If you don’t know what memcache is, the official site says the following:
“memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.”
Basically, what this means is that memcached is a daemon running on a server which allows you to save and retrieve variables in the server memory. This is primarily used to ease database load on websites and applications. The logic for doing something like this is…
- Before running a query, check if the result is available in memcache.
- If the result is in memcache, return the cached result.
- If not, run the query on the database, and store the result in memcache.
To install memcached, simply run the following command:
sudo apt-get install memcached
Once it’s installed, edit /etc/memcached.conf and change the line beginning ‘-m’ which is the amount of memory (in megabytes) to allocate to the server. You can also change the IP address that the server listens on in the line beginning ‘-l’.
Now restart the daemon by running
/etc/init.d/memcached restart
Now you have memcache set up and running on your server. A great feature of memcached is that you can easily cluster servers. If you want to do this, simply install memcache on your other servers before continuing.
At this point, I’d recommend downloading memcache.php which is a php script showing you a lot of useful information about your memcached servers. Once downloaded, put it in a web facing directory on your server, and modify the $MEMCACHE_SERVERS array with your server addresses.
Using memcache in php scripts is very easy. There are either procedural, or object oriented functions already available. Here is an example of a script which will store a simple variable and then retrieve it and display it.
<?php
$memcache = new Memcache;$memcache->addServer(’10.2.0.245′, 11211) or die (“Could not connect”);
$memcache->addServer(’10.2.0.249′, 11211) or die (“Could not connect”);$memcache->set(‘mytestvariable’, “this is the data in my test variable”, false, 60) or die (“Unable to save the data to the server”);
echo “Data has been stored in the cache<br />”;$result = $memcache->get(‘mytestvariable’);
echo “Retrieved data from the server:<br/>”;var_dump($result);
?>
Obviously if you only have one server then only use one addServer line.
The script above will store the test variable in the cache for 60 seconds. If you drop the timeout down to something like 1 second, and then sleep for 3 seconds before attempting to get the data, you will find that the data has expired and nothing will be returned.
It’s also useful to know that you can store anything which can be serialized in memcache. This means it’s safe to store things like arrays in the cache without having to ‘flatten’ the data beforehand.
Obviously how and where you implement a cache is entirely dependant on how your system works. If you have a lot of intense database usage then you will find that even caching with a short timeout will vastly reduce the amount of database queries. You should find that memcache is considerably faster than normal database access, and even faster than the mysql query cache. You also have total control over what data is cached, and how long it is cached for before your queries fall back to checking a database.
Some further reading:
Facebook engineering blog – How facebook use memcache
Fotolog case study
Also, it’s worth checking out the PHP manual page.
