PHP modulo hash and consistent hash operation Memcached distributed cluster

1. Start 4 Memcached service simulation clusters

/usr/local/memcached/bin/memcached -d -p 11211 -u memcached -vv >> /var/log/memcached.11211.log 2>&1
/usr/local/memcached/bin/memcached -d -p 11212 -u memcached -vv >> /var/log/memcached.11212.log 2>&1
/usr/local/memcached/bin/memcached -d -p 11213 -u memcached -vv >> /var/log/memcached.11213.log 2>&1
/usr/local/memcached/bin/memcached -d -p 11214 -u memcached -vv >> /var/log/memcached.11214.log 2>&1

2. Modular hash algorithm

php code

<?php
/**
 * Created by PhpStorm.
 * User: jmsite.cn
 * Date: 2019/1/28
 * Time: 11:38
 */
$memcached = new Memcached();
//设置算法为取模hash
$memcached->setOptions(
    array(
        Memcached::OPT_DISTRIBUTION         => Memcached::DISTRIBUTION_MODULA,
        //Memcached::OPT_LIBKETAMA_COMPATIBLE => true,
        Memcached::OPT_REMOVE_FAILED_SERVERS=> true,
    )
);
//添加服务器
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11211&#39;);
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11212&#39;);
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11213&#39;);
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11214&#39;);
//写入12个key
for ($i =1;$i <= 12;$i++){
    $memcached->set(&#39;key_&#39;.$i, &#39;value_&#39;.$i);
}

Execute the above code and check the log

#memcached.11211.log
<28 new auto-negotiating client connection
28: Client using the ascii protocol
<28 set key_2 0 0 7
>28 STORED
<28 set key_3 0 0 7
>28 STORED
<28 set key_4 0 0 7
>28 STORED
<28 set key_10 0 0 8
>28 STORED
<28 quit
<28 connection closed.
#memcached.11212.log
<28 new auto-negotiating client connection
28: Client using the ascii protocol
<28 set key_1 0 0 7
>28 STORED
<28 set key_6 0 0 7
>28 STORED
<28 set key_9 0 0 7
>28 STORED
<28 set key_12 0 0 8
>28 STORED
<28 quit
<28 connection closed.
#memcached.11213.log
<28 new auto-negotiating client connection
28: Client using the ascii protocol
<28 set key_7 0 0 7
>28 STORED
<28 set key_8 0 0 7
>28 STORED
<28 quit
<28 connection closed.
#memcached.11214.log
<28 new auto-negotiating client connection
28: Client using the ascii protocol
<28 set key_5 0 0 7
>28 STORED
<28 set key_11 0 0 8
>28 STORED
<28 quit
<28 connection closed.

View the key distribution

##Comment out 11214//$memcached->addServer( in the php code '192.168.75.132', '11214');

Execute the php code again
View the distribution of keys

##Compare the distribution of the two keys:

The hits of key_2 and key_10 have not changed and are always in 11211. The hits of the other 10 keys have changed due to the reduction of the server

3. Consistent hash algorithm

PHP code

<?php
/**
 * Created by PhpStorm.
 * User: jmsite.cn
 * Date: 2019/1/28
 * Time: 11:38
 */
$memcached = new Memcached();
//设置算法为一致性hash
$memcached->setOptions(
    array(
        Memcached::OPT_DISTRIBUTION         => Memcached::DISTRIBUTION_CONSISTENT,
        Memcached::OPT_LIBKETAMA_COMPATIBLE => true,
        Memcached::OPT_REMOVE_FAILED_SERVERS=> true,
    )
);
//添加服务器
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11211&#39;);
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11212&#39;);
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11213&#39;);
$memcached->addServer(&#39;192.168.75.132&#39;, &#39;11214&#39;);
//写入12个key
for ($i =1;$i <= 12;$i++){
    $ret = $memcached->set(&#39;key_&#39;.$i, &#39;value_&#39;.$i);
}

Execute the above code and check the log

View the key distribution

Comment out 11214//$ in the PHP code memcached->addServer('192.168.75.132', '11214');

Execute the php code again

View the distribution of keys

Compare the keys twice Distribution:

11211 The original key hit has not changed, and key_4 has been added

11212 The original key hit has not changed

11213 The original key hit No change has occurred, key_12 has been added

There are 2 keys that have changed due to the reduction of server hits

4. Comparison

modulo hash algorithm Reducing one server to 10 key hits has changed.

The consistent hash algorithm has changed to reduce 2 key hits on one server.

Only 12 keys are tested here. The simulated data volume is too small, resulting in uneven distribution of keys. However, the change in key hits caused by the reduction of servers has nothing to do with the size of the simulated data, but is related to the hash algorithm. These tests It reflects the advantages of the consistent hash algorithm. Due to the reduction of servers, the modular hash results of a large number of keys have changed, and the servers hit have also changed; while the consistent hash algorithm key is fixed at a value of 2^32-1 On the hash ring of nodes, the position of the server's key on the hash ring will not change when the server decreases the key. It only affects the hit of the key on the reduced server. Adding a server only affects part of the keys of the server at the next position in the hash ring.

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