基于Redis Sentinel主从切换以及Sharding分片的Redis集群高可用方案

本文主要介绍一种通过Jedis&Sentinel实现Redis集群高可用方案，该方案需要使用Jedis2.2.2及以上版本（强制），Redis2.8及以上版本(可选，Sentinel最早出现在Redis2.4中，Redis2.8中Sentinel更加稳定)，Redis集群是以分片(Sharding)加主从的方式搭建，满足可扩展性的要求；

 

Redis Sentinel介绍

Redis Sentinel是Redis官方提供的集群管理工具，主要有三大功能： 

监控，能持续监控Redis的主从实例是否正常工作； 

通知，当被监控的Redis实例出问题时，能通过API通知系统管理员或其他程序； 

自动故障恢复，如果主实例无法正常工作，Sentinel将启动故障恢复机制把一个从实例提升为主实例，其他的从实例将会被重新配置到新的主实例，且应用程序会得到一个更换新地址的通知。 
Redis Sentinel是一个分布式系统，可以部署多个Sentinel实例来监控同一组Redis实例，它们通过Gossip协议来确定一个主实例宕机，通过Agreement协议来执行故障恢复和配置变更，一般在生产环境中部署多个实例来提高系统可用性，只要有一个Sentinel实例运行正常，就能保证被监控的Redis实例运行正常（类似Zookeeper，通过多个Zookeeper来提高系统可用性）； 

本文不涉及Sentinel的实现细节和工作原理，读者可以阅读其他文章了解；

 

Redis HA方案

HA的关键在于避免单点故障及故障恢复，在Redis Cluster未发布之前，Redis一般以主/从方式部署（这里讨论的应用从实例主要用于备份，主实例提供读写，有不少应用是读写分离的，读写操作需要取不同的Redis实例，该方案也可用于此种应用，原理都是相通的，区别在于数据操作层如何封装），该方式要实现HA主要有如下几种方案： 
1，keepalived：通过keepalived的虚拟IP，提供主从的统一访问，在主出现问题时，通过keepalived运行脚本将从提升为主，待主恢复后先同步后自动变为主，该方案的好处是主从切换后，应用程序不需要知道(因为访问的虚拟IP不变)，坏处是引入keepalived增加部署复杂性； 
2，zookeeper：通过zookeeper来监控主从实例，维护最新有效的IP，应用通过zookeeper取得IP，对Redis进行访问； 
3，sentinel：通过Sentinel监控主从实例，自动进行故障恢复，该方案有个缺陷：因为主从实例地址(IP&PORT)是不同的，当故障发生进行主从切换后，应用程序无法知道新地址，故在Jedis2.2.2中新增了对Sentinel的支持，应用通过redis.clients.jedis.JedisSentinelPool.getResource()取得的Jedis实例会及时更新到新的主实例地址。 

笔者所在的公司先使用了方案1一段时间后，发现keepalived在有些情况下会导致数据丢失，keepalived通过shell脚本进行主从切换，配置复杂，而且keepalived成为新的单点，后来选用了方案3，使用Redis官方解决方案；（方案2需要编写大量的监控代码，没有方案3简便，网上有人使用方案2读者可自行查看）

选用Sentinel出现的问题

Sentinel&Jedis看上去是个完美的解决方案，这句话只说对了一半，在无分片的情况是这样，但我们的应用使用了数据分片-sharing，数据被平均分布到4个不同的实例上，每个实例以主从结构部署，Jedis没有提供基于Sentinel的ShardedJedisPool，也就是说在4个分片中，如果其中一个分片发生主从切换，应用所使用的ShardedJedisPool无法获得通知，所有对那个分片的操作将会失败。 
本文提供一个基于Sentinel的ShardedJedisPool，能及时感知所有分片主从切换行为，进行连接池重建，源码见ShardedJedisSentinelPool.java

 

