All good things must come to an end and Chrome OS devices are no different. At some point, devices will be marked EOL and dropped from the Chromium OS project and all of the source trees. This document is a guide for developers to determine when they can safely move forward in that cleanup process.
You might see the terms EOL and AUE used interchangeably in documents. The official term is Auto Update Expiration (AUE) as it refers to the last release version that is made available through Omaha (our update servers). Practically speaking, they mean the same thing.
First, a milestone version is picked based on the AUE policy and other factors by The Deciders (the group of people high up the food chain who make Decisions) for a hardware platform and the boards that use it. Typically this is done on an SoC basis (e.g. all SandyBridge devices) as support and feature sets are done at that level.
That version is set in GoldenEye for each board under the “Auto Update Expiration Target Milestone”. Omaha uses that data to send AUE notifications to the respective boards which is then presented to the user so they're aware their device needs replacement.
This version makes its way from ToT (canary) to the dev/beta/stable channels like normal.
If something comes up that causes us to push back the AUE version a release or more (e.g. significant stability regressions) we can do it easily in GoldenEye. This means the actual AUE is fluid and not set in stone once decided earlier.
Only after newer versions (AUE + 1) make it to the stable channel for 100% of users do we finally consider the AUE final and people can safely start removing support for it.
Finally the public device page is updated with the AUE version for the board.
Since the AUE version is not guaranteed to “stick” when the branch is first cut for the milestone, we cannot safely delete code right away. Instead, we basically have to wait for 4 to 5 more branches to pass. This also helps ease backporting of security fixes.
For example, stumpy was originally slated to AUE with R64. When R63 was branched, ToT updated to R64, and the AUE version was set in GoldenEye which in turn made it public to users via Omaha.
Then R64 was branched, and started to make its way through the channels. The AUE ended up being delayed to R65, so had we deleted code as soon as R64 branched, we would have had to revert a lot of things.
Eventually R65 made it to the stable channel for everyone including stumpy. We still did not clean things up just yet -- only once R66 was promoted to stable (except for stumpy since it was AUE with R65) did we start cleaning.
Generally we don't go through the codebase and scrub references right away. We let things get cleaned up as people notice them, or as the cruft gets in the way of other things (like code refactoring).
Once the version in the stable channel is newer than the board's AUE version.
You cannot delete support as soon the branch is initially cut in case the AUE version is pushed back.
Follow the same guidance in the previous question about dropping support.