The definitive introduction to D8 and Composer

Note: This blog post was written in 2015 to cover Drupal 8.0, and is no longer up to date. Composer Manager is deprecated and should not be used. Here’s an updated version.

We’re making a new site based on Drupal 8. This site needs to display addresses, and some quick research leads us to the Address module.
However, after downloading and trying to enable Address, we get the following message: Address requires the commerceguys/addressing library.

In the old days a requirement like this would be satisfied by manually downloading and extracting the library into sites/all/libraries/addressing. Nowadays that’s no longer possible, PHP libraries must be installed using Composer.

What’s Composer?

Composer is a tool for managing dependencies on the project level, a project being your site or web application.

Let’s say our (non-Drupal) site needs the commerceguys/intl and commerceguys/addressing libraries (“packages” in Composer-speak). We would create a composer.json file, and list the dependencies there:

Available packages are listed on Packagist, a central repository anyone can add to. Running composer install will download the dependencies:

Notice how Composer has installed two more packages we didn’t ask for.
Those are the dependencies of our dependencies. You can see them listed in the commerceguys/addressing composer.json file.

Once installed, packages can be kept up to date by running composer update.
Composer knows which versions it’s allowed to install/update to by looking at the composer.json file. The dev-master in our examples means that it should always fetch the latest dev release. We could instead use wildcards to allow only releases of a specific branch.

If you’ve ever used Drush Make to download Drupal modules and themes, then all of this sounds familiar. You can think of Composer as the more advanced Drush Make that works for all PHP projects and packages. Compared to Drush Make, Composer has the benefit of being able to recursively resolve dependencies (downloading the dependencies of each dependency) and being able to detect conflicts.
For example, let’s say package A and package B both require Guzzle, the HTTP client. However, package A specifies that it only works with Guzzle5, while package B specifies that it only works with Guzzle6. In this case Composer will abort the install process and warn us.

Composer is clearly an awesome tool, but why do we NEED to use it? Using Drush Make was always optional for site builders. The secret lies in the last line of Composer output: Generating autoload files.
Modern applications such as Drupal 8 consist of many classes, so it would be impractical and costly to manually include each one. Instead, the application includes one special class, called the autoloader which then automatically includes other classes when they are first needed. When Composer runs, it regenerates the autoloader, giving it the locations of the newly downloaded dependencies.

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