ShardedJedisSentinelPool实现分析

构造函数

 类似之前的Jedis Pool的构造方法，需要参数poolConfig提供诸如maxIdle，maxTotal之类的配置，masters是一个List，用来保存所有分片Master在Sentinel中配置的名字（注意master的顺序不能改变，因为Shard算法是依据分片位置进行计算，如果顺序错误将导致数据存储混乱），sentinels是一个Set，其中存放所有Sentinel的地址(格式：IP:PORT，如127.0.0.1:26379)，顺序无关；

初始化连接池

在构造函数中，通过方法



 取得当前所有分片的master地址(IP&PORT)，对每个分片，通过顺次连接Sentinel实例，获取该分片的master地址，如果无法获得，即所有Sentinel都无法连接，将休眠1秒后继续重试，直到取得所有分片的master地址，代码块如下： 



通过



 初始化连接池，到此连接池中的所有连接都指向分片的master；

监控每个Sentinel

在方法



 最后，会为每个Sentinel启动一个Thread来监控Sentinel做出的更改： 



该线程的run方法通过Jedis Pub/Sub API（实现JedisPubSub接口，并通过jedis.subscribe进行订阅）向Sentinel实例订阅“+switch-master”频道，当Sentinel进行主从切换时，该线程会得到新Master地址的通知，通过master name判断哪个分片进行了切换，将新master地址替换原来位置的地址，并调用initPool(List masters)进行Jedis连接池重建；后续所有通过该连接池取得的连接都指向新Master地址，对应用程序透明；

应用示例

maven:

<dependency>
<groupId>redis.clients</groupId>
<artifactId>jedis</artifactId>
<version>2.9.0</version>
</dependency>
<dependency>
<groupId>commons-pool</groupId>
<artifactId>commons-pool</artifactId>
<version>1.6</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-pool2</artifactId>
<version>2.0</version>
<type>jar</type>
<scope>compile</scope>
</dependency>

代码使用如下：

GenericObjectPoolConfig config = new GenericObjectPoolConfig();
config.setMaxTotal(100);
config.setMaxIdle(10);

List<String> masters = new ArrayList<String>();
masters.add("master-1");//sentinel.conf文件中的名字
masters.add("master-2");

Set<String> sentinels = new HashSet<String>();
sentinels.add("192.168.248.127:63792");
sentinels.add("192.168.248.128:63792");

ShardedJedisSentinelPool pool = new ShardedJedisSentinelPool(masters, sentinels, config, 60000);
ShardedJedis j = null;
for (int i = 11; i < 15; i++) {
try {
j = pool.getResource();
j.set("KEY: " + i, "" + i);
pool.returnResource(j);
} catch (JedisConnectionException e) {
e.printStackTrace();
}
}
pool.destroy();

ShardedJedisSentinelPool文件内容如下：

/**
*   2017/4/30.
*/
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.logging.Logger;
import java.util.regex.Pattern;

import org.apache.commons.pool2.PooledObject;
import org.apache.commons.pool2.PooledObjectFactory;
import org.apache.commons.pool2.impl.DefaultPooledObject;
import org.apache.commons.pool2.impl.GenericObjectPoolConfig;
import redis.clients.jedis.*;
import redis.clients.jedis.exceptions.JedisConnectionException;
import redis.clients.util.Hashing;
import redis.clients.util.Pool;

public class ShardedJedisSentinelPool extends Pool<ShardedJedis> {

public static final int MAX_RETRY_SENTINEL = 10;

protected final Logger log = Logger.getLogger(getClass().getName());

protected GenericObjectPoolConfig poolConfig;

protected int timeout = Protocol.DEFAULT_TIMEOUT;

private int sentinelRetry = 0;

protected String password;

protected int database = Protocol.DEFAULT_DATABASE;

protected Set<MasterListener> masterListeners = new HashSet<MasterListener>();

private volatile List<HostAndPort> currentHostMasters;

public ShardedJedisSentinelPool(List<String> masters, Set<String> sentinels) {
this(masters, sentinels, new GenericObjectPoolConfig(),
Protocol.DEFAULT_TIMEOUT, null, Protocol.DEFAULT_DATABASE);
}

