分布式选主 -- 利用Mysql ACID和Lease协议实现选主和高可用
在实际生产开发中，遇到一些多节点共存，需要选主，并且要实现HA自动容错的场景，思考了写方法拿出来和大家分享一下。
- Lease协议，Mysql ACID
- 高可用选主方案设计
- 适用场景
- Java语言实现描述
- 进一步优化
系统中有很多应用场景要类似主从架构，主服务器(Master)对外提供服务，从服务器(Salve)热备份，不提供服务但随时活着，如果Master出现宕机或者网络问题，Slave即可接替Master对外服务，并由Slave提升为Master(新主)。典型的多节点共存，但只能同时存在一个主，并且所有节点的状态能统一维护。
大家一定首先想到了著名的Paxos算法(http://baike.baidu.com/view/8438269.htm)。简单的说，Paxos通过每个节点的投票算法，来决议一个事情，当多余1/2个节点都投票通过时，Paxos产生一个唯一结果的决议，并通知各个节点维护这个信息。例如Paxos的选主，首先产生一个关于某个节点希望当Master的投票，然后各个节点给出反馈，最终Paxos集群维护唯一的Master的结论。Zookeeper就是Paxos的一种实现。这种场景最适合用zookeeper来选主，但zookeeper有个明显的缺点，当存活的节点小于zookeeper集群的1/2时，就不能工作了。比如zk有10各节点，那么必须满足可用的节点大于5才可。
在实际环境中，如果对Master要求不是那么严格的话，可以通过某些改进和取舍来达到目的。比如可能在秒级别允许Master暂时不能访问、选主时间内可能存在一定的冲突但通过再次选主即可。本人设计了一个简易的利用Mysql一致性和简易版Lease来workaround。
Mysql ACID保证了一条数据记录的一致性、完整性，不会出现多进程读写的一致性问题和唯一正确性。Lease协议(协议细节可以Google之)通过向Master发送一个lease(租期)包，Master在这个lease期之内充当主角色，如果lease期到了则再次去申请lease，如果lease期到了，但是网络除了问题，这时Master可以i主动下线，让其他节点去竞选Master。举个例子，三个节点A、B、C经过第一轮选主之后，A成为Master，它获得了10秒的lease，当前时间假设是00:00:00，那么它Master地位可以用到00:00:10，当时间到达00:00:10时，A、B、C会重新进行Master选举，每个节点都有可能成为Master(从工程的角度触发，A继续为Master的概率更大)，如果这时候A的网络断了，不能联通B、C的集群了，那么A会自动下线，不会去竞争，这样就不会出现“脑裂”的现象。
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设计方案如下：(server代表集群中的一台机器，也可看作一个进程，server之间是平等的)
- 各个server之间用ntpserver时间同步(保证服务器之间秒级同步即可)
- 各个server持有一个唯一ID号(ip+进程号)，通过此id唯一标识一个server实例
- 各个server定义一个lease租期，单位为秒
- Mysql唯一表唯一一条记录维护全局Master的信息，ACID保证一致性
- Master Server每半个lease期向Mysql更新如上的唯一一条记录，并更新心跳，维护Master状态
- Slaver Server每半个lease周期从mysql获取Master Server信息，如果数据库中Master的Lease超过了当前时间(heartbeat_time+ lease > current_time)，则申请当Master。
这其中比较棘手的问题是：
1、由于数据库访问和休眠的时间(lease的一半)，有时延的存在，要处理Mysql异常、网络异常。
2、可能存在同时抢占Master的server，这个时候就需要一个验证机制保证为抢到Master的server自动退位为Slaver
下面给出图实例 ：（10.0.0.1为Master）
10.0.0.1 crash了。mysql中维护的10.0.0.1的主信息已过期，其他节点去抢占
各个节点再次读取数据库，查看是否是自己抢占成功了：
之后，10.0.0.3作为Master对外服务。此时如果10.0.0.1重启，可作为Slaver。如果10.0.0.1因为网络分化或者网络异常而不能维护心跳，则在超过自身lease时自动停止服务，不会出现“双Master”的现象。
每个Server遵循如下流程：
数据库设计：
某一时刻，数据库中Master的信息：
当前时间： 45分15秒
当前Master Lease ：6秒
当前Master Lease可用到： 45分21秒
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3、适用的场景
一、生命周期内可使用Mysql、并且各个server之间时间同步。
二、需要集群中选出唯一主对外提供服务，其他节点作为slaver做standby，主lease过期时竞争为Master
三、对比zookeeper，可满足如果集群挂掉一半节点，也可正常工作的情况，比如只有一主一备。
四、允许选主操作在秒级容错的系统，选主的时候可能有lease/2秒的时间窗口，此时服务可能不可用。
五、允许lease/2秒内出现极限双Master情况，但是概率很小。
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4、Java语言实现描述
一些配置信息和时间相关、休眠周期相关的时间变量
final long interval = lease / intervalDivisor;
long waitForLeaseChallenging = 0L;
lease = lease / 1000L;
long challengeFailTimes = 0L;
long takeRest = 0L;
long dbExceptionTimes = 0L;
long offlineTime = 0L;
Random rand = new Random();
Status stateMechine = Status.START;
long activeNodeLease = 0L;
long activeNodeTimeStamp = 0L;
数据库异常的处理：
KeepAlive keepaliveNode = null;
try {
/* first of all get it from mysql */
keepaliveNode = dbService.accquireAliveNode();
if (stateMechine != Status.START && keepaliveNode==null)
throw new Exception();
// recount , avoid network shake
dbExceptionTimes = 0L;
} catch (Exception e) {
log.fatal("[Scanner] Database Exception with times : " + dbExceptionTimes++);
if (stateMechine == Status.OFFLINE) {
log.warn("[Scanner] Database Exception , OFFLINE ");
} else if (dbExceptionTimes >= 3) {
log.fatal("[Scanner] Database Exception , Node Offline Mode Active , uniqueid : " + uniqueID);
