在Redis主从复制架构中，如果master出现了故障，则需要人工将slave提升为master，同时，通知应用侧更新master的地址。这样方式比较低效，对应用侧影响较大。
为了解决这个问题，Redis 2.8中推出了自己的高可用方案Redis Sentinel。
Redis Sentinel架构图如下：
默认情况下，每个Sentinel节点会以每秒一次的频率对Redis节点和其它的Sentinel节点发送PING命令，并通过节点的回复来判断节点是否在线。
如果在down-after-millisecondes毫秒内，没有收到有效的回复，则会判定该节点为主观下线。
如果该节点为master，则该Sentinel节点会通过sentinel is-master-down-by-addr命令向其它sentinel节点询问对该节点的判断，如果超过<quorum>个数的节点判定master不可达，则该sentinel节点会将master判断为客观下线。
这个时候，各个Sentinel会进行协商，选举出一个领头Sentinel，由该领头Sentinel对master节点进行故障转移操作。
故障转移包含如下三个操作：
1. 在所有的slave服务器中，挑选出一个slave，并将其转换为master。
2. 让其它slave服务器，改为复制新的master。
3. 将旧master设置为新master的slave，这样，当旧的master重新上线时，它会成为新master的slave。
以上的所有操作对业务都是透明的，当新的master上线后，Sentinel会自动将这个变化实时通知给业务方。
那么，业务侧又该如何配置，才能扑捉到这个变化呢？
其实，这个主要取决于Redis客户端工具是否支持Redis Sentinel，对于支持的客户端工具来说，如Jedis，
只需将连接字符串设置为Sentinel地址即可。
下面，给出了一个测试代码，并模拟了master发生故障，业务侧是如何处理的？
代码如下：
package com.victor_02;
import Java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;
import org.apache.commons.pool2.impl.GenericObjectPoolConfig;
import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisSentinelPool;
public class JedisSentinelTest {
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
Set<String> sentinels = new HashSet<String>();
sentinels.add("192.168.244.10:26379");
sentinels.add("192.168.244.10:26380");
sentinels.add("192.168.244.10:26381");
JedisSentinelPool jedisSentinelPool = new JedisSentinelPool("mymaster", sentinels);
Jedis jedis = null;
while (true) {
Thread.sleep(1000);
try {
jedis = jedisSentinelPool.getResource();
Date now = new Date();
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
String format_now = dateFormat.format(now);
jedis.set("hello", "world");
String value = jedis.get("hello");
System.out.println(format_now + ' ' + value);
} catch (Exception e) {
System.out.println(e);
} finally {
if (jedis != null)
try {
jedis.close();
} catch (Exception e) {
System.out.println(e);
}
}
}
}
}
模拟故障：
# ./redis-cli -p 6380 127.0.0.1:6380> shutdown
上述代码的输出如下：
四月 16, 2017 10:39:44 下午 redis.clients.jedis.JedisSentinelPool initSentinels 信息: Trying to find master from available Sentinels... 四月 16, 2017 10:39:44 下午 redis.clients.jedis.JedisSentinelPool initSentinels 信息: Redis master running at 192.168.244.10:6380, starting Sentinel listeners... 四月 16, 2017 10:39:44 下午 redis.clients.jedis.JedisSentinelPool initPool 信息: Created JedisPool to master at 192.168.244.10:6380 2017/04/16 22:39:45 world 2017/04/16 22:39:46 world 2017/04/16 22:39:47 world 2017/04/16 22:39:48 world 2017/04/16 22:39:49 world 2017/04/16 22:39:50 world redis.clients.jedis.exceptions.JedisConnectionException: java.net.SocketException: Software caused connection abort: recv failed redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool 四月 16, 2017 10:40:21 下午 redis.clients.jedis.JedisSentinelPool initPool 信息: Created JedisPool to master at 192.168.244.10:6381 四月 16, 2017 10:40:21 下午 redis.clients.jedis.JedisSentinelPool initPool 信息: Created JedisPool to master at 192.168.244.10:6381 redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool 2017/04/16 22:40:22 world 2017/04/16 22:40:23 world 2017/04/16 22:40:24 world 2017/04/16 22:40:25 world 2017/04/16 22:40:26 world
从上述输出可以看出，在master发生故障前，业务侧最后一次正常处理（22:39:50），到再次正常处理是（22:40:22），中间经过了32s。
而其中30s被用来判断master节点是否主观下线（由down-after-milliseconds来指定），整个切换的过程还是比较高效的。
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