public ShardedJedisSentinelPool(List<String> masters, Set<String> sentinels, String password) {
this(masters, sentinels, new GenericObjectPoolConfig(),
Protocol.DEFAULT_TIMEOUT, password);
}

public ShardedJedisSentinelPool(final GenericObjectPoolConfig poolConfig, List<String> masters, Set<String> sentinels) {
this(masters, sentinels, poolConfig, Protocol.DEFAULT_TIMEOUT, null,
Protocol.DEFAULT_DATABASE);
}

public ShardedJedisSentinelPool(List<String> masters, Set<String> sentinels,
final GenericObjectPoolConfig poolConfig, int timeout,
final String password) {
this(masters, sentinels, poolConfig, timeout, password,
Protocol.DEFAULT_DATABASE);
}

public ShardedJedisSentinelPool(List<String> masters, Set<String> sentinels,
final GenericObjectPoolConfig poolConfig, final int timeout) {
this(masters, sentinels, poolConfig, timeout, null,
Protocol.DEFAULT_DATABASE);
}

public ShardedJedisSentinelPool(List<String> masters, Set<String> sentinels,
final GenericObjectPoolConfig poolConfig, final String password) {
this(masters, sentinels, poolConfig, Protocol.DEFAULT_TIMEOUT,
password);
}

public ShardedJedisSentinelPool(List<String> masters, Set<String> sentinels,
final GenericObjectPoolConfig poolConfig, int timeout,
final String password, final int database) {
this.poolConfig = poolConfig;
this.timeout = timeout;
this.password = password;
this.database = database;

List<HostAndPort> masterList = initSentinels(sentinels, masters);
initPool(masterList);
}

public void destroy() {
for (MasterListener m : masterListeners) {
m.shutdown();
}

super.destroy();
}

public List<HostAndPort> getCurrentHostMaster() {
return currentHostMasters;
}

private void initPool(List<HostAndPort> masters) {
if (!equals(currentHostMasters, masters)) {
StringBuffer sb = new StringBuffer();
for (HostAndPort master : masters) {
sb.append(master.toString());
sb.append(" ");
}
log.info("Created ShardedJedisPool to master at [" + sb.toString() + "]");
List<JedisShardInfo> shardMasters = makeShardInfoList(masters);
initPool(poolConfig, new ShardedJedisFactory(shardMasters, Hashing.MURMUR_HASH, null));
currentHostMasters = masters;
}
}

private boolean equals(List<HostAndPort> currentShardMasters, List<HostAndPort> shardMasters) {
if (currentShardMasters != null && shardMasters != null) {
if (currentShardMasters.size() == shardMasters.size()) {
for (int i = 0; i < currentShardMasters.size(); i++) {
if (!currentShardMasters.get(i).equals(shardMasters.get(i))) return false;
}
return true;
}
}
return false;
}

private List<JedisShardInfo> makeShardInfoList(List<HostAndPort> masters) {
List<JedisShardInfo> shardMasters = new ArrayList<JedisShardInfo>();
for (HostAndPort master : masters) {
JedisShardInfo jedisShardInfo = new JedisShardInfo(master.getHost(), master.getPort(), timeout);
jedisShardInfo.setPassword(password);

shardMasters.add(jedisShardInfo);
}
return shardMasters;
}

private List<HostAndPort> initSentinels(Set<String> sentinels, final List<String> masters) {

Map<String, HostAndPort> masterMap = new HashMap<String, HostAndPort>();
List<HostAndPort> shardMasters = new ArrayList<HostAndPort>();

log.info("Trying to find all master from available Sentinels...");

for (String masterName : masters) {
HostAndPort master = null;
boolean fetched = false;

while (!fetched && sentinelRetry < MAX_RETRY_SENTINEL) {
for (String sentinel : sentinels) {
final HostAndPort hap = toHostAndPort(Arrays.asList(sentinel.split(":")));

log.fine("Connecting to Sentinel " + hap);

