  {
    pendingChangesWriteLock.lock();
    try
    {
      final PendingChange change = pendingChanges.get(csn);
      if (change != null && !change.isCommitted())
      {
        change.setOwned(false);
      }
    }
    finally
    {
      pendingChangesWriteLock.unlock();
    }
  }

  /** How long, and how many times, the replay of one change has been failing. */
  static final class ReplayFailure
  {
    private final int attempts;
    private final long failingForMs;

    private ReplayFailure(int attempts, long failingForMs)
    {
      this.attempts = attempts;
      this.failingForMs = failingForMs;
    }

    /**
     * Returns how many deliveries of the change failed to replay it in a row. A delivery
     * is attempted several times in place before it is counted here, so this is a count
     * of deliveries rather than of attempts made on the backend.
     *
     * @return the number of failed deliveries, at least 1
     */
    int getAttempts()
    {
      return attempts;
    }

    /**
     * Returns how long the replay of the change has been failing, that is the time
     * between its first failure and the one which was just recorded.
     *
     * @return the duration in milliseconds, 0 for a first failure
     */
    long getFailingForMs()
    {
      return failingForMs;
    }
  }

  /**
   * Records that the replay of the change with the provided CSN failed once more.
   * <p>
   * The failures are kept on the change itself, which stays listed for as long as it has
   * not been applied, so a change which keeps failing keeps its give-up budget across the
   * deliveries which take over from one another, and a change which is replayed or given
   * up on takes its failures away with it (issue #889).
   *
   * @param csn
   *          the CSN of the change whose replay failed
   * @param nowMs
   *          when it failed, on a clock which only moves forward
   * @return the failures of the change, or {@code null} when it is not listed as an
   *         uncommitted change anymore, which happens when the domain was disabled while
   *         it was being replayed: there is no change left here to give up on
   */
  public ReplayFailure recordReplayFailure(CSN csn, long nowMs)
  {
    pendingChangesWriteLock.lock();
    try
    {
      final PendingChange change = pendingChanges.get(csn);
      if (change == null || change.isCommitted())
      {
        return null;
      }
      if (change.getReplayFailures() == 0)
      {
        // Its first failure: this change joins the ones which are failing right now.
        failingChanges++;
      }
      change.recordReplayFailure(nowMs);
      return new ReplayFailure(change.getReplayFailures(), change.getReplayFailingForMs(nowMs));
    }
    finally
    {
      pendingChangesWriteLock.unlock();
    }
  }

  /**
   * Returns whether the replay of any change listed here is failing right now.
   * <p>
   * A change is failing from its first failed replay until it leaves this map, whether
   * it leaves it applied or given up on. The session restart backoff reads this: a
   * change which was replayed only says that this backend is serving again when it is
   * the last one which was failing, and the successful replays which surround a change
   * this replica can not apply must not keep resetting the wait it has reached on it.
   *
   * @return {@code true} while at least one listed change has a failed replay recorded
   *         against it
   */
  boolean hasFailingChanges()
  {
    pendingChangesReadLock.lock();
    try
    {
      return failingChanges > 0;
    }
    finally
    {
      pendingChangesReadLock.unlock();
    }
  }

  /**
   * Forgets every change listed here, without updating the ServerState.
   * <p>
   * Called when the domain is disabled: its ServerState is saved and cleared from
   * memory, and it is loaded again from the backend when the domain is enabled back, so
   * the bookkeeping which goes with it must not outlive it. A change which stayed here
   * would be discarded as a duplicate when the replication server sends it again, and
   * nothing would ever replay it or record it in the ServerState.
   */
  public void clear()
  {
    pendingChangesWriteLock.lock();
    dependentChangesLock.lock();
    try
    {
      pendingChanges.clear();
      dependentChanges.clear();
      activeAndDependentChanges.clear();
      failingChanges = 0;
    }
    finally
    {
      dependentChangesLock.unlock();
      pendingChangesWriteLock.unlock();
    }
  }

