Memcached is a service that allows entire database tables to be stored in memory, drastically speeding up queries to those tables and alleviating database load. In Drupal, the Memcached module allows you to store all cache tables in memory.
We've covered how to install Memcached before on Debian and on Mac OS X. But server software can vary significantly between sites, and this guide can be used to set up Memcached on Red Hat Enterprise Linux (RHEL) or CentOS, which are architecturally the same.
Install memcached through RPM
The easiest way to install Memcached is through a package manager such as yum or apt. However, Memcached is not available from the default collection of packages, so the first thing we need to do is add a new RPM (Red Hat Package Manager) server so that we can install Memcached through yum.
One of the best 3rd-party RPM servers is provided by Dag Wieers, which will provide us with up-to-date packages that are not provided by Red Hat directly. The one tricky part of setting up an RPM server is making sure you get the repository that matches your server version and architecture (32-bit or 64-bit). So we need to collect that information first.
From a shell prompt, get the CentOS/RedHat version number:
$ cat /etc/redhat-release
CentOS release 5.3 (Final)
Then get the server architecture information. This is a typical response for a 32-bit machine:
$ uname -a
Linux server1.example.com 2.6.18-92.1.13.el5 #1 SMP Wed Sep 24 19:33:52 EDT 2008 i686 i686 i386 GNU/Linux
Or if you have a 64-bit machine you will probably get something like this:
$ uname -a
Linux server.example.com 2.6.18-53.1.21.el5 #1 SMP Tue May 20 09:35:07 EDT 2008 x86_64 x86_64 x86_64 GNU/Linux
Now install the RPM server that matches your architecture and CentOS version from http://dag.wieers.com/rpm/FAQ.php#B2.
The server I was using when I wrote this was a 32-bit machine running CentOS version 5.x. So my particular server was:
http://apt.sw.be/redhat/el5/en/i386/rpmforge/RPMS/rpmforge-release-0.3.6-1.el5.rf.i386.rpm
To install a new RPM server, we can just use the rpm command. Note that you must find the RPM server string that matches your architecture and software. Do not use the URL unless you have a 32-bit machine running CentOS 5.x, instead get the server that's appropriate from http://dag.wieers.com/rpm/FAQ.php#B2.
$ rpm -Uhv http://apt.sw.be/redhat/el5/en/i386/rpmforge/RPMS/rpmforge-release-0.3.6-1.el5.rf.i386.rpm
Now we can simply use yum (or apt) to install Memcached:
$ yum install memcached
Afterwards you can confirm memcached is up and running by calling it.
$ memcached -h
memcached 1.2.6
Install the Memcache PECL Extension
Even though memcached is happily running on the server, it's not accessible from PHP without the PECL extension. Fortunately this is a very easy process, just use the pecl command.
$ pecl install memcache
Then add the memcache extension to your php.ini file, usually at /etc/php.ini.
extension=memcache.so
And finally restart Apache so that it will pick up the new extension:
$ /etc/init.d/apache2 restart
Running phpinfo() on your webserver should now confirm that memcache is installed:
Set up Memcached as a service
Just having memcache installed will not do anything by itself, we need to actually start up some instances of it for our web server to connect to, and we need memcached to automatically start up when the server restarts.
For this we need to install a new script at /etc/init.d/memcached. For this I usually use a custom script that's a bit crude, since it assumes that memcached is being used exclusively for our web server. However, most of the time this is true and it works just fine.
Download the memcached script (rename to just "memcached").
So simply load this script into /etc/init.d. Then set the permissions on it to make it executable:
$ chmod 755 memcached
Then register the script to start up with the server:
$ chkconfig --add memcached
Now you can start up memcached as a service.
$ service memcached start
And you can confirm that memcached has fired up several instances by checking ps.
$ ps -e | grep memcached
22805 ? 00:00:59 memcached
22807 ? 00:00:58 memcached
22809 ? 00:01:16 memcached
22811 ? 00:00:55 memcached
22813 ? 00:00:01 memcached
22815 ? 00:01:02 memcached
22817 ? 00:00:27 memcached
22819 ? 00:00:35 memcached
22821 ? 00:00:01 memcached
22823 ? 00:00:01 memcached
22825 ? 00:00:01 memcached
And that's it! You may need to change the /etc/init.d/memcached file to match your needs depending on what you're using Memcached for. If you're using Memcached with Drupal, you can follow the instructions for changing your settings.php file by following the instructions provided with the Memcache module. Also make sure you configure your Firewall to prevent access to Memcache from external URLs.