stateMechine = Status.OFFLINE;
dbExceptionTimes = 0L;
offlineTime = System.currentTimeMillis();
online = false;
} else
continue;
}
总的循环和状态机的变迁：
while (true) {
SqlSession session = dbConnecction.openSession();
ActionScanMapper dbService = session.getMapper(ActionScanMapper.class);
KeepAlive keepaliveNode = null;
try {
/* first of all get it from mysql */
keepaliveNode = dbService.accquireAliveNode();
if (stateMechine != Status.START && keepaliveNode==null)
throw new Exception();
// recount , avoid network shake
dbExceptionTimes = 0L;
} catch (Exception e) {
log.fatal("[Scanner] Database Exception with times : " + dbExceptionTimes++);
if (stateMechine == Status.OFFLINE) {
log.warn("[Scanner] Database Exception , OFFLINE ");
} else if (dbExceptionTimes >= 3) {
log.fatal("[Scanner] Database Exception , Node Offline Mode Active , uniqueid : " + uniqueID);
stateMechine = Status.OFFLINE;
dbExceptionTimes = 0L;
offlineTime = System.currentTimeMillis();
online = false;
} else
continue;
}
try {
activeNodeLease = keepaliveNode!=null ? keepaliveNode.getLease() : activeNodeLease;
activeNodeTimeStamp = keepaliveNode!=null ? keepaliveNode.getTimestamp() : activeNodeTimeStamp;
takeRest = interval;
switch (stateMechine) {
case START:
if (keepaliveNode == null) {
log.fatal("[START] Accquire node is null , ignore ");
// if no node register here , we challenge it
stateMechine = Status.CHALLENGE_REGISTER;
takeRest = 0;
} else {
// check the lease , wether myself or others
if (activeNodeLease < timestampGap(activeNodeTimeStamp)) {
log.warn("[START] Lease Timeout scanner for uniqueid : " + uniqueID + ", timeout : "
+ timestampGap(activeNodeTimeStamp));
if (keepaliveNode.getStatus().equals(STAT_CHALLENGE))
stateMechine = Status.HEARTBEAT;
else {
stateMechine = Status.CHALLENGE_MASTER;
takeRest = 0;
}
} else if (keepaliveNode.getUniqueID().equals(uniqueID)) {
// I'am restart
log.info("[START] Restart Scanner for uniqueid : " + uniqueID
+ ", timeout : " + timestampGap(activeNodeTimeStamp));
stateMechine = Status.HEARTBEAT;
} else {
log.info("[START] Already Exist Keepalive Node with uniqueid : " + uniqueID);
stateMechine = Status.HEARTBEAT;
}
}
break;
case HEARTBEAT:
/* uniqueID == keepaliveNode.uniqueID */
if (keepaliveNode.getUniqueID().equals(uniqueID)) {
if (activeNodeLease < timestampGap(activeNodeTimeStamp)) {
// we should challenge now , without nessesary to checkout Status[CHALLENGE]
log.warn("[HEARTBEAT] HEART BEAT Lease is timeout for uniqueid : " + uniqueID
+ ", time : " + timestampGap(activeNodeTimeStamp));
stateMechine = Status.CHALLENGE_MASTER;
takeRest = 0;
break;
} else {
// lease ok , just update mysql keepalive status
dbService.updateAliveNode(keepaliveNode.setLease(lease));
online = true;
log.info("[HEARTBEAT] update equaled keepalive node , uniqueid : " + uniqueID
+ ", lease : " + lease + "s, remain_usable : " +
((activeNodeTimeStamp * 1000L + lease * 1000L) - System.currentTimeMillis()) + " ms");
}
} else {
/* It's others , let's check lease */
if (activeNodeLease < timestampGap(activeNodeTimeStamp)) {
if (keepaliveNode.getStatus().equals(STAT_CHALLENGE)) {
waitForLeaseChallenging = (long) (activeNodeLease * awaitFactor);
if ((waitForLeaseChallenging) < timestampGap(activeNodeTimeStamp)) {
