java memcached client源码Qȝ代码量还是很的
主要是如下两个类:
MemcachedClient.java
SockIOPool.java
?先看推荐的测试代?
Java代码 
/**
* Copyright (c) 2008 Greg Whalin
* All rights reserved.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the BSD license
*
* This library is distributed in the hope that it will be
* useful, but WITHOUT ANY WARRANTY; without even the implied
* warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
* PURPOSE.
*
* You should have received a copy of the BSD License along with this
* library.
*
* @author greg whalin <greg@meetup.com>
*/
package com.meetup.memcached.test;
import com.meetup.memcached.*;
import org.apache.log4j.*;
public class TestMemcached {
public static void main(String[] args) {
// memcached should be running on port 11211 but NOT on 11212
BasicConfigurator.configure();
String[] servers = { "localhost:11211"};
SockIOPool pool = SockIOPool.getInstance();
pool.setServers( servers );
pool.setFailover( true );
pool.setInitConn( 10 );
pool.setMinConn( 5 );
pool.setMaxConn( 250 );
pool.setMaintSleep( 30 );
//q是开启一个nagle 法。改法避免|络中充塞小包Q提高网l的利用?nbsp;
pool.setNagle( false );
pool.setSocketTO( 3000 );
pool.setAliveCheck( true );
pool.initialize();
MemcachedClient mcc = new MemcachedClient();
// turn off most memcached client logging:
com.meetup.memcached.Logger.getLogger( MemcachedClient.class.getName() ).setLevel( com.meetup.memcached.Logger.LEVEL_WARN );
for ( int i = 0; i < 10; i++ ) {
boolean success = mcc.set( "" + i, "Hello!" );
String result = (String)mcc.get( "" + i );
System.out.println( String.format( "set( %d ): %s", i, success ) );
System.out.println( String.format( "get( %d ): %s", i, result ) );
}
}
其实 对于我来?我很x(chng)白的是连接池是如何配|的,关键在于 pool.initialize(); q个Ҏ(gu)如何初始化的?nbsp;* Copyright (c) 2008 Greg Whalin
* All rights reserved.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the BSD license
*
* This library is distributed in the hope that it will be
* useful, but WITHOUT ANY WARRANTY; without even the implied
* warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
* PURPOSE.
*
* You should have received a copy of the BSD License along with this
* library.
*
* @author greg whalin <greg@meetup.com>
*/
package com.meetup.memcached.test;
import com.meetup.memcached.*;
import org.apache.log4j.*;
public class TestMemcached {
public static void main(String[] args) {
// memcached should be running on port 11211 but NOT on 11212
BasicConfigurator.configure();
String[] servers = { "localhost:11211"};
SockIOPool pool = SockIOPool.getInstance();
pool.setServers( servers );
pool.setFailover( true );
pool.setInitConn( 10 );
pool.setMinConn( 5 );
pool.setMaxConn( 250 );
pool.setMaintSleep( 30 );
//q是开启一个nagle 法。改法避免|络中充塞小包Q提高网l的利用?nbsp;
pool.setNagle( false );
pool.setSocketTO( 3000 );
pool.setAliveCheck( true );
pool.initialize();
MemcachedClient mcc = new MemcachedClient();
// turn off most memcached client logging:
com.meetup.memcached.Logger.getLogger( MemcachedClient.class.getName() ).setLevel( com.meetup.memcached.Logger.LEVEL_WARN );
for ( int i = 0; i < 10; i++ ) {
boolean success = mcc.set( "" + i, "Hello!" );
String result = (String)mcc.get( "" + i );
System.out.println( String.format( "set( %d ): %s", i, success ) );
System.out.println( String.format( "get( %d ): %s", i, result ) );
}
}
Java代码
/**
* Initializes the pool.
