哨兵的基本概念

哨兵架构Redis Sentinel是对Redis主从架构的一种升级版本

主从架构如果主节点出了问题，可能需要运维人员手工晋升新的主节点

哨兵架构则是具备自动故障发现和故障转移能力，真正实现高可用，当哨兵架构中主节点故障，从节点将会通过一些算法选举出新的主节点，并且能够对外发布谁是新的主节点

哨兵架构的java-api实现

Jedis包下面有一个JedisSentinelPool是专门针对哨兵架构的redis连接池

可以分析下其中部分源码：

构造方法

public JedisSentinelPool(String masterName, Set<HostAndPort> sentinels,
    final GenericObjectPoolConfig<Jedis> poolConfig, final JedisFactory factory,
    final JedisClientConfig sentinelClientConfig) {
  super(poolConfig, factory);
  this.poolConfig = poolConfig;
  this.factory = factory;
  this.sentinelClientConfig = sentinelClientConfig;
  HostAndPort master = initSentinels(sentinels, masterName);
  initMaster(master);
}

可以看到构造哨兵线程池时主要是initSentinels方法生效

initSentinels

private HostAndPort initSentinels(Set<HostAndPort> sentinels, final String masterName) {
  HostAndPort master = null;
  boolean sentinelAvailable = false;
  log.info("Trying to find master from available Sentinels...");
  for (HostAndPort sentinel : sentinels) {
    log.debug("Connecting to Sentinel {}", sentinel);
    try (Jedis jedis = new Jedis(sentinel, sentinelClientConfig)) {
      // 调用Jedis类下面的方法，获取当前主节点的地址
      List<String> masterAddr = jedis.sentinelGetMasterAddrByName(masterName);
      // connected to sentinel...
      sentinelAvailable = true;
      if (masterAddr == null || masterAddr.size() != 2) {
        log.warn("Can not get master addr, master name: {}. Sentinel: {}", masterName, sentinel);
        continue;
      }
      master = toHostAndPort(masterAddr);
      log.debug("Found Redis master at {}", master);
      break;
    } catch (JedisException e) {
      // resolves #1036, it should handle JedisException there's another chance
      // of raising JedisDataException
      log.warn(
          "Cannot get master address from sentinel running @ {}. Reason: {}. Trying next one.",
          sentinel, e);
    }
  }

第一部分是获取主节点地址

这里直接调用Jedis#sentinelGetMasterAddrByName 拿得到的就是当前主节点信息，不需要额外判断，因为其调用Redis中的过sentinel get-master-addr-by-name master-name 这个api，这个api是可以在故障期间提供新的可用主节点地址的

此外，这里连接池中的sentinels是一组ip、port，实际上是redis哨兵架构中所有主从的ip，即任何一个主从ip理应能获取到主节点

for (HostAndPort sentinel : sentinels) {
  MasterListener masterListener = new MasterListener(masterName, sentinel.getHost(), sentinel.getPort());
  // whether MasterListener threads are alive or not, process can be stopped
  masterListener.setDaemon(true);
  masterListeners.add(masterListener);
  masterListener.start();
}
return master;

第二部分是遍历每个哨兵节点，为每个节点单独启动了一个线程，利用redis的发布订阅功能监听当前redis哨兵节点中如果发生故障，新的主节点晋升后发布的消息，从而替换当前Jedis中指向的主节点的ip、port

MasterListener

protected class MasterListener extends Thread {
  ……
  public MasterListener(String masterName, String host, int port) {
    super(String.format("MasterListener-%s-[%s:%d]", masterName, host, port));
    this.masterName = masterName;
    this.host = host;
    this.port = port;
  }
  ……
}

可以看到MasterListener是实现自Thread类，那么看一下它的run方法里面到底写了什么

MasterListener#run

public void run() {
  running.set(true);
  while (running.get()) {
    try {
      ……
          
      final HostAndPort hostPort = new HostAndPort(host, port);
      j = new Jedis(hostPort, sentinelClientConfig);
      // code for active refresh
      List<String> masterAddr = j.sentinelGetMasterAddrByName(masterName);
      if (masterAddr == null || masterAddr.size() != 2) {
        log.warn("Can not get master addr, master name: {}. Sentinel: {}.", masterName,
            hostPort);
      } else {
        initMaster(toHostAndPort(masterAddr));
      }
      j.subscribe(new JedisPubSub() {
        @Override
        public void onMessage(String channel, String message) {
          log.debug("Sentinel {} published: {}.", hostPort, message);
          String[] switchMasterMsg = message.split(" ");
          if (switchMasterMsg.length > 3) {
            if (masterName.equals(switchMasterMsg[0])) {
              initMaster(toHostAndPort(Arrays.asList(switchMasterMsg[3], switchMasterMsg[4])));
            } else {
              log.debug(
                "Ignoring message on +switch-master for master name {}, our master name is {}",
                switchMasterMsg[0], masterName);
            }
          } else {
            log.error("Invalid message received on Sentinel {} on channel +switch-master: {}",
                hostPort, message);
          }
        }
      }, "+switch-master");
    } catch (JedisException e) {
      ……
    } finally {
      if (j != null) {
        j.close();
      }
    }
  }
}

可以看到上面还是使用List<String> masterAddr = j.sentinelGetMasterAddrByName(masterName); 获取当前的主节点

初始化完成后，指向Jedis#subscribe 进行监听

通过masterName.equals(switchMasterMsg[0]) 判断当前Jedis连接中持有的主节点信息是不是发布过来消息中的主节点，如果不是，完成主节点更新

目录CONTENT

Redis哨兵