Mercurial > hg > blitz_stable

annotate src/org/prevayler/implementation/BufferingPrevaylerImpl.java @ 27:511648fa4d64 Version 2.1

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Version to 2.1
author Dan Creswell <dan.creswell@gmail.com>
date Mon, 04 Jan 2010 13:00:40 +0000
parents 3dc0c5604566
children
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0
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1 package org.prevayler.implementation;
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2
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3 import java.util.LinkedList;
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4 import java.util.Date;
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5 import java.io.*;
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6
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7 import org.prevayler.*;
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8
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9 import org.dancres.blitz.ActiveObject;
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10 import org.dancres.blitz.ActiveObjectRegistry;
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11
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12 /**
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13 <p>Prevayler which guarentees that all logged changes will be made
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14 persistent within a certain fixed amount of time - e.g. after 5 seconds.
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15 When a snapshot is requested, any logged changes will be flushed to disk and
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16 then a snapshot will be prepared. This provides correct recovery should
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17 we crash before we save the snapshot (which could be a while) but after
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18 we've flushed the log.</p>
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19
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20 <p>This Prevayler is designed to work in co-operation with Blitz's
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21 OpCountingCheckpointTrigger and the ActiveObjectRegistry.</p>
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22
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23 @see org.dancres.blitz.txn.OpCountingCheckpointTrigger
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24 @see org.dancres.blitz.ActiveObjectRegistry
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25 */
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26 public class BufferingPrevaylerImpl implements SnapshotPrevayler {
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27
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28 private final PrevalentSystem system;
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29 private final SystemClock clock;
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30 private final CommandOutputStream output;
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31
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32 private LinkedList theBuffer = new LinkedList();
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33
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34 /** Returns a new Prevayler for the given PrevalentSystem.
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35 * "PrevalenceBase" shall be the directory where the snapshot and log files shall be created and read.
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36 * @param newSystem The newly started, "empty" PrevalentSystem that will be used as a starting point for every system startup, until the first snapshot is taken.
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37 * @param shouldReset Whether to issue resets in the underlying OOS
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38 */
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39 public BufferingPrevaylerImpl(PrevalentSystem newSystem,
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40 boolean shouldReset, boolean shouldClean,
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41 int aBufferSize)
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42 throws IOException, ClassNotFoundException {
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43
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44 this(newSystem, "PrevalenceBase", shouldReset, shouldClean,
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45 aBufferSize);
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46 }
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47
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48
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49 /** Returns a new Prevayler for the given PrevalentSystem.
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50 * @param newSystem The newly started, "empty" PrevalentSystem that will be used as a starting point for every system startup, until the first snapshot is taken.
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51 * @param directory The full path of the directory where the snapshot and log files shall be created and read.
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52 * @param shouldReset Whether to issue resets in the underlying OOS
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53 */
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54 public BufferingPrevaylerImpl(PrevalentSystem newSystem, String directory,
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55 boolean shouldReset, boolean shouldClean,
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56 int aBufferSize)
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57 throws IOException, ClassNotFoundException {
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58
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59 newSystem.clock(new SystemClock());
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60 CommandInputStream input = new CommandInputStream(directory);
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61
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62 PrevalentSystem savedSystem = input.readLastSnapshot();
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63 system = (savedSystem == null)
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64 ? newSystem
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65 : savedSystem;
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66
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67 recoverCommands(input);
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68
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69 output = input.commandOutputStream(shouldReset, shouldClean,
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70 aBufferSize);
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71 clock = (SystemClock)system.clock();
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72 clock.resume();
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73 }
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74
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75
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76 /** Returns the underlying PrevalentSystem.
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77 */
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78 public PrevalentSystem system() {
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79 return system;
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80 }
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81
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82 /** Logs the received command for crash or shutdown recovery and executes it on the underlying PrevalentSystem.
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83 * @see system()
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84 * @return The serializable object that was returned by the execution of command.
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85 * @throws IOException if there is trouble writing the command to the log.
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86 * @throws Exception if command.execute() throws an exception.
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87 */
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88 public synchronized Serializable executeCommand(Command command) throws Exception {
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89 clock.pause(); //To be deterministic, the system must know exactly at what time the command is being executed.
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90 try {
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91 theBuffer.add(new ClockRecoveryCommand(command, clock.time()));
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92
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93 return command.execute(system);
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94
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95 } finally {
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96 clock.resume();
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97 }
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98 }
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99
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100 public synchronized Serializable executeCommand(Command command,
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101 boolean doSync)
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102 throws Exception {
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103
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104 clock.pause();
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105 try {
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106 theBuffer.add(new ClockRecoveryCommand(command, clock.time()));
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107
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108 return command.execute(system);
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109
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110 } finally {
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111 clock.resume();
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112 }
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113 }
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114
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115 /**
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116 * This method prepares a snapshot of the system and returns it in a
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117 * Snapshotter instance which can be used to save the snapshot to disk
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118 * once dirty state has been sync'd to disk. If your application has no
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119 * additional state, you can simply invoke on the Snapshotter immediately.
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120 * @return Snapshotter to be used to save an appropriate snapshot post
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121 * sync'ing of dirty state to disk.
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122 * @see system()
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123 * @throws IOException if there is trouble preparing the snapshot file.
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124 */
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125 public synchronized Snapshotter takeSnapshot() throws IOException {
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126 clock.pause();
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127 try {
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128 flush();
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129
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130 return output.writeSnapshot(system);
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131 } finally {
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132 clock.resume();
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133 }
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134 }
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136
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137 private synchronized void flush() throws IOException {
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138 while (! theBuffer.isEmpty()) {
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139 Command myCommand = (Command) theBuffer.remove(0);
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140
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141 output.writeCommand(myCommand, theBuffer.isEmpty());
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142 }
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143 }
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144
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145 private void recoverCommands(CommandInputStream input) throws IOException, ClassNotFoundException {
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146 Command command;
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147 while(true) {
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148 try {
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149 command = input.readCommand();
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150 } catch (EOFException eof) {
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151 break;
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152 }
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153
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154 try {
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155 command.execute(system);
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156 } catch (Exception e) {
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157 //Don't do anything at all. Commands may throw exceptions normally.
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158 System.err.println("Command threw exception");
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159 e.printStackTrace(System.err);
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160 }
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161 }
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162 }
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163 }
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164