*/
public void initialize() {
synchronized( this ) {
// check to see if already initialized
if ( initialized
&& ( buckets != null || consistentBuckets != null )
&& ( availPool != null )
&& ( busyPool != null ) ) {
log.error( "++++ trying to initialize an already initialized pool" );
return;
}
// pools
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();
// mark pool as initialized
this.initialized = true;
// start maint thread
if ( this.maintSleep > 0 )
this.startMaintThread();
}
}
如上代码程如下Q?nbsp;* Initializes the pool.
*/
public void initialize() {
synchronized( this ) {
// check to see if already initialized
if ( initialized
&& ( buckets != null || consistentBuckets != null )
&& ( availPool != null )
&& ( busyPool != null ) ) {
log.error( "++++ trying to initialize an already initialized pool" );
return;
}
// pools
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();
// mark pool as initialized
this.initialized = true;
// start maint thread
if ( this.maintSleep > 0 )
this.startMaintThread();
}
}
1 (g)是否已l被初始?nbsp;
2 定义可用链接 Q繁忙链接池
3 判断是否一致性hash法 q是普通的法
4 定义一个后台线E?Q来l护
?Q首先来分析下一致性hash法?nbsp;
从如下代码来分析 Q?nbsp;
Java代码
if (
this.hashingAlg == CONSISTENT_HASH )
populateConsistentBuckets();
}
this.hashingAlg == CONSISTENT_HASH )
populateConsistentBuckets();
}
Java代码
/** serverdC致性hash??2?nbsp;圆环 **/
private void populateConsistentBuckets() {
if ( log.isDebugEnabled() )
log.debug( "++++ initializing internal hashing structure for consistent hashing" );
// store buckets in tree map
this.consistentBuckets = new TreeMap<Long,String>();
MessageDigest md5 = MD5.get();
//得到ȝ权重
if ( this.totalWeight <= 0 && this.weights != null ) {
for ( int i = 0; i < this.weights.length; i++ )
this.totalWeight += ( this.weights[i] == null ) ? 1 : this.weights[i];
}
else if ( this.weights == null ) {
this.totalWeight = this.servers.length;
}
for ( int i = 0; i < servers.length; i++ ) {
//每台服务器的权重
int thisWeight = 1;
if ( this.weights != null && this.weights[i] != null ) {
thisWeight = this.weights[i];
}
//有兴的朋友可以参考^衡Hash 法的另一个指标是q?nbsp;(Balance) Q定义如下：(x) q性 q性是指哈希的l果能够可能分布到所有的~冲中去Q这样可以得所有的~冲I间都得到利?nbsp;
//?jin)解册U情况， consistent hashing 引入?#8220;虚拟节点”的概念，它可以如下定义：(x) “虚拟节点”Q?nbsp;virtual node Q是实际节点?nbsp;hash I间的复制品Q?nbsp;replica Q，一实际个节点对应了(jin)若干?#8220;虚拟节点”Q这个对应个C成ؓ(f)“复制个数”Q?#8220;虚拟节点”?nbsp;hash I间中以 hash 值排列?nbsp;
double factor = Math.floor(((double)(40 * this.servers.length * thisWeight)) / (double)this.totalWeight);
