pastpolicy.java

pastry的java实现的2.0b版

/*************************************************************************

"FreePastry" Peer-to-Peer Application Development Substrate 

Copyright 2002, Rice University. All rights reserved.

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********************************************************************************/

package rice.p2p.past;

import rice.*;
import rice.Continuation.*;
import rice.p2p.commonapi.*;
import rice.p2p.past.messaging.*;
import rice.persistence.*;

/**
 * @(#) PastPolicy.java This interface represents a policy for Past, which is
 * asked whenever the local node is told to replicate or validate an item. This
 * allows for applications to control replication and object validate behavior,
 * permitting behavior specific for mutable or self-authenticating data.
 *
 * @version $Id: PastPolicy.java 2302 2005-03-11 00:58:26Z jeffh $
 * @author Alan Mislove
 */
public interface PastPolicy {

  /**
   * This method is called when Past is told to fetch a key. This method allows
   * the application to specify how a replica is fetched and authenticated. The
   * client should fetch the object (possibly using the past instance provided)
   * and then return the object to the provided continuation. The client *MUST*
   * call the continuation at some point in the future, even if the request is
   * lost.
   *
   * @param id The id to fetch
   * @param past The local past instance
   * @param backup The backup cache, where the object *might* be located
   * @param command The command to call with the replica to store
   * @param hint DESCRIBE THE PARAMETER
   */
  public void fetch(Id id, NodeHandle hint, Cache backup, Past past, Continuation command);

  /**
   * This method is call before an insert() is processed on the local node. This
   * allows applications to make a decision on whether or not to store the
   * replica. Unless you know what you are doing, don't return anything but
   * 'true' here.
   *
   * @param content The content about to be stored
   * @return Whether the insert should be allowed
   */
  public boolean allowInsert(PastContent content);

  /**
   * The default policy for Past, which fetches any available copy of a
   * replicated object and always allows inserts locally.
   *
   * @version $Id: pretty.settings 2305 2005-03-11 20:22:33Z jeffh $
   * @author Alan Mislove
   */
  public static class DefaultPastPolicy implements PastPolicy {

    /**
     * This method fetches the object via a lookup() call.
     *
     * @param id The id to fetch
     * @param hint A hint as to where the key might be
     * @param backup The backup cache, where the object *might* be located
     * @param past The local past instance
     * @param command The command to call with the replica to store
     */
    public void fetch(final Id id, final NodeHandle hint, final Cache backup, final Past past, Continuation command) {
      if ((backup != null) && backup.exists(id)) {
        backup.getObject(id, command);
      } else {
        past.lookup(id, false,
          new StandardContinuation(command) {
            public void receiveResult(Object o) {
              if (o != null) {
                parent.receiveResult(o);
              } else {
                past.lookupHandle(id, hint,
                  new StandardContinuation(parent) {
                    public void receiveResult(Object o) {
                      if (o != null) {
                        past.fetch((PastContentHandle) o, parent);
                      } else {
                        parent.receiveResult(null);
                      }
                    }
                  });
              }
            }
          });
      }
    }

    /**
     * This method always return true;
     *
     * @param content The content about to be stored
     * @return Whether the insert should be allowed
     */
    public boolean allowInsert(PastContent content) {
      return true;
    }
  }
}