log.info("[HEARTBEAT] Lease Expired , Diff[" + timestampGap(activeNodeTimeStamp) + "] , Lease[" + activeNodeLease + "]");
stateMechine = Status.CHALLENGE_MASTER;
takeRest = 0;
} else {
log.info("[HEARTBEAT] Other Node Challenging , We wait for a moment ...");
}
} else {
log.info("[HEARTBEAT] Lease Expired , Diff[" + timestampGap(activeNodeTimeStamp) + "] , lease[" + activeNodeLease + "]");
stateMechine = Status.CHALLENGE_MASTER;
takeRest = 0;
}
} else {
online = false;
log.info("[HEARTBEAT] Exist Active Node On The Way with uniqueid : "
+ keepaliveNode.getUniqueID() + ", lease : " + keepaliveNode.getLease());
}
}
break;
case CHALLENGE_MASTER:
dbService.challengeAliveNode(new KeepAlive().setUniqueID(uniqueID).setLease(lease));
online = false;
// wait for the expired node offline automatic
// and others also have changce to challenge
takeRest = activeNodeLease;
stateMechine = Status.CHALLENGE_COMPLETE;
log.info("[CHALLENGE_MASTER] Other Node is timeout["
+ timestampGap(activeNodeTimeStamp) + "s] , I challenge with uniqueid : " + uniqueID
+ ", lease : " + lease + ", wait : " + lease);
break;
case CHALLENGE_REGISTER:
dbService.registerNewNode(new KeepAlive().setUniqueID(uniqueID).setLease(lease));
online = false;
// wait for the expired node offline automatic
// and others also have changce to challenge
takeRest = activeNodeLease;
stateMechine = Status.CHALLENGE_COMPLETE;
log.info("[CHALLENGE_REGISTER] Regiter Keepalive uniqueid : " + uniqueID + ", lease : " + lease);
break;
case CHALLENGE_COMPLETE :
if (keepaliveNode.getUniqueID().equals(uniqueID)) {
dbService.updateAliveNode(keepaliveNode.setLease(lease));
online = true;
log.info("[CHALLENGE_COMPLETE] I Will be the Master uniqueid : " + uniqueID);
// make the uptime correct
stateMechine = Status.HEARTBEAT;
} else {
online = false;
log.warn("[CHALLENGE_COMPLETE] So unlucky , Challenge Failed By Other Node with uniqueid : " + keepaliveNode.getUniqueID());
if (challengeFailTimes++ >= (rand.nextLong() % maxChallenge) + minChallenge) {
// need't challenge anymore in a long time
takeRest=maxChallengeAwaitInterval;
stateMechine = Status.HEARTBEAT;
challengeFailTimes = 0L;
log.info("[CHALLENGE_COMPLETE] Challenge Try Times Used Up , let's take a long rest !");
} else {
stateMechine = Status.HEARTBEAT;
log.info("[CHALLENGE_COMPLETE] Challenge Times : " + challengeFailTimes + ", Never Give Up , to[" + stateMechine + "]");
}
}
break;
case OFFLINE :
log.fatal("[Scanner] Offline Mode Node with uniqueid : " + uniqueID);
if (System.currentTimeMillis() - offlineTime >= maxOfflineFrozen) {
// I am relive forcely
log.info("[Scanner] I am relive to activie node , uniqueid : " + uniqueID);
stateMechine = Status.HEARTBEAT;
offlineTime = 0L;
} else if (keepaliveNode != null) {
// db is reconnected
stateMechine = Status.HEARTBEAT;
offlineTime = 0L;
log.info("[Scanner] I am relive to activie node , uniqueid : " + uniqueID);
}
break;
default :
System.exit(0);
}
session.commit();
session.close();
if (takeRest != 0)
Thread.sleep(takeRest);
log.info("[Scanner] State Stage [" + stateMechine + "]");
} catch (InterruptedException e) {
log.fatal("[System] Thread InterruptedException : " + e.getMessage());
} finally {
log.info("[Scanner] UniqueID : " + uniqueID + ", Mode : " + (online?"online":"offline"));
}
}
}
enum Status {
START, HEARTBEAT, CHALLENGE_MASTER, CHALLENGE_REGISTER, CHALLENGE_COMPLETE, OFFLINE
}