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
- 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
- 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
- // 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
- 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
- SockIOPool.SockIO sock = pool.getSock( key, hashCode );
看一看pool.getSock的代码
- // 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
- 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。