Redis Jedis 之 Sharded 源码分析

r囧r小猫 2022-06-07 01:48 280阅读 0赞

## 概述 ##

Jedis是Redis官方推荐的Java客户端，当业务的数据量非常庞大时，需要考虑将数据存储到多个缓存节点上，如何定位数据应该存储的节点，一般用的是**一致性哈希算法**。Jedis在客户端角度实现了一致性哈希算法，对数据进行分片，存储到对应的不同的redis实例中。

Jedis对Sharded的实现主要是在`ShardedJedis.java`和`ShardedJedisPool.java`中，ShardedJedisPool是基于apache的common-pool2的对象池实现。

## 继承关系 ##

ShardedJedis—>BinaryShardedJedis—>Sharded

## 构造函数 ##

public ShardedJedis(List<JedisShardInfo> shards, Hashing algo, Pattern keyTagPattern) {
        super(shards, algo, keyTagPattern);
    }

参数解释：

*  shards是一个JedisShardInfo的列表，一个JedisShardedInfo类代表一个数据分片的主体。
 *  algo是用来进行数据分片的算法
 *  keyTagPattern 自定义分片算法所依据的key的形式，例如，可以不针对整个key的字符串做哈希计算，而是类似对`thisisa{key}`中包含在大括号内的字符串进行哈希计算。

JedisShardInfo是什么样的？

public class JedisShardInfo extends ShardInfo<Jedis> {
    
      public String toString() {
        return host + ":" + port + "*" + getWeight();
      }
    
      private int connectionTimeout;
      private int soTimeout;
      private String host;
      private int port;
      private String password = null;
      private String name = null;
      // Default Redis DB
      private int db = 0;
    
      public String getHost() {
        return host;
      }
    
      public int getPort() {
        return port;
      }
    
      public JedisShardInfo(String host) {
        super(Sharded.DEFAULT_WEIGHT);
        URI uri = URI.create(host);
        if (JedisURIHelper.isValid(uri)) {
          this.host = uri.getHost();
          this.port = uri.getPort();
          this.password = JedisURIHelper.getPassword(uri);
          this.db = JedisURIHelper.getDBIndex(uri);
        } else {
          this.host = host;
          this.port = Protocol.DEFAULT_PORT;
        }
      }
    
      public JedisShardInfo(String host, String name) {
        this(host, Protocol.DEFAULT_PORT, name);
      }
    
      public JedisShardInfo(String host, int port) {
        this(host, port, 2000);
      }
    
      public JedisShardInfo(String host, int port, String name) {
        this(host, port, 2000, name);
      }
    
      public JedisShardInfo(String host, int port, int timeout) {
        this(host, port, timeout, timeout, Sharded.DEFAULT_WEIGHT);
      }
    
      public JedisShardInfo(String host, int port, int timeout, String name) {
        this(host, port, timeout, timeout, Sharded.DEFAULT_WEIGHT);
        this.name = name;
      }
    
      public JedisShardInfo(String host, int port, int connectionTimeout, int soTimeout, int weight) {
        super(weight);
        this.host = host;
        this.port = port;
        this.connectionTimeout = connectionTimeout;
        this.soTimeout = soTimeout;
      }
    
      public JedisShardInfo(String host, String name, int port, int timeout, int weight) {
        super(weight);
        this.host = host;
        this.name = name;
        this.port = port;
        this.connectionTimeout = timeout;
        this.soTimeout = timeout;
      }
    
      public JedisShardInfo(URI uri) {
        super(Sharded.DEFAULT_WEIGHT);
        if (!JedisURIHelper.isValid(uri)) {
          throw new InvalidURIException(String.format(
            "Cannot open Redis connection due invalid URI. %s", uri.toString()));
        }
    
        this.host = uri.getHost();
        this.port = uri.getPort();
        this.password = JedisURIHelper.getPassword(uri);
        this.db = JedisURIHelper.getDBIndex(uri);
      }
    
    @Override
      public Jedis createResource() {
        return new Jedis(this);
      }
        /** * 省略setters和getters **/
    }

