最近在糾結memcache的負載均衡問題，下面這個貌似靠譜，面試完看下

原文出自

memcached是針對數據庫的緩存軟件 能有效降低數據庫的負載
下面是何如將多個memcached做成集羣負載均衡,讓memcached變的更高可用

一、安裝步驟：
1、編譯安裝libevent：

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tar zxvf libevent-1.4.9-stable.tar.gz
cd libevent-1.4.9-stable/
./configure --prefix=/usr
make && make install<span id="more-105"></span>

2、編譯安裝Memcached：

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tar zxvf memcached-1.2.6.tar.gz
cd memcached-1.2.6/
./configure --with-libevent=/usr
make && make install

3、編譯安裝magent：

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mkdir magent
cd magent/
tar zxvf magent-0.5.tar.gz
/sbin/ldconfig
sed -i "s#LIBS = -levent#LIBS = -levent -lm#g" Makefile
make
cp magent /usr/bin/magent

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二、實例：

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memcached -m 1 -u root -d -l 10.1.94.100 -p 11211
memcached -m 1 -u root -d -l 10.1.94.101 -p 11211
memcached -m 1 -u root -d -l 10.1.94.102 -p 11211
magent -u root -n 51200 -l 127.0.0.1 -p 12000 -s 10.1.94.100:11211 -s 10.1.94.101:11211 -b 10.1.94.102:11211

1、分別在10.1.94.100 – 10.1.94.102 3台機器的11211端口啓動3個Memcached進程，在12000端口開啓magent代理程序；(以此類推可在不同機器上啓動memcached)

2、10.1.94.100和101為主Memcached，10.1.94.102為備份Memcached；

3、連接上12000的magent，set key1和set key2，根據哈希算法，key1被寫入10.1.94.100和10.1.94.102的Memcached，key2被寫入
   10.1.94.101和10.1.94.102的Memcached；

4、當10.1.94.100和101的Memcached死掉，連接到12000端口的magent取數據，數據會從10.1.94.102的Memcached取出；

5、當10.1.94.100和101的Memcached重啓復活，連接到12000端口，magent會從10.1.94.100或10.1.94.101的Memcached取數據，
   由於這兩台Memcached重啓後無數據，因此magent取得的將是空值，儘管10.1.94.102的Memcached還有數據(此問題尚待改進)。

三、整個測試流程：

先在magent中添加1個key 如下:

telnet 127.0.0.1 12000
Trying 127.0.0.1…
Connected to 127.0.0.1.
Escape character is ‘^]’.
stats
memcached agent v0.4
matrix 1 -> 127.0.0.1:11211, pool size 0
matrix 2 -> 127.0.0.1:11212, pool size 0
END
set key1 0 0 8
wangqi
STORED
set key2 0 0 8
wangqi
STORED
quit
Connection closed by foreign host.

然後在2個輪詢的memcached中get剛才添加的key是否返回正確:

telnet 10.1.94.100 11211
Trying 10.1.94.100…
Connected to 10.1.94.100.
Escape character is ‘^]’.
get key1
END
get key2
VALUE key2 0 8
wangqi
END
quit
Connection closed by foreign host.
telnet 10.1.94.101 11211
Trying 10.1.94.101…
Connected to 10.1.94.101.
Escape character is ‘^]’.
get key1
VALUE key1 0 8
wangqi
END
get key2
END
quit
Connection closed by foreign host.
telnet 10.1.94.103 11211
Trying 10.1.94.103…
Connected to 10.1.94.103.
Escape character is ‘^]’.
get key1
VALUE key1 0 8
wangqi
END
get key2
VALUE key2 0 8
wangqi
END
quit
Connection closed by foreign host.
模擬10.1.94.100和101的Memcached死掉
直接kill進程然後:

telnet 127.0.0.1 12000
Trying 127.0.0.1…
Connected to 127.0.0.1.
Escape character is ‘^]’.
get key1
VALUE key1 0 8
wangqi
END
get key2
VALUE key2 0 8
wangqi
END
quit
Connection closed by foreign host.
模擬10.1.94.100和101的Memcached重啓復活
telnet 127.0.0.1 12000
Trying 127.0.0.1…
Connected to 127.0.0.1.
Escape character is ‘^]’.
get key1
END
get key2
END
quit
Connection closed by foreign host.
magent -u root(啓動用户uid) -n 51200(最大連接數) -l 127.0.0.1(ip地址) -p 12000(監聽端口) -s 127.0.0.1:11211(memcached的ip) -s 127.0.0.1:11212(memcached的ip) -b 127.0.0.1:11213(備份memcached的ip)

如果不需要輪詢hash的話 只有一主一備就可以了 但是在主當掉後備份memcached接手 然後在主服務恢復後數據會根據主服務器走….