for ( long j = 0; j < factor; j++ ) { //加密规则cM 127.0.0.1_1
byte[] d = md5.digest( ( servers[i] + "-" + j ).getBytes() ); //转化?6位的字节数组
//16位二q制数组?位ؓ(f)一l，每组W?个值左U?4位，W三个值左U?6位，W二个值左U?位，W一个gUM。进行或q算Q得C个小??2 ơ方的long?nbsp;
for ( int h = 0 ; h < 4; h++ ) { //因ؓ(f)?6?nbsp;
Long k = //实际上每个字节进行了(jin)q算
((long)(d[3+h*4]&0xFF) << 24)
| ((long)(d[2+h*4]&0xFF) << 16)
| ((long)(d[1+h*4]&0xFF) << 8)
| ((long)(d[0+h*4]&0xFF));
consistentBuckets.put( k, servers[i] );
if ( log.isDebugEnabled() )
log.debug( "++++ added " + servers[i] + " to server bucket" );
}
}
// create initial connections
if ( log.isDebugEnabled() )
log.debug( "+++ creating initial connections (" + initConn + ") for host: " + servers[i] );
//创徏链接
for ( int j = 0; j < initConn; j++ ) {
SockIO socket = createSocket( servers[i] );
if ( socket == null ) {
log.error( "++++ failed to create connection to: " + servers[i] + " -- only " + j + " created." );
break;
}
//加入socket到连接池 q里慢慢?nbsp;
addSocketToPool( availPool, servers[i], socket );
if ( log.isDebugEnabled() )
log.debug( "++++ created and added socket: " + socket.toString() + " for host " + servers[i] );
}
}
}
?比如?我们调用?如下代码: private void populateConsistentBuckets() {
if ( log.isDebugEnabled() )
log.debug( "++++ initializing internal hashing structure for consistent hashing" );
// store buckets in tree map
this.consistentBuckets = new TreeMap<Long,String>();
MessageDigest md5 = MD5.get();
//得到ȝ权重
if ( this.totalWeight <= 0 && this.weights != null ) {
for ( int i = 0; i < this.weights.length; i++ )
this.totalWeight += ( this.weights[i] == null ) ? 1 : this.weights[i];
}
else if ( this.weights == null ) {
this.totalWeight = this.servers.length;
}
for ( int i = 0; i < servers.length; i++ ) {
//每台服务器的权重
int thisWeight = 1;
if ( this.weights != null && this.weights[i] != null ) {
thisWeight = this.weights[i];
}
//有兴的朋友可以参考^衡Hash 法的另一个指标是q?nbsp;(Balance) Q定义如下：(x) q性 q性是指哈希的l果能够可能分布到所有的~冲中去Q这样可以得所有的~冲I间都得到利?nbsp;
//?jin)解册U情况， consistent hashing 引入?#8220;虚拟节点”的概念，它可以如下定义：(x) “虚拟节点”Q?nbsp;virtual node Q是实际节点?nbsp;hash I间的复制品Q?nbsp;replica Q，一实际个节点对应了(jin)若干?#8220;虚拟节点”Q这个对应个C成ؓ(f)“复制个数”Q?#8220;虚拟节点”?nbsp;hash I间中以 hash 值排列?nbsp;
double factor = Math.floor(((double)(40 * this.servers.length * thisWeight)) / (double)this.totalWeight);
for ( long j = 0; j < factor; j++ ) { //加密规则cM 127.0.0.1_1
byte[] d = md5.digest( ( servers[i] + "-" + j ).getBytes() ); //转化?6位的字节数组
//16位二q制数组?位ؓ(f)一l，每组W?个值左U?4位，W三个值左U?6位，W二个值左U?位，W一个gUM。进行或q算Q得C个小??2 ơ方的long?nbsp;
