Codelines, Branches, and Releases

The Android Open Source Project (AOSP) maintains a complete software stack to be ported by OEMs and other device implementors and run on their own hardware. To maintain the quality of Android, Google has contributed full-time engineers, product managers, user interface designers, quality assurance testers, and all the other roles required to bring modern devices to market.

Accordingly, we maintain a number of "code lines" to clearly separate the current stable version of Android from unstable experimental work. We roll the open source administration and maintenance of the Android code lines into the larger product development cycle.

The chart below depicts at a conceptual level how AOSP manages code and releases. We're referring to these as "code lines" instead of "branches" simply because at any given moment there may be more than one branch for a given "code line". For instance, when a release is cut, it may or may not become a new branch based on the needs of the moment.

  1. At any given moment, there is a current latest release of the Android platform. This typically takes the form of a branch in the tree.

  2. Device builders and contributors work with the current latest release, fixing bugs, launching new devices, experimenting with new features, and so on.

  3. In parallel, Google works internally on the next version of the Android platform and framework according to the product's needs and goals. We develop the next version of Android by working with a device partner on a flagship device whose specifications are chosen to push Android in the direction we believe it should go.

  4. When the "n+1"th version is ready, it will be published to the public source tree and become the new latest release.

code-line diagram

Figure 1. AOSP code and releases

Terms and Caveats

  • A release corresponds to a formal version of the Android platform, such as 1.5, 2.1, and so on. Generally speaking, a release of the platform corresponds to the version in the SdkVersion field of AndroidManifest.xml files and defined within frameworks/base/api in the source tree.

  • An upstream project is an open source project from which the Android stack is pulling code. These include obvious projects such as the Linux kernel and WebKit. Over time we are migrating some of the semi-autonomous Android projects (such as ART, the Android SDK tools, Bionic, and so on) to work as "upstream" projects. Generally, these projects are developed entirely in the public tree. For some upstream projects, development is done by contributing directly to the upstream project itself. See Upstream Projects for details. In both cases, snapshots will be periodically pulled into releases.

  • At all times, a release code-line (which may actually consist of more than one actual branch in git) is considered the sole canonical source code for a given Android platform version. OEMs and other groups building devices should pull only from a release branch.

  • "Experimental" code-lines are established to capture changes from the community so they can be iterated on with an eye toward stability.

  • Changes that prove stable will eventually be pulled into a release branch. Note this applies only to bug fixes, application improvements, and other changes that do not affect the APIs of the platform.

  • Changes will be pulled into release branches from upstream projects (including the Android "upstream" projects) as necessary.

  • The "n+1"th version (that is, next major version of the framework and platform APIs) will be developed by Google internally. See About Private Codelines for details.

  • Changes will be pulled from upstream, release, and experimental branches into Google's private branch as necessary.

  • When the platform APIs for the next version have stabilized and been fully tested, Google will cut a release of the next platform version. (This specifically refers to a new SdkVersion.) This will also correspond to the internal code-line being made a public release branch, and the new current platform code-line.

  • When a new platform version is cut, a corresponding experimental code-line will be created at the same time.

About Private Codelines

The source management strategy above includes a code-line that Google will keep private. The reason for this is to focus attention on the current public version of Android.

OEMs and other device builders naturally want to ship devices with the latest version of Android. Similarly, application developers don't want to deal with more platform versions than strictly necessary. Meanwhile, Google retains responsibility for the strategic direction of Android as a platform and a product. Our approach focuses on a small number of flagship devices to drive features while securing protections of Android-related intellectual property.

As a result, Google frequently has possession of confidential information from third parties. And we must refrain from revealing sensitive features until we've secured the appropriate protections. In addition, there are real risks to the platform arising from having too many platform versions extant at once. For these reasons, we have structured the open source project -- including third-party contributions -- to focus on the currently-public stable version of Android. "Deep development" on the next version of the platform will happen in private until it's ready to become an official release.

We recognize many contributors will disagree with this approach. We respect others may have a different point of view; however, this is the approach we feel is best, and the one we've chosen to implement.