  /**
   * Marks the change of the provided message as being replayed.
   *
   * @param msg
   *          the message whose change is being replayed
   * @return {@code false} if this message is not the delivery which is listed as
   *         pending, which happens when the session was restarted after a failed replay
   *         while this message was still waiting in the replay queue: the replication
   *         server delivered the change again and that delivery took over from this one,
   *         or the domain was disabled and forgot the change, so this copy must not be
   *         replayed.
   */
  public boolean markInProgress(LDAPUpdateMsg msg)
  {
    pendingChangesWriteLock.lock();
    try
    {
      final PendingChange change = pendingChanges.get(msg.getCSN());
      if (change == null || change.isCommitted() || change.getLDAPUpdateMsg() != msg)
      {
        return false;
      }
      change.setOwned(true);
      activeAndDependentChanges.add(change);
      return true;
    }
    finally
    {
      pendingChangesWriteLock.unlock();
    }
  }
  /**
   * Get the first update in the list that have some dependencies cleared.
   *
   * @return The LDAPUpdateMsg to be handled.
   */
  public LDAPUpdateMsg getNextUpdate()
  {
    pendingChangesReadLock.lock();
    dependentChangesLock.lock();
    try
    {
      if (!dependentChanges.isEmpty() && !pendingChanges.isEmpty())
      {
        PendingChange firstDependentChange = dependentChanges.first();
        if (pendingChanges.firstKey().isNewerThanOrEqualTo(firstDependentChange.getCSN()))
        {
          dependentChanges.remove(firstDependentChange);
          return firstDependentChange.getLDAPUpdateMsg();
        }
      }
      return null;
    }
    finally
    {
      dependentChangesLock.unlock();
      pendingChangesReadLock.unlock();
    }
  }

  /**
   * Mark the first pendingChange as dependent on the second PendingChange.
   * @param dependentChange The PendingChange that depend on the second
   *                        PendingChange.
   */
  private void addDependency(PendingChange dependentChange)
  {
    pendingChangesReadLock.lock();
    dependentChangesLock.lock();
    try
    {
      /*
       * A change which is not listed as pending anymore - the domain was disabled while
       * its dependencies were computed - must not be listed as dependent either:
       * getNextUpdate() reads both and would hand out a change nothing owns. The
       * replication server sends it again when the domain is enabled back.
       */
      if (pendingChanges.containsKey(dependentChange.getCSN()))
      {
        dependentChanges.add(dependentChange);
      }
    }
    finally
    {
      dependentChangesLock.unlock();
      pendingChangesReadLock.unlock();
    }
  }

  private PendingChange getPendingChange(CSN csn)
  {
    pendingChangesReadLock.lock();
    try
    {
      return pendingChanges.get(csn);
    }
    finally
    {
      pendingChangesReadLock.unlock();
    }
  }

  /**
   * Check if the given AddOperation has some dependencies on any
   * currently running previous operation.
   * Update the dependency list in the associated PendingChange if
   * there are some dependencies.
   * AddOperation depends on
   *
   * - DeleteOperation done on the same DN
   * - ModifyDnOperation with the same target DN as the ADD DN
   * - ModifyDnOperation with new DN equals to the ADD DN parent
   * - AddOperation done on the parent DN of the ADD DN
   *
   * @param op The AddOperation to be checked.
   *
   * @return A boolean indicating if this operation has some dependencies.
   */
  public boolean checkDependencies(AddOperation op)
  {
    final CSN csn = OperationContext.getCSN(op);
    final PendingChange change = getPendingChange(csn);
    if (change == null)
    {
      return false;
    }

