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versioning.md

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Starboard Versioning

Motivation

When a porter implements Starboard for a platform, it is more precise to say that they have implemented support for a certain version of Starboard. Changes to the Starboard API are associated with Starboard versions. Any usage of new Starboard APIs must also be protected by a compile-time check for the Starboard version it belongs to. This decoupling of Cobalt and Starboard versions ensures that a porter can update to a newer version of Cobalt, but not be required to implement new Starboard APIs, if the version of Starboard they have implemented is still supported.

Starboard API version vs. Starboard application version

The Starboard version describes the set of Starboard APIs available to Starboard applications. It will be incremented with every open-source release that includes changes to the Starboard API. Reasons to increment the Starboard API version include:

  • New Starboard APIs
  • Removed Starboard APIs
  • Modified semantics to existing APIs

Some notable cases that do not justify incrementing the Starboard version include:

  • More descriptive or clearer comments for existing APIs that do not change the semantics
  • New utility classes that are built on top of Starboard APIs, such as starboard::ScopedFile
  • Changes that affect the upward API to Starboard applications, but not the downward API to porters. For example, defining new upward APIs in terms of existing Starboard APIs.

A particular Starboard application may be versioned independently of the Starboard API. A given version of a Starboard application may support a range of Starboard versions. It may be the case that some new functionality in a Starboard application requires Starboard APIs that were added to a particular API version. If a porter wants to use such a feature, they must therefore also implement the required version of the Starboard API. For example, Voice Search was added to Cobalt version 5 and requires the SbMicrophone APIs which were added to Starboard version 2. Platforms that implemented Starboard version 1 continued to build and run Cobalt 5 correctly, but the Voice Search feature would be unavailable.

Range of supported Starboard versions

The minimum supported API version is defined by the SB_MINIMUM_API_VERSION macro, which is defined in starboard/configuration.h. Likewise, the SB_MAXIMUM_API_VERSION macro defines the maximum supported API version. All platforms must declare a SB_API_VERSION macro in the platform’s configuration.h to declare the starboard version the platform has implemented. Declaring implementation for an API version outside this range will result in an error at compilation time. Generally Starboard applications will not support all versions of the Starboard API indefinitely. Starboard application owners may increment the minimum required Starboard version at their discretion.

Using new Starboard APIs from Starboard Applications

Usage of a Starboard API that is not available in all supported Starboard API versions must be guarded with a check for SB_API_VERSION. Starboard applications must continue to function correctly and must not disable existing functionality if this check evaluates to false, but it’s acceptable to disable new functionality in Starboard applications if this evaluates to false.

Adding and using new Starboard APIs

"Frozen" Starboard versions

All Starboard versions that are less than the SB_MAXIMUM_API_VERSION version are considered frozen. Any Starboard APIs in a frozen version MUST not change.

Communicating Starboard API changes to porters

When a new version of Starboard is released, starboard/CHANGELOG.md should be updated with the feature define comments for all features enabled in that version.