Zookeeper 05 示例代码-主备节点切换

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主备节点的切换，是分布式应用的基本要求。现在用 Zookeeper 实现主备节点自动切换功能。

基本思路：

1 多个服务启动后，都尝试在 Zookeeper中创建一个 EPHEMERAL 类型的节点，Zookeeper本身会保证，只有一个服务会创建成功，其他服务抛出异常。

2 成功创建节点的服务，作为主节点，继续运行

3 其他服务设置一个Watcher监控节点状态，

4 如果主节点消失，其他服务会接到通知，再次尝试创建EPHEMERAL 类型的节点。

 

1. public class Master implements Watcher {

2. private static final Logger LOG = LoggerFactory.getLogger(Master.class);

3. enum MasterStates {RUNNING, ELECTED, NOTELECTED};

4. private volatile MasterStates state = MasterStates.RUNNING;

5.  

6. MasterStates getState() {

7. return state;

8. }

9.  

10. private static final int SESSION_TIMEOUT = 5000;

11. private static final String CONNECTION_STRING = "10.58.69.142:2181";

12. private static final String ZNODE_NAME = "/master";

13. private Random random = new Random(System.currentTimeMillis());

14. private ZooKeeper zk;

15. private String serverId = Integer.toHexString(random.nextInt());

16.  

17. private volatile boolean connected = false;

18. private volatile boolean expired = false;

19.  

20. public void startZk() throws IOException {

21. zk = new ZooKeeper(CONNECTION_STRING, SESSION_TIMEOUT, this);

22. }

23.  

24. public void stopZk() {

25. if (zk != null) {

26. try {

27. zk.close();

28. } catch (InterruptedException e) {

29. LOG.warn("Interrupted while closing ZooKeeper session.", e);

30. }

31. }

32. }

33.  

34. /**

35. * 抢注节点

36. */

37. public void enroll() {

38. zk.create(ZNODE_NAME,

39. serverId.getBytes(),

40. ZooDefs.Ids.OPEN_ACL_UNSAFE,

41. CreateMode.EPHEMERAL,

42. masterCreateCallBack, null);

43. }

44.  

45. AsyncCallback.StringCallback masterCreateCallBack = new AsyncCallback.StringCallback() {

46. @Override

47. public void processResult(int rc, String path, Object ctx, String name) {

48. switch (KeeperException.Code.get(rc)) {

49. case CONNECTIONLOSS:

50. //网络问题，需要检查节点是否创建成功

51. checkMaster();

52. return;

53. case OK:

54. state = MasterStates.ELECTED;

55. break;

56. case NODEEXISTS:

57. state = MasterStates.NOTELECTED;

58. // 添加Watcher

59. addMasterWatcher();

60. break;

61. default:

62. state = MasterStates.NOTELECTED;

63. LOG.error("Something went wrong when running for master.",

64. KeeperException.create(KeeperException.Code.get(rc), path));

65. }

66. LOG.info("I'm " + (state == MasterStates.ELECTED ? "" : "not ") + "the leader " + serverId);

67. }

68. };

69.  

70. public void checkMaster() {

71. zk.getData(ZNODE_NAME, false, masterCheckCallBack, null);

72. }

73.  

74. AsyncCallback.DataCallback masterCheckCallBack = new AsyncCallback.DataCallback() {

75. @Override

76. public void processResult(int rc, String path, Object ctx, byte[] data, Stat stat) {

77. switch (KeeperException.Code.get(rc)) {

78. case CONNECTIONLOSS:

79. checkMaster();

80. return;

81. case NONODE:

82. // 节点未创建，再次注册

83. enroll();

84. return;

85. case OK:

86. if (serverId.equals(new String(data))) {

87. state = MasterStates.ELECTED;

88. } else {

89. state = MasterStates.NOTELECTED;

90. addMasterWatcher();

91. }

92. break;

93. default:

94. LOG.error("Error when reading data.",KeeperException.create(KeeperException.Code.get(rc), path));

95. }

96. }

97. };

98.  

99. void addMasterWatcher() {

100. zk.exists(ZNODE_NAME,

101. masterExistsWatcher,

102. masterExistsCallback,

103. null);

104. }

105. Watcher masterExistsWatcher = new Watcher() {

106. @Override

107. public void process(WatchedEvent event) {

108. if(event.getType() == Event.EventType.NodeDeleted){

109. assert ZNODE_NAME.equals(event.getPath());

110. enroll();

111. }

112. }

113. };

114. AsyncCallback.StatCallback masterExistsCallback = new AsyncCallback.StatCallback() {

115. public void processResult(int rc, String path, Object ctx, Stat stat) {

116. switch (KeeperException.Code.get(rc)) {

117. case CONNECTIONLOSS:

118. addMasterWatcher();

119. break;

120. case OK:

121. break;

122. case NONODE:

123. state = MasterStates.RUNNING;

124. enroll();

125. LOG.info("It sounds like the previous master is gone, " +

126. "so let's run for master again.");

127. break;

128. default:

129. checkMaster();

130. break;

131. }

132. }

133. };

134.  

135. public static void main(String[] args) throws InterruptedException, IOException {

136. Master m = new Master( );

137. m.startZk();

138.  

139. while (!m.isConnected()) {

140. Thread.sleep(100);

141. }

142. m.enroll();

143. while (!m.isExpired()) {

144. Thread.sleep(1000);

145. }

146.  

147. m.stopZk();

148. }

149. boolean isConnected() {

150. return connected;

151. }

152.  

153. boolean isExpired() {

154. return expired;

155. }

156.  

157. @Override

158. public void process(WatchedEvent e) {

159. LOG.info("Processing event: " + e.toString());

160. if (e.getType() == Event.EventType.None) {

161. switch (e.getState()) {

162. case SyncConnected:

163. connected = true;

164. break;

165. case Disconnected:

166. connected = false;

167. break;

168. case Expired:

169. expired = true;

170. connected = false;

171. LOG.error("Session expiration");

172. default:

173. break;

174. }

175. }

176. }

177.