Improving Web App Performance With Memcached

When we think of storage in a web application our first thought is usually a traditional database like MySQL. This is great for long term storage and data analysis, but there's a better option for many short-term needs: memcached. It's a great choice for saving bits of information between page requests and increasing performance. In this introduction we'll show you how to start using memcached with PHP.

Introduction

Memcached is simply a server with an interface to let you store things in memory. This can run on the same machine as your web server, but scalability comes from distributing instances across multiple servers. All you need is the memcached daemon running and PHP provides a simple interface with a PECL library. On a Debian-based Linux system this is as easy as:

$ sudo apt-get install memcached
$ sudo service memcached start
$ sudo pecl install memcached
$ sudo service httpd restart

Now we should mention there are technically two PHP libraries available to work with memcached. The older library is called "memcache" and is lacking in some features. The newer "memcached" library uses libmemcached and is generally preferred.

The first step in PHP is to connect to the server. Connections can be persisted across requests, which is nice for performance. Then add to the server list as needed. In this case we'll use a locally running instance on the default port:

function get_memcached() {
    // Set a persistent connection ID
    $mc = new Memcached('webapp');
    // Set a short timeout (in milliseconds) so if the server goes down
    // it doesn't take down our site with it
    $mc->setOption(Memcached::OPT_CONNECT_TIMEOUT, 1000);
    if ( !$mc->getServerList() ) {
        if ( !$mc->addServer('localhost', 11211) ) {
            error_log('Could not add memcached server.');
            return false;
        }
    }
    return $mc;
}

Now you can read and write PHP variables to memcached with simple functions based on keys you define. They'll be serialized and deserialized automatically. You can't write resources like database connections or result sets, but you can turn those result sets into arrays and store those.

$mc->set('list', array(1, 2, 3));
$list = $mc->get('list');

Storing Data

Let's say we want to store a list of recently visited URLs for each logged in user. We could use sessions but that wouldn't work across devices and it'll disappear as soon as the session is cleared. We could use a database but that's slow for this kind of data that's most likely not critical to our system. Using memcached this is easy:

$user_id = 123;  // The current user's ID

$recents = array();
if ($mc = get_memcached()) {
    // Get any stored in memcached
    $recents = $mc->get("recents-$user_id");
    if (!$recents) {
        $recents = array();
    }
    $recents[] = $_SERVER['REQUEST_URI'];
    $recents = array_unique($recents);
    $mc->set("recents-$user_id", $recents);
}

print_r($recents);

Caching

Now let's really boost your web application by caching database results. Database queries are usually the biggest bottleneck in server processing, so avoiding duplicate queries by caching results in memory can provide a huge performance gain. The simplest method is to query and store just with primary keys. Typically it's best that the cached value be deleted when the database record is updated so a user never sees any out-of-date values.

function store_product($id, $name, $price) {
    // Create or update the product in the database
    $db = get_db();
    $qry = $db->prepare('REPLACE INTO product (id, name, price) VALUES (:id, :name, :price)');
    $qry->bindParam(':id', $id);
    $qry->bindParam(':name', $name);
    $qry->bindParam(':price', $price);
    $qry->execute();

    // Remove outdated values from cache if it's there
    if ($mc = get_memcached()) {
        $mc->delete("product-$id");
    }
}

function get_product($id) {
    $product = null;

    // First check the cache
    if ($mc = get_memcached()) {
        $product = $mc->get("product-$id");
    }

    if (!$product) {
        // Not in the cache; check the db
        $qry = $db->prepare('SELECT * FROM product WHERE id = $id');
        $qry->bindParam(':id', $id);
        if ($qry->execute()) {
            $product = $qry->fetch();

            // Cache for future use
            if ($mc) {
                $mc->set("product-$id", $product);
            }
        }
    }

    return $product;
}

Caveats

As with any technology choice there are limitations and things to watch out for:

• The max length of a key is 250 bytes. Keep your keys simple and short.
• The default max value size is about 1MB. This isn't the right place to store large values.
• Storage isn't locked for reading or writing in the way database records can be locked. Just be aware that any web request can update any value at any time.
• Be sure your memcached servers have enough combined RAM.

Next Steps

There's more you can do with memcached:

• Cached values can have timeouts. This is useful when data should be cached for a set period of time instead of deleted manually.
• Simple increment and decrement methods are useful for keeping quick counters between requests.
• With memcached configured correctly you can share data between applications written in many programming languages.

Give memcached a try. In the right scenarios it's a very simple and effective solution to maximize your web app performance.

About Matthew Schwartz

Matthew is a software architect and lead developer at Reflexions Data in NYC. In his spare time he runs DocForge.com, cooks, and shoots a mean game of pocket billiards.

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