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1   /**
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10   *     http://www.apache.org/licenses/LICENSE-2.0
11   *
12   * Unless required by applicable law or agreed to in writing, software
13   * distributed under the License is distributed on an "AS IS" BASIS,
14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15   * See the License for the specific language governing permissions and
16   * limitations under the License.
17   */
18  package org.apache.hadoop.hbase.errorhandling;
19  
20  import java.util.ArrayList;
21  import java.util.List;
22  
23  import org.apache.commons.logging.Log;
24  import org.apache.commons.logging.LogFactory;
25  import org.apache.hadoop.classification.InterfaceAudience;
26  
27  /**
28   * The dispatcher acts as the state holding entity for foreign error handling.  The first
29   * exception received by the dispatcher get passed directly to the listeners.  Subsequent
30   * exceptions are dropped.
31   * <p>
32   * If there are multiple dispatchers that are all in the same foreign exception monitoring group,
33   * ideally all these monitors are "peers" -- any error on one dispatcher should get propagated to
34   * all others (via rpc, or some other mechanism).  Due to racing error conditions the exact reason
35   * for failure may be different on different peers, but the fact that they are in error state
36   * should eventually hold on all.
37   * <p>
38   * This is thread-safe and must be because this is expected to be used to propagate exceptions
39   * from foreign threads.
40   */
41  @InterfaceAudience.Private
42  public class ForeignExceptionDispatcher implements ForeignExceptionListener, ForeignExceptionSnare {
43    public static final Log LOG = LogFactory.getLog(ForeignExceptionDispatcher.class);
44    protected final String name;
45    protected final List<ForeignExceptionListener> listeners =
46        new ArrayList<ForeignExceptionListener>();
47    private ForeignException exception;
48  
49    public ForeignExceptionDispatcher(String name) {
50      this.name = name;
51    }
52  
53    public ForeignExceptionDispatcher() {
54      this("");
55    }
56  
57    public String getName() {
58      return name;
59    }
60  
61    @Override
62    public synchronized void receive(ForeignException e) {
63      // if we already have an exception, then ignore it
64      if (exception != null) return;
65  
66      LOG.debug(name + " accepting received exception" , e);
67      // mark that we got the error
68      if (e != null) {
69        exception = e;
70      } else {
71        exception = new ForeignException(name, "");
72      }
73  
74      // notify all the listeners
75      dispatch(e);
76    }
77  
78    @Override
79    public synchronized void rethrowException() throws ForeignException {
80      if (exception != null) {
81        // This gets the stack where this is caused, (instead of where it was deserialized).
82        // This is much more useful for debugging
83        throw new ForeignException(exception.getSource(), exception.getCause());
84      }
85    }
86  
87    @Override
88    public synchronized boolean hasException() {
89      return exception != null;
90    }
91  
92    @Override
93    synchronized public ForeignException getException() {
94      return exception;
95    }
96  
97    /**
98     * Sends an exception to all listeners.
99     * @param message human readable message passed to the listener
100    * @param e {@link ForeignException} containing the cause.  Can be null.
101    */
102   private void dispatch(ForeignException e) {
103     // update all the listeners with the passed error
104     for (ForeignExceptionListener l: listeners) {
105       l.receive(e);
106     }
107   }
108 
109   /**
110    * Listen for failures to a given process.  This method should only be used during
111    * initialization and not added to after exceptions are accepted.
112    * @param errorable listener for the errors.  may be null.
113    */
114   public synchronized void addListener(ForeignExceptionListener errorable) {
115     this.listeners.add(errorable);
116   }
117 }