Setting up and using memcached & memcache on Linux CentOS 5/Plesk 9

The second part of my tutorial on optimizing your server for hosting high traffic websites: installing and configuring memcached and the memcache php extension for your server. This is a little easier than the first step (setting up nginx as reverse proxy, see article below) and can be applied to any kind of dynamic website.


What's the point ?
On highly dynamic websites such as forums, news sites or any user content based website, the database server load is often very high. The more traffic you get, the more cluttered your database server becomes, sometimes rendering your website completely unavailable to visitors. Using a data caching daemon will allow you to save some data in memory instead of fetching the data from the database every time. You should know that memcached is used by major websites such as Wikipedia, SourceForge, SlashDot... need I say more?

What is memcached ?
Memcached is the daemon running on your server. Its usage is extremely simple, there are no configuration files, all you do is start the daemon on a given port, and your websites will connect to this daemon to store data in memory. Yes, the data is stored in your RAM. So when starting memcached you'll have to decide how much RAM memcached will be allowed to use. If you start memcached with a 1GB memory space, memcached will store this much data; when the cache is full some of the older data will begin to disappear from the cache.

What is memcache ?
Memcache, in our case, is the PHP extension that will allow us to connect to and make use of Memcached. This PHP extension is not part of the default ones so you'll have to download and install it (see step 2). It provides classes as well as functions that I must admit are very easy to use and understandable. In this article, I provide a mysql+memcache wrapper class for anyone to use.

What is the difference between memcached and memcache ?
Well if you've read the two points above, you should already know. In short, memcached is the daemon running on your machine; memcache is the PHP extension allowing you to make use of memcached.


1. Setting up memcached

I haven't found memcached in my repositories (might aswell try # yum install memcached just in case?) so I'll download the source and compile it. First go get the latest version from the official website.

# wget http://memcached.googlecode.com/files/memcached-1.4.1.tar.gz

# tar zxvf memcached-1.4.1.tar.gz

# cd memcached-1.4.1

# ./configure

If like me you get this message "libevent is missing" or something, you can run this command:

# yum install libevent-devel

And then run configure again:

# ./configure

Install memcached:
# make install
That's it, you're set! That was pretty easy wasn't it? We'll now see the command line arguments for starting memcached:
# memcached -d -m 1024 -l 127.0.0.1 -p 11211 -u nobody
The arguments are:
-d : start as daemon, running in the background
-m 1024: allow memcached to use up to 1024 MB of RAM (1GB)
-l 127.0.0.1: listen on local interface
-p 11211: listen on port 11211
-u nobody: run as user "nobody"

If you're not sure how much memory you should allocate to memcached, try running this command first:

# free

It will tell you how much free RAM you've got left.
Note that upon starting memcached, if all is OK, you will see no output message. To see if memcached is correctly started, run this command:

# ps aux | grep memcached

You should be seeing something like this:

nobody   13133  0.0  0.0  43580   732 ?        Ssl  07:11   0:00 memcached -d -m 128 -l 127.0.0.1 -p 11211 -u nobody
user     13143  0.0  0.0   4152   648 pts/0    R+   07:11   0:00 grep memcached

2. Setting up memcache PHP extension
The memcache PHP extension should be found in the classic repositories, so try this command:

# yum install php-pecl-memcache

If you're lucky (why should you be unlucky anyway?) the install will work fine and you'll be seeing these messages:
Installed: php-pecl-memcache.i386 0:2.2.3-1.el5_2
Dependency Installed: php-pear.noarch 1:1.4.9-4.el5.1
Complete!
Just for reference, here's a link to the official memcache website, if you need to grab the sources.

 Let's see if memcache was installed properly. First restart the httpd:
 # service httpd restart
Then place a simple php file on your website containing the following code:

phpinfo(); 

Open the PHP file in your browser (eg. http://mydomain.com/phpinfo.php ) and have a look at the output. If you can find a "memcache" section looking like the following picture, it means memcache was successfully installed.

We will now have a look at the memcache configuration file. First locate your php module configuration files folder, in my case /etc/php.d/ . You should find the newly installed "memcache.ini" configuration file. Open it up to see a list of configuration keys and their meaning.