    boolean hasDependencies = false;
    final DN targetDN = op.getEntryDN();
    for (PendingChange pendingChange : activeAndDependentChanges)
    {
      if (pendingChange.getCSN().isNewerThanOrEqualTo(csn))
      {
        // From now on, the dependency should be for newer changes to be dependent on this one, so we can stop for now.
        break;
      }
      final LDAPUpdateMsg pendingMsg = pendingChange.getLDAPUpdateMsg();
      if (pendingMsg instanceof DeleteMsg)
      {
        if (pendingMsg.getDN().equals(targetDN))
        {
          // it is a deleteOperation on the same DN
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof AddMsg)
      {
        if (pendingMsg.getDN().isSuperiorOrEqualTo(targetDN))
        {
          // it is an addOperation on a parent of the current AddOperation
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof ModifyDNMsg)
      {
        // it is a ModifyDnOperation with the same target DN as the ADD DN
        // or a ModifyDnOperation with new DN equals to the ADD DN parent?
        if (pendingMsg.getDN().equals(targetDN))
        {
          hasDependencies = true;
          addDependency(change);
        }
        else
        {
          final ModifyDNMsg pendingModDn = (ModifyDNMsg) pendingMsg;
          if (pendingModDn.newDNIsParent(targetDN))
          {
            hasDependencies = true;
            addDependency(change);
          }
        }
      }
    }
    return hasDependencies;
  }

  /**
   * Check if the given ModifyOperation has some dependencies on any
   * currently running previous operation.
   * Update the dependency list in the associated PendingChange if
   * there are some dependencies.
   *
   * ModifyOperation depends on
   * - AddOperation done on the same DN
   * - ModifyDNOperation having newDN the same as targetDN
   *
   * @param op The ModifyOperation to be checked.
   *
   * @return A boolean indicating if this operation has some dependencies.
   */
  public boolean checkDependencies(ModifyOperation op)
  {
    final CSN csn = OperationContext.getCSN(op);
    final PendingChange change = getPendingChange(csn);
    if (change == null)
    {
        return false;
    }

    boolean hasDependencies = false;
    final DN targetDN = change.getLDAPUpdateMsg().getDN();
    for (PendingChange pendingChange : activeAndDependentChanges)
    {
      if (pendingChange.getCSN().isNewerThanOrEqualTo(csn))
      {
        // From now on, the dependency should be for newer changes to be dependent on this one, so we can stop for now.
        break;
      }
      final LDAPUpdateMsg pendingMsg = pendingChange.getLDAPUpdateMsg();
      if (pendingMsg instanceof AddMsg)
      {
        if (pendingMsg.getDN().equals(targetDN))
        {
          // it is an addOperation on a same DN
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof ModifyDNMsg)
      {
        if (((ModifyDNMsg) pendingMsg).newDNIsEqual(targetDN))
        {
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof ModifyMsg)
      {
        if (pendingMsg.getDN().equals(targetDN))
        {
          // it is another modify on the same DN, they depend
          hasDependencies = true;
          addDependency(change);
        }
      }
    }
    return hasDependencies;
  }

  /**
   * Check if the given ModifyDNMsg has some dependencies on any
   * currently running previous operation.
   * Update the dependency list in the associated PendingChange if
   * there are some dependencies.
   *
   * Modify DN Operation depends on
   * - AddOperation done on the same DN as the target DN of the MODDN operation
   * - AddOperation done on the new parent of the MODDN  operation
   * - DeleteOperation done on the new DN of the MODDN operation
   * - ModifyDNOperation done from the new DN of the MODDN operation
   *
   * TODO: Consider cases where there is a rename A -> B then rename B -> C. Second change depends on first
   *
   * @param msg The ModifyDNMsg to be checked.
   *
   * @return A boolean indicating if this operation has some dependencies.
   */
  public boolean checkDependencies(ModifyDNMsg msg)
  {
    final CSN csn = msg.getCSN();
    final PendingChange change = getPendingChange(csn);
    if (change == null)
    {
      return false;
    }