try {
Jedis jedis = new Jedis(hap.getHost(), hap.getPort());
master = masterMap.get(masterName);
if (master == null) {
List<String> hostAndPort = jedis.sentinelGetMasterAddrByName(masterName);
if (hostAndPort != null && hostAndPort.size() > 0) {
master = toHostAndPort(hostAndPort);
log.fine("Found Redis master at " + master);
shardMasters.add(master);
masterMap.put(masterName, master);
fetched = true;
jedis.disconnect();
break;
}
}
} catch (JedisConnectionException e) {
log.warning("Cannot connect to sentinel running @ " + hap + ". Trying next one.");
}
}

if (null == master) {
try {
log.severe("All sentinels down, cannot determine where is "
+ masterName + " master is running... sleeping 1000ms, Will try again.");
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
fetched = false;
sentinelRetry++;
}
}

// Try MAX_RETRY_SENTINEL times.
if (!fetched && sentinelRetry >= MAX_RETRY_SENTINEL) {
log.severe("All sentinels down and try " + MAX_RETRY_SENTINEL + " times, Abort.");
throw new JedisConnectionException("Cannot connect all sentinels, Abort.");
}
}

// All shards master must been accessed.
if (masters.size() != 0 && masters.size() == shardMasters.size()) {

log.info("Starting Sentinel listeners...");
for (String sentinel : sentinels) {
final HostAndPort hap = toHostAndPort(Arrays.asList(sentinel.split(":")));
MasterListener masterListener = new MasterListener(masters, hap.getHost(), hap.getPort());
masterListeners.add(masterListener);
masterListener.start();
}
}

return shardMasters;
}

private HostAndPort toHostAndPort(List<String> getMasterAddrByNameResult) {
String host = getMasterAddrByNameResult.get(0);
int port = Integer.parseInt(getMasterAddrByNameResult.get(1));

return new HostAndPort(host, port);
}

/**
* PoolableObjectFactory custom impl.
*/
protected static class ShardedJedisFactory implements PooledObjectFactory<ShardedJedis> {
private List<JedisShardInfo> shards;
private Hashing algo;
private Pattern keyTagPattern;

public ShardedJedisFactory(List<JedisShardInfo> shards, Hashing algo, Pattern keyTagPattern) {
this.shards = shards;
this.algo = algo;
this.keyTagPattern = keyTagPattern;
}

public PooledObject<ShardedJedis> makeObject() throws Exception {
ShardedJedis jedis = new ShardedJedis(shards, algo, keyTagPattern);
return new DefaultPooledObject<ShardedJedis>(jedis);
}

public void destroyObject(PooledObject<ShardedJedis> pooledShardedJedis) throws Exception {
final ShardedJedis shardedJedis = pooledShardedJedis.getObject();
for (Jedis jedis : shardedJedis.getAllShards()) {
try {
try {
jedis.quit();
} catch (Exception e) {

}
jedis.disconnect();
} catch (Exception e) {

}
}
}

public boolean validateObject(PooledObject<ShardedJedis> pooledShardedJedis) {
try {
ShardedJedis jedis = pooledShardedJedis.getObject();
for (Jedis shard : jedis.getAllShards()) {
if (!shard.ping().equals("PONG")) {
return false;
}
}
return true;
} catch (Exception ex) {
return false;
}
}

public void activateObject(PooledObject<ShardedJedis> p) throws Exception {

}

public void passivateObject(PooledObject<ShardedJedis> p) throws Exception {

}
}

protected class JedisPubSubAdapter extends JedisPubSub {
@Override
public void onMessage(String channel, String message) {
}

@Override
public void onPMessage(String pattern, String channel, String message) {
}