The default options are just fine, but if you're interested, you should know that memcache offers load-balancing features through the "allow_failover" configuration key. I'm not going to make use of this feature so I will not be editing any of the settings.


3. Using memcache in your code
Unfortunately, installing both components isn't enough. You'll have to edit your code in order to make use of the caching features. Be reassured though, it couldn't be easier! There are a couple of functions you'll need to use, nothing complex.
If you want to find out the complete listing of the memcache php functions, visit the official website. Basically we'll be using 5 methods:
- Memcache::connect($host, $port, $timeout): connect to your daemon
- Memcache::get($key) : fetch data from your cache
- Memcache::set($key, $var, $flag, $expire): store data in your cache
- Memcache::delete($key): remove data from your cache
- Memcache::close(): disconnect.

You can cache any data that you want:

$mc = new Memcache;

$mc->connect("localhost", 11211);

$saved_data = $mc->get("saved_data");

if (!$saved_data) {

  $saved_data = file_get_contents("myfile.txt");

  $mc->set("saved_data", $saved_data, MEMCACHE_COMPRESSED, 60*60*24*7); // store for 7 days

}

echo $saved_data;

$mc->close();

Applied to MySQL queries:

$mc = new Memcache;

$mc->connect("localhost", 11211);

$news_articles = $mc->get("news_articles");

if (!$news_articles) {

  $news_articles = array();

  $query = "SELECT * FROM news_articles ORDER BY article_id DESC LIMIT 0,10";

  $result = mysql_query($query);

  while($row = mysql_fetch_assoc($result)) $news_articles[] = $row;

  $mc->set("news_articles", serialize($news_articles), MEMCACHE_COMPRESSED, 60*60*24*7); // store for 7 days, but don't forget to rebuild the cache when a new article is posted!

} else {

  $news_articles = unserialize($news_articles);

}

// Display articles..

$mc->close();
As you can see in the example above, I use the "serialize" and "unserialize" php functions. Why is that? The reason is because the Memcache::get() function always returns a string. So if you want to store an array of data (or an object), you'll have to serialize said array, and unserialize it after having read it from the database.
If you know a better workaround for this problem please feel free to leave a comment.


4. Wrapper class for memcache & mysql
I have just written a simple wrapper class for MySQL, making use of the powerful caching system offered by memcache. You can download the class here, I included a simple example for testing the class.
The principle is very simple: when executing a query, the script will check if the query result is already in the cache. If the data is in the cache, it is returned immediatly (no query executed). if the data is not in the cache, the query is executed, and the results are then placed in the cache with the specified "time to live".

Here are the wrapper class functions:
function MySQLMemcache($mysql_info, $memcache_info, $autoconnect=true, $enable_logging=true);
function connect();
function disconnect();
function dataQuery($query, $usecache=true, $ttl=0);
function nonDataQuery($query);
function fieldDataQuery($query, $field, $usecache=true, $ttl=0);

More documentation is provided inside the actual php file.

Thank you for reading, feel free to leave a comment if this article has been helpful to you.
I'm finished with my server optimization thematic.
TTFN!

The final solution: nginx+apache2 and memcached

Been a while!
But I'll make up for the huge time gap: this post will probably be one of the most useful I'll ever post.

I happen to be running a high traffic website, have been running it for about 5 years now. Over the past few years though, my website has known a major traffic increase which resulted in my servers being regularly cluttered and my website inaccessible. My website profile: an Invision Powerd Board based website (heavily modded though), running under PHP 5 and MySQL 5. Servers are hosted in France at OVH.com.
At first, my reasoning was quite simple: spend more money on a more powerful server. I ran about 5 or 6 server upgrades over the years. I must say it worked at first, since I was running low-end servers. But for the last couple of months the traffic became way too high, which resulted in my website being completely inaccessible for a part of the world (for visitors in remote countries such as Canada, connections frequently timed out) and just plain slow for everyone else. At that time the traffic was: nearly 60K unique visitors/day, about 10 million visits/month.
My server setup: a quad-core with 8GB of ram for Apache, and a quad-core with 4GB of ram for MySQL, both using SATA2 RAID0 HDDs. Connected to eachother with a 1 Gb link.

Well, I've finally settled for a solution that seems to be working great. The website's fast for everyone, even for me over there in China.


