聊聊Election Algorithms

序

本文主要研究一下Election Algorithms

Election Algorithms

Election Algorithms大体有两类，一类是Garcia-Molina提出的Bully Election，一类是Chang & Roberts's Token Ring Election algorithm；对于大多数的election algorithms一般有以下几个假定：

• 完整的topology，信息能够在topology的nodes之间传递
• 每一个node有惟一的id，并且对整个topology其余node可见
• 全部的通讯网络是可靠的，只容许node fail，要求消息不丢失，不重复，不损坏
• 要求已经有fail detector机制来处理node fail的状况

Bully Election

• 当有node检测到leader fail以后，就发送election request给其余node，election request中带上本身的id
• 当node接收到election request时判断若是本身的id大于request中的id，则能够"bully"覆盖request中的id，若是小于则不改动，而后发送election request给其余node；当有node接收到election request的id是本身的node id时，则代表本身是leader，因而给其余node发送leader request
• 当node接收到leader request时设置本地leader id，同时判断若是leader id不是本身的node id时则转发该leader request给其余node

Token Ring Election

• 当有node检测到leader fail以后，就发送election request给其余node，election request中带上本身的id
• 当node接收到election request时，则判断本身的node id是否在里面，不在的话则追加本身的node id到election request中；若是本身的node id已经在该election request中时则提取这些node id，取出id最大的做为leader，而后给其余node发送leader request
• 当node接收到leader request时设置本地leader id，同时判断若是leader id不是本身的node id时则转发该leader request给其余node

实例

这里采用distributedLeaderElection的实现

Bully Election

public void onMessage(String message) {
		
		String messageHeader = message.split(":")[0];
		String messageBody = message.split(":")[1];
		
		if (messageHeader.equals("Election")){
			if (Integer.parseInt(messageBody.trim()) < Node.getMyID() // If we are a better candidate
					&& !participant){
				System.out.println("I " + Node.getMyID() + " am a better candidate than "+messageBody);
				Node.sendMsgToNextNode("Election" + ":" + Node.getMyID()); // Suggest us for election
			}
			else if (Integer.parseInt(messageBody.trim()) == Node.getMyID())  { // If this is our ID
				System.out.println("I " + Node.getMyID() + " received my own pebble, so I am the new leader");
				Node.sendMsgToNextNode("Leader" + ":" + Node.getMyID()); // Announce us as the new leader
			}
			else { // The current candidate is better
				System.out.println("I " + Node.getMyID() + " am a worse candidate than "+messageBody);
				Node.sendMsgToNextNode(message); // Forward his candidancy
			}
			participant = true;
		}
		
		else if (messageHeader.equals("Leader")){
			System.out.println(messageBody + " is the new leader");
			leaderID = messageBody;
			if (Integer.parseInt(messageBody.trim()) != Node.getMyID()) Node.sendMsgToNextNode(message);
		}
	}

• 能够看到bully算法在看到election request中node id小于本身node id时，直接bully覆盖该node id；当走了一圈发现请求中node id是本身的node id时，则选举本身为leader

Token Ring Election

public void onMessage(String message) {

		String messageHeader = message.split(":")[0];
		List<String> messageBody = Arrays.asList(message.replace(messageHeader+":", "").split(":"));
		
		if (messageHeader.equals("Election")){
			if (!messageBody.contains(Node.getMyID()+"")){ // If we are not contained in the list
				System.out.println("I " + Node.getMyID() + " am not contained in this message "+message);
				Node.sendMsgToNextNode(message + ":" + Node.getMyID()); // Suggest us for election
			}
			else { // If we are in the list
				System.out.println("I " + Node.getMyID() + " am contained in this message");
				String newLeader = findLeaderInBody(messageBody);
				Node.sendMsgToNextNode("Leader" + ":" + newLeader); // Announce the new leader
			}
		}
		
		else if (messageHeader.equals("Leader")){
			String leaderBody = message.split(":")[1];
			System.out.println(leaderBody + " is the new leader");
			leaderID = leaderBody;
			if (Integer.parseInt(leaderBody.trim()) != Node.getMyID()) Node.sendMsgToNextNode(message);
		}
	}

	private String findLeaderInBody(List<String> messageBody) {
		int maxID = 0;
		if (messageBody.size() > 0){
			for (String leaderCandidate : messageBody){
				if (Integer.parseInt(leaderCandidate.trim()) > maxID) {
					maxID = Integer.parseInt(leaderCandidate.trim());
				}
			}
		}
		return maxID+"";
	}

• 能够看到ring算法是在请求中追加本身的node id；当走了一圈发现本身的node id已经在其中时，经过findLeaderInBody从这些node id中取出最大的那个，选举该node为leader

小结

• Election Algorithms大体有两类，一类是Garcia-Molina提出的Bully Election，一类是Chang & Roberts's Token Ring Election algorithm
• 对于大多数的election algorithms一般有以下几个假定：
  • 完整的topology，信息能够在topology的nodes之间传递
  • 每一个node有惟一的id，并且对整个topology其余node可见
  • 全部的通讯网络是可靠的，只容许node fail，要求消息不丢失，不重复，不损坏
  • 要求已经有fail detector机制来处理node fail的状况
• bully算法与ring算法的共同点是对比node id，在具体的实例中咱们能够看到，bully算法顾名思义就是若是本身的node id比较大，则能够覆盖request中的node，最后node id最大的为leader；而ring算法则是采起追加node id方式，最后在从中选取node id最大的为leader

doc

• Election Algorithms
• The Bully Election Algorithm
• Bully Election Algorithm Example
• A Token Ring Election Algorithm
• Token Ring Election Algorithm Example
• distributedLeaderElection