Memcache Consistent Hashing, Cluster, PHP code, Ketama and all about it
I have been trying for the whole day to understand and code for Memcache with PHP but I am getting confused at few points. I have gone through many articles and almost every SO questions related this but could not find exact answers.
1) What would be the code to create Consistent Hashed Key in PHP? What libraries I have to install and what I really need to do? Any good article to go through?
2) Suppose, I have successfully stored a Consistent Hashed Key, now if my any of server is down or added a new server would it make any difference even though I am using Consistent Hashed Key etc?
3) Will using Memcached::addServers() instead of Memcached::addServer() make any difference in the case of Consistent Hashing as stated in http://ru.php.net/manual/en/memcached.addserver.php if not then what means?
$m = new Memcached();
$m->setOption(Memcached::OPT_DISTRIBUTION, Memcached::DISTRIBUTION_CONSISTENT);
$m->addServers($servers);
4) Is using above code is enough for Consistent Hashing and then adding/removing servers would not make any difference to keys?
5) What is Ketama Library? and why to use it if Memcached::DISTRIBUTION_CONSISTENT can work better? following http://www.last.fm/user/RJ/journal/2007/04/10/rz_libketama_-_a_consistent_hashing_algo_for_memcache_clients
6) Do I have to hash my keys in some way or just provide my key and let the Memcached handle the rest?
Please guys I need your real support to understand and implement it my production environment as soon as possible. Your answers would let me understand what should I code for better.
Answer
Well those are many questions in once let me try my best to answer one by one.
1) What would be the code to create Consistent Hashed Key in PHP? What libraries I have to install and what I really need to do? Any good article to go through?
Well as you put your code in question this code is enough for Consistent Hashing in PHP. You just need to use your LibMemcached Client library to use Consistent Hashing with Memcached. Just add the following line below your code
$m->setOption(Memcached::OPT_LIBKETAMA_COMPATIBLE, true);
It is highly recommended to enable this option if you want to use consistent hashing, and it may be enabled by default in future releases. Follow this for bunch of Constants and their definition for better understanding http://www.php.net/manual/en/memcached.constants.php
Though a better aproach can be for more better performance by setting globally in php.ini (I haven't tested yet.)
memcache.hash_strategy = consistent;
as suggested in http://blog.fedecarg.com/2008/12/24/memcached-consistent-hashing-mechanism/ then you don't need to specify individually in each memcached calling. Default value is Standard and that uses Modulus calculation which will not be helpful if you add or remove servers.
2) Suppose, I have successfully stored a Consistent Hashed Key, now if my any of server is down or added a new server would it make any difference even though I am using Consistent Hashed Key etc?
Though as the lsmooth said There will always be an impact when servers are removed or added but minimal as suppose adding 1 server into 3 servers it will approximately 1/4 = 25% loss of keys so as many servers as less the chances of losing keys.
3) Will using Memcached::addServers() instead of Memcached::addServer() make any difference in the case of Consistent Hashing as stated in http://ru.php.net/manual/en/memcached.addserver.php if not then what means?
As said by Ismooth. He is correct. It's preferred to use addServers. Note that «addServers()» must be called after all options are set with setOption, else the options won't apply to those servers.
4) Is using above code is enough for Consistent Hashing and then adding/removing servers would not make any difference to keys?
Already answered in Question No 1 answer.
5) What is Ketama Library? and why to use it if Memcached::DISTRIBUTION_CONSISTENT can work better? http://www.last.fm/user/RJ/journal/2007/04/10/rz_libketama_-_a_consistent_hashing_algo_for_memcache_clients
LibKetama is the library on which Consistent Hashing key distribution algorithm is based on. So, using Consistent Hashing in Memcached means using LibKetama and that's what it is.
6) Do I have to hash my keys in some way or just provide my key and let the Memcached handle the rest?
Said by Yvan as "Memcached client will hash your keys automatically. Say you have 3 servers, A, B and C and 3 keys «K1» to «K9». For example, the client hash algorithm will store as follow : K1/K2/K3 stored on A, K4/K5/K6 stored on B, K7/K8/K9 stored on C. If your server B goes down, its keys (K4/K5/K6) will be stored evenly on the 2 remaining servers (A and C). For example, K4 will go to A, and K5/K6 will go to server C.
That's just an example, not the real algorithm. You can find out which key goes on which server with the function $memcached->getServerByKey( 'K4'). Then make one server go down, and see what the getServerByKey() sends you after this failure". at http://www.dugwood.com/895442.html#dwCmtForm.