Background

Chromium is a big project, and it makes sense to modularize. Many sub-teams work largely independently, such as the sandbox and V8 teams, and the WebKit code is merged on regular schedules. It makes sense to modularize such that these teams can work independently, yet still share common code.

For an overview of the modules that exist, please see Chromium code layout.

Quick start

Say you need to make a change that adds or changes something in base for the benefit of something in chrome.

First you make your change, get it reviewed and checked in. At this point, Chromium is still pulling a specific, older revision of base without your changes. It does this by pulling in a specific version of webkit, and then specifying that it should use the version of base that the webkit module is currently using (see the "From" syntax below). Nobody will see your change because of this.

Then you update the webkit module to pull in your new version of base. First you need to figure out which version your new version is. It's a good idea to first do svn update in base/ to make sure everything is up-to-date, then do svn info. You'll see a Revision line that tells you the current revision number of the current module. Put this revision number into the webkit/DEPS file for the base module. Get this change reviewed and submitted.

At this point, chrome is still pulling a specific version of webkit that pulls base without your change. Repeat the same procedure as before, but this time, make chrome pull the latest version of webkit. Along with this deps file changes should go your corresponding changes to Chromium. This allows you to atomically pull in the most recent version and update Chromium to be compatible with it.

Dependencies

Each project depends on a variety of modules. For example, the WebKit project depends on the networking code, our shared base code, and some third-party libraries. Chromium and V8 in turn both depend on WebKit. (In practice, Chromium depends on everything since we are in the Chromium tree). Each project must list the projects it depends on, and which version it wants to pull. We do not support transitive dependencies, so each project must list all of the projects it depends on.

A user's client specifies a set of target solutions to check out. (Yes, please excuse the Visual Studio jargon.) In the simplest case, a user's client only specifies a single solution, say the chrome solution. The root directory of each solution has a text file named DEPS that defines the set of dependencies for the project. Users can override these dependencies if desired.

The contents of the DEPS file is a python associative array, which looks something like the following:

deps = {
  "breakpad" : "http://google-breakpad.googlecode.com/svn/trunk@189",
  "webkit"   : "http://src.chromium.org/chrome/trunk/src/webkit@3395",
  "v8"       : "http://v8.googlecode.com/svn/trunk@77829",
  ...
}

The DEPS file can easily be used to express a dependency on a subversion tag and other subversion servers:

deps = {
  ...
  "breakpad" : "http://google-breakpad.googlecode.com/svn/tags/1.0.29.0",
  ...
}

While it is possible for a DEPS file to specify that the trunk of a dependency be used, it is intended that a DEPS file instead specify a known good revision number or tag. This ensures that Chromium developers, for example, are insulated from activity on the trunk of a dependency. When the maintainer(s) of the dependency decide to make Chromium use the new revision of the dependency, they just need to contribute a change to the chrome/DEPS file.

The "From" keyword

Given that dependencies are not computed recursively, it can be a pain to maintain complex dependency trees manually, especially if modules have multiple overlapping dependencies. To simplify such situations, gclient supports the From keyword which can be used to express a dependency in terms of the DEPS file of another module. For example, you could have:

deps = {
  "breakpad" : "http://google-breakpad.googlecode.com/svn/trunk@189",
  "webkit"   : "http://src.chromium.org/chrome/trunk/src/webkit@3395",
  "v8"       : From("webkit"),
  ...
}

The use of the From keyword above indicates that the version of "v8" to be pulled should be determined by inspecting webkit/DEPS.

Test Data

Some modules like webkit have very large amounts of test data. For these modules, a good convention is to define a separate module for the test data. In the case of the webkit module, webkit/DEPS can define a dependency on the webkit test data module, but chrome/DEPS can exclude this dependency. As a result, Chromium developers are exempted from having to checkout the WebKit test data by default, but this also does not inconvenience WebKit developers who prefer to check out the test data. Also, by having the test data in a parallel directory, the cost of manually updating just the webkit directory is minimized.

Tooling

To facilitate working with a bunch of separately versioned modules, some tooling is needed.

Installation

Checkout the depot_tools package, which includes gclient, gcl, and svn:

$ git clone https://chromium.googlesource.com/chromium/tools/depot_tools

Add the depot_tools directory to your PATH.

Basic Usage

To checkout Chromium the first time, follow these instructions. 

Advanced Usage

The contents of a default .gclient file looks something like:

solutions = [
  { "name"        : "chrome",
    "url"         : "http://src.chromium.org/chrome/trunk/src/chrome",
    "custom_deps" : {}
  }
]

An element of the solutions array (a solution) describes a repository directory that will be checked out into your working copy. Each solution may optionally define additional dependencies (via its DEPS file) to be checked out alongside the solution's directory. A solution may also specify custom dependencies (via the custom_deps property) that override or augment the dependencies specified by the DEPS file.

Users can edit this file to add new solutions or alter dependencies for a particular solution.

For example, a V8 developer may wish to checkout the V8 trunk alongside a stable version of Chromium. So, they might setup a .gclient file like so:

solutions = [
  { "name"        : "chrome",
    "url"         : "http://src.chromium.org/chrome/trunk/src/chrome@5000",
    "custom_deps" : {
      "v8" : "http://v8.googlecode.com/svn/trunk"
    }
  }
]

The above specifies that a particular revision of Chromium, r5000, should be used instead of the Chromium trunk. It then specifies that the V8 trunk should be used instead of the version of V8 specified by chrome/DEPS.

The V8 example is somewhat simple since V8 does not itself have other dependencies. For modules like webkit, specifying custom dependencies for each of WebKit's dependencies is tedious and could potentially get out of sync with what other developers are using (as specified in webkit/DEPS). So, an alternative approach to working with the trunk of another module is to set up a second solution:

solutions = [
  { "name"        : "chrome",
    "url"         : "http://src.chromium.org/chrome/trunk/src/chrome@5000",
    "custom_deps" : {
      "skia"      : None,
      "webkit"    : None,
      "v8"        : None
    }
  },
  { "name"        : "webkit",
    "url"         : "http://src.chromium.org/chrome/trunk/src/webkit",
    "custom_deps" : {}
  }
]

The above .gclient file specifies that the svn URLs given in chrome/DEPS for the skia, webkit, and v8 modules should be ignored. Those modules are then fetched according to webkit/DEPS. Both chrome/DEPS and webkit/DEPS, in this example, specify other common dependencies such as base. The tool will complain if the two solutions specify conflicting dependencies. A user must explicitly ignore a dependency that conflicts.