for ( int h = 0 ; h < 4; h++ ) { //因ؓ(f)?6?nbsp;
Long k = //实际上每个字节进行了(jin)q算
((long)(d[3+h*4]&0xFF) << 24)
| ((long)(d[2+h*4]&0xFF) << 16)
| ((long)(d[1+h*4]&0xFF) << 8)
| ((long)(d[0+h*4]&0xFF));
consistentBuckets.put( k, servers[i] );
if ( log.isDebugEnabled() )
log.debug( "++++ added " + servers[i] + " to server bucket" );
}
}
// create initial connections
if ( log.isDebugEnabled() )
log.debug( "+++ creating initial connections (" + initConn + ") for host: " + servers[i] );
//创徏链接
for ( int j = 0; j < initConn; j++ ) {
SockIO socket = createSocket( servers[i] );
if ( socket == null ) {
log.error( "++++ failed to create connection to: " + servers[i] + " -- only " + j + " created." );
break;
}
//加入socket到连接池 q里慢慢?nbsp;
addSocketToPool( availPool, servers[i], socket );
if ( log.isDebugEnabled() )
log.debug( "++++ created and added socket: " + socket.toString() + " for host " + servers[i] );
}
}
}
Java代码
MemcachedClient mcc = new MemcachedClient();
mcc.set("6", 1);
mcc.set("6", 1);
q里key 如何定位C台server呢？我先把一致性hash法的定位方法说下?nbsp;
Java代码
//得到定位server的Socket装对象
SockIOPool.SockIO sock = pool.getSock( key, hashCode );
SockIOPool.SockIO sock = pool.getSock( key, hashCode );
Java代码
//计算出key对应的hash?md5) Q然?nbsp;
long bucket = getBucket( key, hashCode );
long bucket = getBucket( key, hashCode );
Java代码
//得到大于hash的mapQ因为treemap已经排好序了(jin)。调用tailMap可以得到大于{于q个hash的对?nbsp;Q然后调用firstKey得到圆环上的hash?nbsp;
SortedMap<Long,String> tmap =
this.consistentBuckets.tailMap( hv );
return ( tmap.isEmpty() ) ? this.consistentBuckets.firstKey() : tmap.firstKey();
SortedMap<Long,String> tmap =
this.consistentBuckets.tailMap( hv );
return ( tmap.isEmpty() ) ? this.consistentBuckets.firstKey() : tmap.firstKey();
]]>
Memcached是danga.comQ运营LiveJournal的技术团队）(j)开发的一套分布式内存对象~存pȝQ用于在动态系l中减少数据库负载，提升性能。LJ每秒动态页面访问量几千ơ，用户700万。Memcached数据库负蝲大幅度降低，更好的分配资源，更快速访问?/span>
关于q个东西Q相信很多h都用q，本文意在通过对memcached的实现及(qing)代码分析Q获得对q个?gu)的开源Y件更深入的了(jin)解，q可以根据我们的需要对其进行更q一步的优化。末?jin)将通过对BSM_Memcache扩展的分析，加深?memcached的用方式理解。?/span>
1.Memcached是什?/span>
在阐q这个问题(sh)前，我们首先要清楚它“不是什?#8221;。很多h把它当作和SharedMemory那种形式的存储蝲体来使用Q虽然memcached使用?jin)同L(fng)“Key=>Value”方式l织数据Q但是它和共享内存、APC{本地缓存有非常大的区别。Memcached是分布式的，也就是说它不是本地的。它Z|络q接Q当然它也可以用localhostQ方式完成服务，本n它是一个独立于应用的程序或守护q程QDaemon方式Q。即Memcached是高性能的，分布式的内存对象~存pȝQ用于在动态应用中减少数据库负载，提升讉K速度?/span>