    boolean hasDependencies = false;
    final DN targetDN = change.getLDAPUpdateMsg().getDN();
    for (PendingChange pendingChange : activeAndDependentChanges)
    {
      if (pendingChange.getCSN().isNewerThanOrEqualTo(csn))
      {
        // From now on, the dependency should be for newer changes to be dependent on this one, so we can stop for now.
        break;
      }
      final LDAPUpdateMsg pendingMsg = pendingChange.getLDAPUpdateMsg();
      if (pendingMsg instanceof DeleteMsg)
      {
        // Check if the target of the Delete is the same
        // as the new DN of this ModifyDN
        if (msg.newDNIsEqual(pendingMsg.getDN()))
        {
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof AddMsg)
      {
        // Check if the Add Operation was done on the new parent of
        // the MODDN  operation
        if (msg.newParentIsEqual(pendingMsg.getDN()))
        {
          hasDependencies = true;
          addDependency(change);
        }
        // Check if the AddOperation was done on the same DN as the
        // target DN of the MODDN operation
        if (pendingMsg.getDN().equals(targetDN))
        {
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof ModifyDNMsg)
      {
        if (msg.newDNIsEqual(pendingMsg.getDN()))
        {
          // the ModifyDNOperation was done from the new DN of the MODDN operation
          hasDependencies = true;
          addDependency(change);
        }
      }
    }
    return hasDependencies;
  }

  /**
   * Check if the given DeleteOperation has some dependencies on any
   * currently running previous operation.
   * Update the dependency list in the associated PendingChange if
   * there are some dependencies.
   *
   * DeleteOperation depends on
   * - DeleteOperation done on children DN
   * - ModifyDnOperation with target DN that are children of the DEL DN
   * - AddOperation done on the same DN
   *
   *
   * @param op The DeleteOperation to be checked.
   *
   * @return A boolean indicating if this operation has some dependencies.
   */
  public boolean checkDependencies(DeleteOperation op)
  {
    final CSN csn = OperationContext.getCSN(op);
    final PendingChange change = getPendingChange(csn);
    if (change == null)
    {
      return false;
    }

    boolean hasDependencies = false;
    final DN targetDN = op.getEntryDN();
    for (PendingChange pendingChange : activeAndDependentChanges)
    {
      if (pendingChange.getCSN().isNewerThanOrEqualTo(csn))
      {
        // From now on, the dependency should be for newer changes to be dependent on this one, so we can stop for now.
        break;
      }
      final LDAPUpdateMsg pendingMsg = pendingChange.getLDAPUpdateMsg();
      if (pendingMsg instanceof DeleteMsg)
      {
          /*
           * Check if the operation to be run is a deleteOperation on a
           * children of the current DeleteOperation.
           */
        if (pendingMsg.getDN().isSubordinateOrEqualTo(targetDN))
        {
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof AddMsg)
      {
          /*
           * Check if the operation to be run is an addOperation on a
           * parent of the current DeleteOperation.
           */
        if (pendingMsg.getDN().equals(targetDN))
        {
          hasDependencies = true;
          addDependency(change);
        }
      }
      else if (pendingMsg instanceof ModifyDNMsg)
      {
        final ModifyDNMsg pendingModDn = (ModifyDNMsg) pendingMsg;
          /*
           * Check if the operation to be run is an ModifyDNOperation
           * on a children of the current DeleteOperation
           */
        if (pendingMsg.getDN().isSubordinateOrEqualTo(targetDN) || pendingModDn.newDNIsParent(targetDN))
        {
          hasDependencies = true;
          addDependency(change);
        }
      }
    }
    return hasDependencies;
  }

  /**
   * Check the dependencies of a given Operation/UpdateMsg.
   *
   * @param op   The Operation for which dependencies must be checked.
   * @param msg  The LocalizableMessage for which dependencies must be checked.
   * @return     A boolean indicating if an operation cannot be replayed
   *             because of dependencies.
   */
  public boolean checkDependencies(Operation op, LDAPUpdateMsg msg)
  {
    if (op instanceof ModifyOperation)
    {
      return checkDependencies((ModifyOperation) op);
    }
    else if (op instanceof DeleteOperation)
    {
      return checkDependencies((DeleteOperation) op);
    }
    else if (op instanceof AddOperation)
    {
      return checkDependencies((AddOperation) op);
    }
    else if (op instanceof ModifyDNOperationBasis)
    {
      return checkDependencies((ModifyDNMsg) msg);
    }
    else
    {
      return true;  // unknown type of operation ?!
    }
  }
}