@Override
public void onPSubscribe(String pattern, int subscribedChannels) {
}

@Override
public void onPUnsubscribe(String pattern, int subscribedChannels) {
}

@Override
public void onSubscribe(String channel, int subscribedChannels) {
}

@Override
public void onUnsubscribe(String channel, int subscribedChannels) {
}
}

protected class MasterListener extends Thread {

protected List<String> masters;
protected String host;
protected int port;
protected long subscribeRetryWaitTimeMillis = 5000;
protected Jedis jedis;
protected AtomicBoolean running = new AtomicBoolean(false);

protected MasterListener() {
}

public MasterListener(List<String> masters, String host, int port) {
this.masters = masters;
this.host = host;
this.port = port;
}

public MasterListener(List<String> masters, String host, int port,
long subscribeRetryWaitTimeMillis) {
this(masters, host, port);
this.subscribeRetryWaitTimeMillis = subscribeRetryWaitTimeMillis;
}

public void run() {

running.set(true);

while (running.get()) {

jedis = new Jedis(host, port);

try {
jedis.subscribe(new JedisPubSubAdapter() {
@Override
public void onMessage(String channel, String message) {
log.fine("Sentinel " + host + ":" + port + " published: " + message + ".");

String[] switchMasterMsg = message.split(" ");

if (switchMasterMsg.length > 3) {

int index = masters.indexOf(switchMasterMsg[0]);
if (index >= 0) {
HostAndPort newHostMaster = toHostAndPort(Arrays.asList(switchMasterMsg[3], switchMasterMsg[4]));
List<HostAndPort> newHostMasters = new ArrayList<HostAndPort>();
for (int i = 0; i < masters.size(); i++) {
newHostMasters.add(null);
}
Collections.copy(newHostMasters, currentHostMasters);
newHostMasters.set(index, newHostMaster);

initPool(newHostMasters);
} else {
StringBuffer sb = new StringBuffer();
for (String masterName : masters) {
sb.append(masterName);
sb.append(",");
}
log.fine("Ignoring message on +switch-master for master name "
+ switchMasterMsg[0]
+ ", our monitor master name are ["
+ sb + "]");
}

} else {
log.severe("Invalid message received on Sentinel "
+ host
+ ":"
+ port
+ " on channel +switch-master: "
+ message);
}
}
}, "+switch-master");

} catch (JedisConnectionException e) {

if (running.get()) {
log.severe("Lost connection to Sentinel at " + host
+ ":" + port
+ ". Sleeping 5000ms and retrying.");
try {
Thread.sleep(subscribeRetryWaitTimeMillis);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
} else {
log.fine("Unsubscribing from Sentinel at " + host + ":"
+ port);
}
}
}
}

public void shutdown() {
try {
log.fine("Shutting down listener on " + host + ":" + port);
running.set(false);
// This isn't good, the Jedis object is not thread safe
jedis.disconnect();
} catch (Exception e) {
log.severe("Caught exception while shutting down: " + e.getMessage());
}
}
}
}

总结

本文通过现实中遇到的问题，即在Redis数据分片的情况下，在使用Sentinel做HA时，如何做到主从的切换对应用程序透明，通过Jedis的Pub/Sub功能，能同时监控多个分片的主从切换情况，并通过监听到的新地址重新构造连接池，后续从连接池中取得的所有连接都指向新地址。该方案的关键是：使用sentinel做HA，Jedis版本必须2.2.2及以上，所有访问Redis实例的连接都必须从连接池中获取；

该项目的GitHub主页: https://github.com/warmbreeze/sharded-jedis-sentinel-pool




==================================以下有代验证==============================================

==================================以下有代验证==============================================

连接泄露Bug

循环时发现ShardedJedisSentinelPool连接池有时竟然满了！毕竟默认是8个连接，但为什么会这样？查了下代码，果然是这个原因！这个新扩展的pool没有为getResource()出来的ShardedJedis设置DataSource，所以关闭ShardedJedis时没法正常还回到连接池中。

补丁很简单，就是参照Jedis的ShardedJedisPool，覆写一些资源管理的方法。

@Override
public ShardedJedis getResource() {
ShardedJedis jedis = super.getResource();
jedis.setDataSource(this);
return jedis;
}

@Override
public void returnBrokenResource(final ShardedJedis resource) {
if (resource != null) {
returnBrokenResourceObject(resource);
}
}

@Override
public void returnResource(final ShardedJedis resource) {
if (resource != null) {
resource.resetState();
returnResourceObject(resource);
}
}