Memcached 使用libevent库实现网l连接服务，理论上可以处理无限多的连接，但是它和Apache不同Q它更多的时候是面向E_的持l连接的Q所以它实际的ƈ发能力是有限制的。在保守情况下memcached的最大同时连接数?00Q这和LinuxU程能力有关p，q个数值是可以调整的。关?libevent可以参考相x(chng)档?Memcached内存?sh)用方式也和APC不同。APC是基于共享内存和MMAP的，memcachd有自q内存分配法和管理方式，它和׃n内存没有关系Q也没有׃n内存的限Ӟ通常情况下，每个memcachedq程可以理2GB的内存空_(d)如果需要更多的I间Q可以增加进E数?/span>
2. Memcached适合什么场?/span>
在很多时候，memcached都被滥用?jin)，q当然少不了(jin)对它的抱怨。我l常在论坛上看见有h发脓(chung)Q类g“如何提高效率”Q回复是“用memcached”Q至于怎么用，用在哪里Q用来干什么一句没有。memcached不是万能的，它也不是适用在所有场合?/span>
Memcached ?#8220;分布?#8221;的内存对象缓存系l，那么是_(d)那些不需?#8220;分布”的，不需要共享的Q或者干脆规模小到只有一台服务器的应用， memcached不会(x)带来M好处Q相反还?sh)(x)拖慢系l效率，因ؓ(f)|络q接同样需要资源，即是UNIX本地q接也一栗在我之前的试数据中显C， memcached本地d速度要比直接PHP内存数组慢几十倍，而APC、共享内存方式都和直接数l差不多。可见，如果只是本地U缓存，使用 memcached是非怸划算的?/span>
Memcached在很多时候都是作为数据库前端cache使用的。因为它比数据库了(jin)很多SQL解析、磁盘操作等开销Q而且它是使用内存来管理数据的Q所以它可以提供比直接读取数据库更好的性能Q在大型pȝ中，讉K同样的数据是很频J的Q?memcached可以大大降低数据库压力，使系l执行效率提升。另外，memcached也经怽为服务器之间数据׃n的储媒介Q例如在SSOpȝ中保存系l单点登陆状态的数据可以保存在memcached中，被多个应用共享?/span>
需要注意的是，memcached使用内存理数据Q所以它是易qQ当服务器重启，或者memcachedq程中止Q数据便?x)丢失，所?memcached不能用来持久保存数据。很多h的错误理解，memcached的性能非常好，好到?jin)内存和盘的对比程度，其实memcached使用内存q不?x)得到成百上千的d速度提高Q它的实际瓶颈在于网l连接，它和使用盘的数据库pȝ相比Q好处在于它本n非常“?#8221;Q因为没有过多的开销和直接的d方式Q它可以L应付非常大的数据交换量，所以经怼(x)出现两条千兆|络带宽都满负荷?jin)，memcachedq程本nq不占用多少CPU资源的情c(din)?/span>
通常的网늼存方式有动态缓存和?rn)态缓存等几种Q在ASP.NET中已l可以实现对面局部进行缓存，而用memcached的缓存比 ASP.NET的局部缓存更加灵z，可以~存?sh)L的对象，不管是否在页面上输出。而memcached最大的优点是可以分布式的部|Ԍq对于大规模应用来说也是必不可少的要求?/span>
LiveJournal.com使用?jin)memcached在前端进行缓存，取得?jin)良好的效果Q而像wikipedia,sourceforge{也采用?jin)或卛_采用memcached作ؓ(f)~存工具。memcached可以大规模网站应用发挥巨大的作用?/span>
2.1 memcached 的工作原?/span>
首先 memcached 是以守护E序方式q行于一个或多个服务器中Q随时接受客L(fng)的连接操作，客户端可以由各种语言~写Q目前已知的客户?API 包括 Perl/PHP/Python/Ruby/Java/C#/C {等。PHP {客L(fng)在与 memcached 服务建立q接之后Q接下来的事情就是存取对象了(jin)Q每个被存取的对象都有一个唯一的标识符 keyQ存取操作均通过q个 key q行Q保存到 memcached 中的对象实际上是攄内存?sh)的Qƈ不是保存?cache 文g中的Q这也是Z?memcached 能够如此高效快速的原因。注意，q些对象q不是持久的Q服务停止之后，里边的数据就?x)丢失?/span>
.2 memcached 安装
首先是下?memcached ?jin)，目前最新版本是 1.1.12Q直接从官方|站卛_下蝲?memcached-1.1.12.tar.gz。除此之外，memcached 用到?libeventQ我下蝲的是 libevent-1.1a.tar.gz
接下来是分别?libevent-1.1a.tar.gz ?memcached-1.1.12.tar.gz 解开包、编译、安装：(x)
# tar -xzf libevent-1.1a.tar.gz
# cd libevent-1.1a
# ./configure --prefix=/usr
# make
# make install
# cd ..
# tar -xzf memcached-1.1.12.tar.gz
# cd memcached-1.1.12
# ./configure --prefix=/usr
# make
# make install
# cd libevent-1.1a
# ./configure --prefix=/usr
# make
# make install
# cd ..
# tar -xzf memcached-1.1.12.tar.gz
# cd memcached-1.1.12
# ./configure --prefix=/usr
# make
# make install
安装完成之后Qmemcached 应该?/usr/bin/memcached?/span>
3.如何使用memcached-Server?
