FailoverArbiter


1   package com.ubermq.jms.server.journal.impl;
2   
3   import com.ubermq.jms.server.journal.*;
4   import com.ubermq.jms.common.routing.impl.*;
5   import com.ubermq.kernel.*;
6   import java.io.*;
7   import java.util.*;
8   
9   /**
10   * This arbiter implementation supports failover,
11   * also known as hot/standby or active/backup
12   * deployment topologies. <P>
13   *
14   * The arbiter works such that the first connection
15   * receives all messages until that connection
16   * disconnects or fail, at which point the next most-
17   * recently connected output node will begin
18   * to receive messages.
19   */
20  public class FailoverArbiter
21      implements DurableConnectionArbiter
22  {
23      private transient DatagramSink active;
24      private transient LinkedList cxns;
25  
26      public static final long serialVersionUID = 1L;
27  
28      /**
29       * Constructs a failover arbiter.
30       */
31      public FailoverArbiter()
32      {
33          cxns = new LinkedList();
34      }
35  
36      private void readObject(ObjectInputStream ois)
37          throws IOException, ClassNotFoundException  
38      {
39          ois.defaultReadObject();
40          cxns = new LinkedList();
41      }
42  
43      /**
44       * Switches the active connection to the next one
45       * from the list of connections, if any,
46       * and recovers the durable subscriber to re-output
47       * unacknowledged messages.
48       */
49      private void nextConnection()
50      {
51          if (cxns.size() > 0) {
52              active = (DatagramSink)cxns.removeFirst();
53          } else {
54              active = null;
55          }
56      }
57  
58      /**
59       * Outputs a datagram. If the datagram causes an overflow,
60       * the given overflow handler is called to determine whether the
61       * operation should be retried.
62       * @param d a datagram
63       * @param h the overflow handler to use
64       */
65      public void output(IDatagram d,
66                         IOverflowHandler h)
67          throws IOException
68      {
69          if (isOpen()) {
70              try {
71                  active.output(d, h);
72              } catch(IOException iox) {
73                  // move to next connection if we
74                  // fail here.
75                  nextConnection();
76                  throw iox;
77              }
78          }
79          else
80              throw new IOException("not open");
81      }
82  
83      /**
84       * Determines whether the destination node is available for output.
85       * @return true if the output dest node is available
86       */
87      public boolean isOpen()
88      {
89          return (active != null);
90      }
91  
92      /**
93       * Disconnects an output node from the arbiter.
94       */
95      public void disconnect(DatagramSink cdn)
96      {
97          if (cdn.equals(active)) {
98              nextConnection();
99          } else {
100             cxns.remove(cdn);
101         }
102     }
103 
104     /**
105      * Disconnects all output nodes from the arbiter.
106      */
107     public void disconnectAll()
108     {
109         active = null;
110         cxns.clear();
111     }
112 
113     /**
114      * Connects a new output destination node to the
115      * arbiter.
116      */
117     public void connect(DatagramSink cdn)
118     {
119         if (active == null)
120             active = cdn;
121         else {
122             cxns.addLast(cdn);
123         }
124     }
125 
126     public String   toString()
127     {
128         return "Active Proxy: " + active + " Backups: " + cxns;
129     }
130 
131     public String   toHtml()
132     {
133         if (active != null)
134         {
135             StringBuffer   sb = new StringBuffer  ();
136             sb.append(AbstractMultipleArbiter.getDisplayName(active));
137             sb.append(" <I>(Active)</I><br>");
138 
139             Iterator iter = cxns.iterator();
140             while (iter.hasNext())
141             {
142                 sb.append(AbstractMultipleArbiter.getDisplayName((DatagramSink)iter.next()));
143                 sb.append("<br>");
144             }
145 
146             return sb.toString();
147         }
148         else
149         {
150             return "<b>Disconnected</b>";
151         }
152     }
153 }
154
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