Symfony Service Container: The Need for Speed

During the first five articles of this series on Dependency Injection, we have progressively introduced the main concepts behind this simple and useful design pattern. We have also talked about the implementation of a lightweight PHP container that will be used for Symfony 2.

But with the introduction of the XML and YAML configuration files, you might have became a bit sceptic about the performance of the container itself. Even if services are lazy loading, reading a bunch of XML or YAML files on each request and creating objects by using introspection is probably not very efficient in PHP. And because the container is almost always the corner stone of any application using it, its speed does matter a lot.

On the one hand, using XML or YAML to describe services and their configuration is very powerful and flexible:

<container xmlns="http://symfony-project.org/2.0/container">
  <parameters>
    <parameter key="mailer.username">foo</parameter>
    <parameter key="mailer.password">bar</parameter>
    <parameter key="mailer.class">Zend_Mail</parameter>
  </parameters>
  <services>
    <service id="mail.transport" class="Zend_Mail_Transport_Smtp" shared="false">
      <argument>smtp.gmail.com</argument>
      <argument type="collection">
        <argument key="auth">login</argument>
        <argument key="username">%mailer.username%</argument>
        <argument key="password">%mailer.password%</argument>
        <argument key="ssl">ssl</argument>
        <argument key="port">true</argument>
      </argument>
    </service>
    <service id="mailer" class="%mailer.class%">
      <call method="setDefaultTransport">
        <argument type="service" id="mail.transport" />
      </call>
    </service>
  </services>
</container>

But, on the other hand, defining the service container as a plain PHP class gives you the full speed, as seen during the second article of this series:

class Container extends sfServiceContainer
{
  static protected $shared = array();

  protected function getMailTransportService()
  {
    return new Zend_Mail_Transport_Smtp('smtp.gmail.com', array(
      'auth'     => 'login',
      'username' => $this['mailer.username'],
      'password' => $this['mailer.password'],
      'ssl'      => 'ssl',
      'port'     => 465,
    ));
  }

  protected function getMailerService()
  {
    if (isset(self::$shared['mailer']))
    {
      return self::$shared['mailer'];
    }

    $class = $this['mailer.class'];

    $mailer = new $class();
    $mailer->setDefaultTransport($this->getMailTransportService());

    return self::$shared['mailer'] = $mailer;
  }
}

The above code does the bare minimum to provide flexibility thanks to the configuration variables, and still be very fast.

How can you have the best of both world? That’s quite simply. The Symfony Dependency Injection component provides yet another built-in dumper: a PHP dumper. This dumper can convert any service container to plain PHP code. That’s right, it is able to generate the code you could have written by hand in the first place.

Let’s use again our Zend_Mail example and for brevity’s sake, let’s use the XML definition file created in the previous article:

$sc = new sfServiceContainerBuilder();

$loader = new sfServiceContainerLoaderFileXml($sc);
$loader->load('/somewhere/container.xml');

$dumper = new sfServiceContainerDumperPhp($sc);

$code = $dumper->dump(array('class' => 'Container'));

file_put_contents('/somewhere/container.php', $code);

As for any other dumper, the sfServiceContainerDumperPhp class takes a container as its constructor first argument. The dump() method takes an array of options, and one of them is the name of the class to generate.

Here is the generated code:

class Container extends sfServiceContainer
{
  protected $shared = array();

  public function __construct()
  {
    parent::__construct($this->getDefaultParameters());
  }

  protected function getMailTransportService()
  {
    $instance = new Zend_Mail_Transport_Smtp('smtp.gmail.com', array(
      'auth' => 'login',
      'username' => $this->getParameter('mailer.username'),
      'password' => $this->getParameter('mailer.password'),
      'ssl' => 'ssl',
      'port' => 465
    ));

    return $instance;
  }

  protected function getMailerService()
  {
    if (isset($this->shared['mailer'])) return $this->shared['mailer'];

    $class = $this->getParameter('mailer.class');
    $instance = new $class();
    $instance->setDefaultTransport($this->getMailTransportService());

    return $this->shared['mailer'] = $instance;
  }

  protected function getDefaultParameters()
  {
    return array (
      'mailer.username' => 'foo',
      'mailer.password' => 'bar',
      'mailer.class' => 'Zend_Mail',
    );
  }
}

If you have a closer look at the code generated by the dumper, you will notice that the code is very similar to the one we wrote by hand.

By using the sfServiceContainerDumperPhp dumper, you can have the best of both world: the flexibility of the XML or YAML format to describe and configure your services, and the speed of an optimized and auto-generated PHP file.

Of course, as projects have almost always different settings for different environments, you can of course generate different container classes, based on the environment or on a debugging setting. Here is a small snippet of PHP code that illustrates how to build the container dynamically for the very first request, and use a cached one for all other requests when not in debugging mode:

$name = 'Project'.md5($appDir.$isDebug.$environment).'ServiceContainer';
$file = sys_get_temp_dir().'/'.$name.'.php';

if (!$isDebug && file_exists($file))
{
  require_once $file;
  $sc = new $name();
}
else
{
  // build the service container dynamically
  $sc = new sfServiceContainerBuilder();
  $loader = new sfServiceContainerLoaderFileXml($sc);
  $loader->load('/somewhere/container.xml');

  if (!$isDebug)
  {
    $dumper = new sfServiceContainerDumperPhp($sc);

    file_put_contents($file, $dumper->dump(array('class' => $name));
  }
}

That wraps up our tour of the Symfony 2 Dependency Injection Container.

Before closing this series, I want to show you yet another great feature of the dumpers. Dumpers can do a lot of different things, and to demonstrate the decoupling of the implementation of the component, I have implemented a Graphviz dumper. What for? To help you visualize your services and their dependencies.

First, let’s see how to use it on our example container:

$dumper = new sfServiceContainerDumperGraphviz($sc);
file_put_contents('/somewhere/container.dot', $dumper->dump());

The Graphviz dumper generates a dot representation of your container:

digraph sc {
  ratio="compress"
  node [fontsize="11" fontname="Myriad" shape="record"];
  edge [fontsize="9" fontname="Myriad" color="grey" arrowhead="open" arrowsize="0.5"];

  node_service_container [label="service_container\nsfServiceContainerBuilder\n", shape=record, fillcolor="#9999ff", style="filled"];
  node_mail_transport [label="mail.transport\nZend_Mail_Transport_Smtp\n", shape=record, fillcolor="#eeeeee", style="dotted"];
  node_mailer [label="mailer\nZend_Mail\n", shape=record, fillcolor="#eeeeee", style="filled"];
  node_mailer -> node_mail_transport [label="setDefaultTransport()" style="dashed"];
}

This representation can be converted to an image by using the dot program:

$ dot -Tpng /somewhere/container.dot > /somewhere/container.png

For this simple example, the visualization has no real added value, but as soon as you start having more than a few services, it can be quite useful… and beautiful.

As a teaser for the next Symfony component that will be released later this month, here is the graph for an hypothetic CMS using the Symfony 2 new Templating Framework:

That’s all for this series on Dependency Injection. I hope you have learned something reading these articles. I also hope you will try the Symfony 2 Service Container component soon and give me feedback about your usage. Also, if you create “recipes” for some existing Open-Source libraries, consider sharing them with the community. You can also send them to me and I will host them along side the container component to ease reusing.