在服务端q行Q?/span>
# ./memcached -d -m 2048 -l 10.0.0.40 -p 11211 -u httpd
-d 以守护程序（daemonQ方式运?memcachedQ?/span>
-m 讄 memcached 可以使用的内存大，单位?MQ?/span>
-l 讄监听?IP&nb
sp;地址Q如果是本机的话Q通常可以不设|此参数Q?/span>
-p 讄监听的端口，默认?11211Q所以也可以不设|此参数Q?/span>
-u 指定用户Q如果当前ؓ(f) root 的话Q需要用此参数指定用户?/span>
q将?x)启动一个占?G内存的进E，q打开11211端口用于接收h。由?2位系l只能处?G内存的寻址Q所以在大于4G内存?sh)用PAE?2位服务器上可以运?-3个进E，q在不同端口q行监听?/span>
4. 如何使用memcached-Client?
在应用端包含一个用于描qClient的Class后，可以直接用，非常单?/span>
PHP Example:
$options["debug"] = false;
$memc = new MemCachedClient($options);
$myarr = array("one","two", 3);
$memc->set("key_one", $myarr);
$options["servers"] = array("192.168.1.41:11211", "192.168.1.42:11212");
$val = $memc->get("key_one");
print $val[0]."\n"; // prints 'one‘
print $val[1]."\n"; // prints 'two‘
print $val[2]."\n"; // prints 3
$memc = new MemCachedClient($options);
$myarr = array("one","two", 3);
$memc->set("key_one", $myarr);
$options["servers"] = array("192.168.1.41:11211", "192.168.1.42:11212");
$val = $memc->get("key_one");
print $val[0]."\n"; // prints 'one‘
print $val[1]."\n"; // prints 'two‘
print $val[2]."\n"; // prints 3
5.Z么不使用数据库做q些Q?/span>
暂且不考虑使用什么样的数据库(MS-SQL, Oracle, Postgres, MysQL-InnoDB, etc..), 实现事务(ACIDQAtomicity, Consistency, Isolation, and Durability )需要大量开销Q特别当使用到硬盘的时候，q就意味着查询可能?x)阻塞。当使用不包含事务的数据库（例如Mysql-MyISAMQ，上面的开销不存在，但读U程又可能会(x)被写U程d。Memcached从不dQ速度非常快?/span>
6.Z么不使用׃n内存?
最初的~存做法是在U程内对对象q行~存Q但q样q程间就无法׃n~存Q命中率非常低，D~存效率极低。后来出C(jin)׃n内存的缓存，多个q程或者线E共享同一块缓存，但毕竟还是只能局限在一台机器上Q多台机器做相同的缓存同h一U资源的费Q而且命中率也比较低?/span>
Memcached Server和Clients共同工作Q实现跨服务器分布式的全局的缓存。ƈ且可以与Web Server共同工作QW(xu)eb Server对CPU要求高，对内存要求低QMemcached Server对CPU要求低，对内存要求高Q所以可以搭配用?/span>
7.Mysql 4.x的缓存怎么?
