首先分析一下Java client 启动时的部分代码 Memcached 支持直接设置多个servers属性 来实现多个memcahced均衡,对应还有一个属性是weights,字面意思就是权重,分析了一下代码,和我想的是一样的 启动memcached的代码通常是这样的
Java代码 SockIOPool pool = SockIOPool.getInstance(poolname); pool.setServers(servers); pool.setWeights(weights); pool.setInitConn(initConn); pool.setMinConn(minConn); pool.setMaxConn(maxConn); pool.setMaxIdle(maxIdle); pool.setMaxBusyTime(maxBusyTime); pool.setMaintSleep(maintSleep); pool.setSocketTO(socketTO); pool.setSocketConnectTO(socketConnectTO); pool.setNagle(nagle); pool.setHashingAlg(SockIOPool.NEW_COMPAT_HASH); pool.initialize(); MemCachedClient client = new MemCachedClient(poolname); servers 和 weights 都是一个数组,就是说可以同时设置多个server 然后看一下 pool.initialize() 做了什么 Java代码 availPool = new HashMap<String,Map<SockIO,Long>>( servers.length * initConn ); busyPool = new HashMap<String,Map<SockIO,Long>>( servers.length * initConn ); deadPool = new IdentityHashMap<SockIO,Integer>(); hostDeadDur = new HashMap<String,Long>(); hostDead = new HashMap<String,Date>(); maxCreate = (poolMultiplier > minConn) ? minConn : minConn / poolMultiplier; // only create up to maxCreate connections at once if ( log.isDebugEnabled() ) { log.debug( "++++ initializing pool with following settings:" ); log.debug( "++++ initial size: " + initConn ); log.debug( "++++ min spare : " + minConn ); log.debug( "++++ max spare : " + maxConn ); } // if servers is not set, or it empty, then // throw a runtime exception if ( servers == null || servers.length <= 0 ) { log.error( "++++ trying to initialize with no servers" ); throw new IllegalStateException( "++++ trying to initialize with no servers" ); } // initalize our internal hashing structures if ( this.hashingAlg == CONSISTENT_HASH ) populateConsistentBuckets(); else populateBuckets(); 看到这里就是开辟一些连接池的空间,然后调用了根据我们选择的hash 算法 执行populateBuckets();或者populateConsistentBuckets(); hash算法共有4种 Java代码 // native String.hashCode(); public static final int NATIVE_HASH = 0; // original compatibility hashing algorithm (works with other clients) public static final int OLD_COMPAT_HASH = 1; // new CRC32 based compatibility hashing algorithm (works with other clients) public static final int NEW_COMPAT_HASH = 2; // MD5 Based -- Stops thrashing when a server added or removed public static final int CONSISTENT_HASH = 3; 我们通常用的是 NEW_COMPAT_HASH,这个保证可以wokrs with other clients 所以看一下populateBuckets()做了什么 Java代码 this.buckets = new ArrayList<String>(); for ( int i = 0; i < servers.length; i++ ) { if ( this.weights != null && this.weights.length > i ) { for ( int k = 0; k < this.weights[i].intValue(); k++ ) { this.buckets.add( servers[i] ); if ( log.isDebugEnabled() ) log.debug( "++++ added " + servers[i] + " to server bucket" ); } } else { this.buckets.add( servers[i] ); } // create initial connections for ( int j = 0; j < initConn; j++ ) { SockIO socket = createSocket( servers[i] ); if ( socket == null ) { break; } addSocketToPool( availPool, servers[i], socket ); } } 假如我们设置的servers是 192.168.0.1:44444和192.168.0.2:22222 然后我们设置了weights是 5和3 那么 buckets list的值最终会是 [ 192.168.0.1:44444, 192.168.0.1:44444, 192.168.0.1:44444, 192.168.0.1:44444, 192.168.0.1:44444, 192.168.0.2:22222, 192.168.0.2:22222. 192.168.0.2:22222. ] 然后就开始根据initCon初始连接数按servers分别创建socket 那么究竟这个buckets做什么用呢? 在我们使用set存放对象时会调用 Java代码 SockIOPool.SockIO sock = pool.getSock( key, hashCode ); 看一看pool.getSock的代码 Java代码 // get initial bucket long bucket = getBucket( key, hashCode ); String server = ( this.hashingAlg == CONSISTENT_HASH ) ? consistentBuckets.get( bucket ) : buckets.get( (int)bucket ); 其中有段代码是这样的,看看getBucket Java代码 private long getBucket( String key, Integer hashCode ) { long hc = getHash( key, hashCode ); if ( this.hashingAlg == CONSISTENT_HASH ) { return findPointFor( hc ); } else { long bucket = hc % buckets.size(); if ( bucket < 0 ) bucket *= -1; return bucket; } }先不管key和hashCode,我们看到首先算出一个hc值后会直接做hc%buckets.size()实际上就是根据buckets的数量散列,最终值一定是buckets.size()范围里的一个值 然后最终server值就根据buckets.get( (int)bucket )得到,那么假如我们得到bucket是3,则参照上面buckets 里的值,得到 list.get(3)=192.168.0.1:44444 所以会根据weight设置的值的不同得到不同的server ,如果 weights设置10:1 那buckets里就是10个相同的server和另一个不同的,将来散列得到的server很大可能性是servers里设置的第一个server。
原文地址:http://bachmozart.javaeye.com/blog/211836