可见JedisShardInfo包含了一个redis节点ip地址，端口号，name，密码等等相关信息。要构造一个ShardedJedis，提供一个或多个JedisShardInfo。

最终构造函数的实现在其父类Sharded里面

public Sharded(List<S> shards, Hashing algo, Pattern tagPattern) {
        this.algo = algo;
        this.tagPattern = tagPattern;
        initialize(shards);
    }

## 哈希环的初始化 ##

Sharded类里面维护了一个**TreeMap**，基于红黑树实现，用来盛放经过一致性哈希计算后的redis节点，另外维护了一个**LinkedHashMap**，用来保存ShardInfo与Jedis实例的对应关系

定位的流程如下

先在TreeMap中找到对应key所对应的ShardInfo，然后通过ShardInfo在LinkedHashMap中找到对应的Jedis实例。

Sharded类对这些实例变量的定义如下所示：

public static final int DEFAULT_WEIGHT = 1;
    private TreeMap<Long, S> nodes;
    private final Hashing algo;
    private final Map<ShardInfo<R>, R> resources = new LinkedHashMap<ShardInfo<R>, R>();
    
    /** * The default pattern used for extracting a key tag. The pattern must have * a group (between parenthesis), which delimits the tag to be hashed. A * null pattern avoids applying the regular expression for each lookup, * improving performance a little bit is key tags aren't being used. */
    private Pattern tagPattern = null;
    // the tag is anything between {}
    public static final Pattern DEFAULT_KEY_TAG_PATTERN = Pattern.compile("\\{(.+?)\\}");

接下来查看其构造函数中的initialize方法

private void initialize(List<S> shards) {
        nodes = new TreeMap<Long, S>();
    
        for (int i = 0; i != shards.size(); ++i) {
            final S shardInfo = shards.get(i);
            if (shardInfo.getName() == null)
                for (int n = 0; n < 160 * shardInfo.getWeight(); n++) {
                    nodes.put(this.algo.hash("SHARD-" + i + "-NODE-" + n), shardInfo);
                }
            else
                for (int n = 0; n < 160 * shardInfo.getWeight(); n++) {
                    nodes.put(
                            this.algo.hash(shardInfo.getName() + "*"
                                    + shardInfo.getWeight() + n), shardInfo);
                }
            resources.put(shardInfo, shardInfo.createResource());
        }
    }

可以看到，它对每一个ShardInfo通过一定规则计算其哈希值，然后存到TreeMap中，这里它实现了一致性哈希算法中虚拟节点的概念，因为我们可以看到同一个ShardInfo不止一次被放到TreeMap中，数量是，权重\*160。  
增加了虚拟节点的一致性哈希有很多好处，能避免数据在redis节点间分布不均匀。

然后，在LinkedHashMap中放入ShardInfo以及其对应的Jedis实例，通过调用其自身的createSource()来得到jedis实例。

## 数据定位 ##

从ShardedJedis的代码中可以看到，无论进行什么操作，都要先根据key来找到对应的Redis，然后返回一个可供操作的Jedis实例。

例如其set方法：

public String set(String key, String value) { Jedis j = getShard(key);
        return j.set(key, value);
    }

而getShard方法则在Sharded.java中实现，其源代码如下所示：

public R getShard(byte[] key) {
        return resources.get(getShardInfo(key));
    }
    
    public R getShard(String key) {
        return resources.get(getShardInfo(key));
    }
    
    public S getShardInfo(byte[] key) {
        SortedMap<Long, S> tail = nodes.tailMap(algo.hash(key));
        if (tail.isEmpty()) {
            return nodes.get(nodes.firstKey());
        }
        return tail.get(tail.firstKey());
    }
    
    public S getShardInfo(String key) {
        return getShardInfo(SafeEncoder.encode(getKeyTag(key)));
    }

可以看到，先通过getShardInfo方法从TreeMap中获得对应的ShardInfo，然后根据这个ShardInfo就能够再LinkedHashMap中获得对应的Jedis实例了。

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