Mysql查询~存?sh)是很理惻I因ؓ(f)以下几点Q?/span>
当指定的表发生更新后Q查询缓存(sh)(x)被清I。在一个大负蝲的系l上q样的事情发生的非常频繁Q导致查询缓存效率非怽Q有的情况下甚至q(sh)如不开Q因为它对cache的管理还是会(x)有开销?/span>
?2位机器上QMysql对内存的操作q是被限制在4G以内Q但memcached可以分布开Q内存规模理Z不受限制?/span>
Mysql上的是查询缓存，而不是对象缓存，如果在查询后q需要大量其它操作，查询~存帮不上忙了(jin)?/span>
如果要缓存的数据不大Qƈ且查询的不是非常频繁Q这L(fng)情况下可以用Mysql 查询~存Q不然的话memcached更好?/span>
8.数据库同步怎么P
q里的数据库同步是指的类似Mysql Master-Slave模式的靠日志同步实现数据库同步的机制?/span>
你可以分布读操作Q但无法分布写操作，但写操作的同步需要消耗大量的资源Q而且q个开销是随着slave服务器的增长而不断增长的?/span>
下一步是要对数据库进行水q_分，从而让不同的数据分布到不同的数据库服务器组上，从而实现分布的dQ这需要在应用中实现根据不同的数据q接不同的数据库?/span>
当这一模式工作后（我们也推荐这样做Q，更多的数据库D更多的让人头疼的g错误?/span>
Memcached可以有效的降低对数据库的讉KQ让数据库用主要的精力来做不频繁的写操作Q而这是数据库自己控制的，很少?x)自己阻?自己?/span>
9.Memcached快吗Q?/span>
非常快，它用libeventQ可以应付(sh)Q意数量打开的连接（使用epollQ而非pollQ，使用非阻塞网lIOQ分布式散列对象C同的服务器，查询复杂度是O(1)?/span>
10.memcached的相x(chng)象类
public abstract class BaseManager<T extends BaseObject> implements Manager<T> 
{
private MemcachedClient memcached;
protected int default_cache_time_second = 3600;
public void setMemcached(MemcachedClient memcached){
this.memcached = memcached;
}
public MemcachedClient getMemcached(){
return memcached;
}
public void setDefault_cache_time_second(int default_cache_time_second){
this.default_cache_time_second = default_cache_time_second;
} public Object getCacheValueFromMemcached(String key){
return getCacheValueFromMemcached(key, null);
}
public Object getCacheValueFromMemcached(String key, Object obj){
Object new_obj = null;
try{
new_obj = memcached.get(key);
} catch (Exception e){
if (logger.isWarnEnabled()){
logger.warn("Failed to get from MeMCache", e);
}
}
return (new_obj != null) ? new_obj : obj;
}
public void setCacheValueToMemcached(String cacheKey, int time_to_live, Serializable obj){
if (null != memcached.get(cacheKey)){
memcached.replace(cacheKey, time_to_live, obj);
} else{
memcached.add(cacheKey, time_to_live, obj);
}
}
}
{ private MemcachedClient memcached;
protected int default_cache_time_second = 3600;
public void setMemcached(MemcachedClient memcached)
{ this.memcached = memcached;
}
public MemcachedClient getMemcached()
{ return memcached;
}
public void setDefault_cache_time_second(int default_cache_time_second)
{ this.default_cache_time_second = default_cache_time_second;
} public Object getCacheValueFromMemcached(String key)
{return getCacheValueFromMemcached(key, null);
}
public Object getCacheValueFromMemcached(String key, Object obj)
{Object new_obj = null;
try
{new_obj = memcached.get(key);
} catch (Exception e)
{if (logger.isWarnEnabled())
{logger.warn("Failed to get from MeMCache", e);
}
}
return (new_obj != null) ? new_obj : obj;
}
public void setCacheValueToMemcached(String cacheKey, int time_to_live, Serializable obj)
{ if (null != memcached.get(cacheKey))
{ memcached.replace(cacheKey, time_to_live, obj);
} else
{ memcached.add(cacheKey, time_to_live, obj);
}
}
}
11.service调用memcache的一个例子：(x)
public List<Integer> getLastModifyAlbumMember(int limit) {
String cacheKey = this.createCachekey(new
Object[]{"schedule","lastmodifyalbumember",limit});
List list = (List) this.getCacheValueFromMemcached(cacheKey);
List<Integer> list1 = new ArrayList<Integer>();
if(null!=list&&list.size()>0){
for (Object aList : list){
list1.add(Integer.parseInt(String.valueOf(aList)));
}
}
return list1;
}
{String cacheKey = this.createCachekey(new
Object[]
{"schedule","lastmodifyalbumember",limit});List list = (List) this.getCacheValueFromMemcached(cacheKey);
List<Integer> list1 = new ArrayList<Integer>();
if(null!=list&&list.size()>0)
{for (Object aList : list)
{list1.add(Integer.parseInt(String.valueOf(aList)));
}
}
return list1;
}
参考资料：(x)
http://www.danga.com/
http://www.linuxjournal.com/article/7451
]]>