客户端实现多个Memcached服务集群

客户端实现多个Memcached服务集群

Memcached服务器之间是独立的，没有任何关联，不能进行集群，Memcached集群是从客户端实现的。

应用系统使用统一的访问Memcached配置,在调用client包来set(key,value), get(key)时，使用相同选取服务器节点的算法，这样就实现了取到先前存入的对象。

选取服务器节点的算法思路是key的哈希值与节点数取模，比如，有Memcached服务器n0,n1,n2,n3,n4，key为"user_name",那么就是 hash(username)%5,如果结果为0就选取n0,依次类推。

选取服务器接点的算法一般封装在memcached client包中。

MemcachedSession session = (MemcachedSession) this.findSessionByKey(msg				.getKey());

//默认使用这个会话定位器
private MemcachedSessionLocator sessionLocator = new ArrayMemcachedSessionLocator();

// 使用一致性哈希算法（Consistent Hash Strategy）  建议使用。
builder.setSessionLocator(new KetamaMemcachedSessionLocator());  

ArrayMemcachedSessionLocator:

public final long getHash(int size, String key) {
	long hash = this.hashAlgorighm.hash(key);
	return hash % size;
}

public final Session getSessionByKey(final String key) {
		if (this.sessions == null || this.sessions.size() == 0) {
			return null;
		}
		// Copy on read
		List<List<Session>> sessionList = this.sessions;
		int size = sessionList.size();
		if (size == 0) {
			return null;
		}
		long start = this.getHash(size, key);
		List<Session> sessions = sessionList.get((int) start);
		Session session = getRandomSession(sessions);

		// If it is not failure mode,get next available session
		if (!this.failureMode && (session == null || session.isClosed())) {
			long next = this.getNext(size, start);
			while ((session == null || session.isClosed()) && next != start) {
				sessions = sessionList.get((int) next);
				next = this.getNext(size, next);
				session = getRandomSession(sessions);
			}
		}
		return session;
	}

KetamaMemcachedSessionLocator：

public final Session getSessionByKey(final String key) {
		if (this.ketamaSessions == null || this.ketamaSessions.size() == 0) {
			return null;
		}
		long hash = this.hashAlg.hash(key);
		Session rv = this.getSessionByHash(hash);
		int tries = 0;
		while (!this.failureMode && (rv == null || rv.isClosed())
				&& tries++ < this.maxTries) {
			hash = this.nextHash(hash, key, tries);
			rv = this.getSessionByHash(hash);
		}
		return rv;
	}

	public final Session getSessionByHash(final long hash) {
		TreeMap<Long, List<Session>> sessionMap = this.ketamaSessions;
		if (sessionMap.size() == 0) {
			return null;
		}
		Long resultHash = hash;
		if (!sessionMap.containsKey(hash)) {
			// Java 1.6 adds a ceilingKey method, but xmemcached is compatible
			// with jdk5,So use tailMap method to do this.
			SortedMap<Long, List<Session>> tailMap = sessionMap.tailMap(hash);
			if (tailMap.isEmpty()) {
				resultHash = sessionMap.firstKey();
			} else {
				resultHash = tailMap.firstKey();
			}
		}
		//
		// if (!sessionMap.containsKey(resultHash)) {
		// resultHash = sessionMap.ceilingKey(resultHash);
		// if (resultHash == null && sessionMap.size() > 0) {
		// resultHash = sessionMap.firstKey();
		// }
		// }
		List<Session> sessionList = sessionMap.get(resultHash);
		if (sessionList == null || sessionList.size() == 0) {
			return null;
		}
		int size = sessionList.size();
		return sessionList.get(this.random.nextInt(size));
	}