1. Running PHP with Fast-CGI

My website is a community based website, which means the site is strongly dynamic. Every page served is PHP, strictly no HTML. The "default" option for serving PHP is to use PHP as an Apache module. The problem with this solution is that for every page served, a new Apache/httpd process has to be loaded in memory. With high traffic website this isn't necessarily a good solution especially if your server doesn't have much RAM.
So the first thing I did was to switch PHP from Apache module to Fast-CGI module.

Those tutorials should help you set up PHP as fast-cgi module for Apache:
[english] http://www.fastcgi.com/drupal/node/5?q=node/10
[french] http://www.sos-dedie.com/2009/01/15/apache-2-worker-et-php-fastcgi/


2. Setting up nginx for serving static content

Apache will be serving dynamic content via PHP as FastCGI module. But on top of Apache, we'll be using another webserver, a very lightweight one, for serving static content. Basically, this means php pages will be served by Apache, but other content (images, javascript, css, static html...) is served by nginx, which is extremely fast and reliable for such things.
How to put such a set up in production? Simple as that: nginx on port 80, Apache on port 8080 (or another) and nginx is configured to redirect all dynamic content to Apache. It's called "using nginx as reverse-proxy".

Here are a few articles about it:
[english] http://kovyrin.net/2006/05/18/nginx-as-reverse-proxy/
[english] http://wiki.joyent.com/accelerators:nginx_apache_proxy
[french] http://www.papygeek.com/software/optimiser-son-serveur-web-avec-nginx/
Both english articles don't cover vhosts issues, so if I ever get comments asking me how to proceed, I'll post a new article about it.


3. Memcached and memcache

I had a bit of trouble figuring that one out as I couldn't really find any article explaining the difference between memcache and memcached. So here's the deal.
- Basically "memcached" (note the trailing "d", which stands for "daemon") is a process that runs on your machine and that allows you to easily cache data in memory--RAM. It's basically a simple and efficient cache manager. It listens on a given port and you can connect to it via...
- memcache: this is the name of the PHP module that allows you to make use of memcached. You're going to need to install this because it doesn't come with PHP! Memcached and memcache can be found in the usual repositories (eg. rpmforge)

[english] http://www.danga.com/memcached/ < Memcached official website
[english] http://www.php.net/memcache < Memcache (PHP module) official website

Installing and configuring both isn't the only thing you have to do. You're going to have to make use of the memcache PHP module functions. That's the trick! But I'll guide you through it.

Here are a couple of handy functions:
memcache_connect ($host, $port, $timeout) : connects to the memcache server you've set up on your machine.
memcache_get ($key) : gets a string from the cache. Returns null if the string was not found.
memcache_set ($key, $data, $flag, $ttl) : save a string into the cache.
memcache_delete ($key) : deletes the string from the cache

Now how to use these functions: this couldn't be any simpler.
- Begin by connecting to the memcached server using memcache_connect()
- Before running any SQL query, ask yourself: can this query be cached? In theory, most queries can be. In my case, I used the memcache functions to optimize my portal page (index.php) which is basically a simple news article display. In other words, the content almost never changes, so this kind of query can definitely be cached.
Here is a simple code example:

// Retrieve data from cache
$articles = memcache_get( "news_articles");

// if $articles is null, it means the data isn't in the cache store
if (!$articles) {
$articles = array();
$result = mysql_query("SELECT * FROM news ORDER BY news_id DESC LIMIT 0,10");
while ($row = mysql_fetch_assoc($result)) $articles[] = $row;

// Save the data into the cache.
// Since memcache_get can only return a string, you'll have to serialize the data before saving it into the cache.
// The data is saved for 1 week as defined with the last parameter.
memcache_set( "news_articles", serialize($articles), MEMCACHE_COMPRESSED, 60*60*24*7);
}
// $articles is not null
else {
$articles = unserialize($articles); // unserialize the data
}
// You now have a fully loaded $articles array, ready for display!


Conclusion

I managed to reduce the amount of SQL queries of my main page from an average of 15 to... 5. Every single page of my website loads nearly instantly even during high influx of visitors. Lately I had about 3000 users online simultaneously, and I didn't notice any slowdowns.
So I can safely say that these 3 points described above actually solved all my issues.

What a relief!
Clem