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5264bd0 Dec 9, 2016
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AMP versioning policy

Basics

  • We use semver terminology, but see below for changes from simple semver.
  • AMP and its extensions are versioned. There is also an AMP release that corresponds to the precise code. Each major and minor version may correspond to many AMP releases.
  • The AMP JS library is distributed via its major version only. The major version is expected to change rarely if ever (on the order of less than once per year).
  • Each AMP extension is versioned independently. Publishers of AMP documents can choose the major and minor version of an extension. They cannot choose the patch version.
  • AMP is released about once per weak and each release has a unique version number. With each release, AMP pages are rendered with the latest code for the respective major/minor versions of the AMP JS library and extensions.

Goals

  • Reduce overall JS URL entropy. AMP aims to provide as few versions of the JS library and extensions as possible to improve cache hit rates for these requests.
  • Avoid breaking existing pages.
  • Keep all users of AMP on secure versions of the library.

Non-breaking changes

  • If a change is backward compatible, no version should be changed.
  • This means that, for now, there is no known case under which the minor version of an extension would be changed.

Breaking changes

Breaking changes are:

  • Changes to the semantics of supported attributes and other exposed APIs.
  • Changes to CSS included in AMP that are not backward compatible.
  • Changes to the DOM structure (elements, attributes and attribute values) generated by AMP extensions that are not backward compatible.

The following special cases are not considered breaking changes:

  • Changes to elements and their children including their CSS if the element name starts with i-.
  • Changes to attribute and class names starting with -. But this doesn't mean, CSS backward compatibility may be broken with this mechanism, where it would otherwise be considered a breaking change.
  • Changes required to maintain the security of AMP pages.
  • Changes that break fewer than 0.1% of crawler accessible AMP pages.

For breaking changes the major version of an extension (or more rarely) the main AMP JS library should be increased.

Deprecation process

In rare cases the AMP project may decide to deprecate existing functionality. Such changes must be publicly discussed and provide significant user benefit that justifies additional work for page developers.

The process is

  • Create "Intent to change" issue.
  • Allow for at least 2 weeks of open discussion.
  • Wait for 3 LGTMs from core committers.
  • (Announce)[#Announcements] change on the mailing list.
  • Start warning for pages that might break via the developer console.
  • Give developers 6 weeks to apply changes.
  • Apply new behavior to old content in a way that minimizes breakage.

Announcements

Changes that may break existing pages (including those where the number of broken pages is assumed to be very small) should be announced via the amphtml-announce@googlegroups.com mailing list. Sometimes such changes are not anticipated upfront in which case announcements should still be sent to let websites know.

Security fixes may in some cases only be announced after the change was rolled out.

Removing old versions

Old versions of extensions may be removed if they are used by fewer than 0.1% of crawler accessible AMP pages.