[Bug 1231628] Re: Support for phased updates

[Barry Warsaw]

** Changed in: system-image (Ubuntu)
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/1231628

Title:
  Support for phased updates

Status in Ubuntu system image (server/client/updater):
  Fix Released
Status in “system-image” package in Ubuntu:
  Fix Released

Bug description:
  In order to progressively deploy an image to our users and to be able
  to block an update in case of an issue, we need the client to support
  the phased-percentage field.

  This field is part of an image definition in index.json and is an integer between 0 and 100.
  The client needs to hash the version number with something unique to the device (MAC, EMEI, ...) and use that to seed a random number generator and then get a value between 1 and 100. If that value is higher than phased-percentage the entry needs to be ignored in the upgrade path resolution.

  Only the latest version number can have phased-percentage set but the client doesn't need to care about this, the server will enforce it on its side.
  All the client needs to do is check phased-percentage against its own random value and drop them from the path resolution if the local value is higher than the one in the index (same algorithm as phased package updates in the archive).

  
  In the case an image is known to be bad, it'll have its phased-percentage value set to "0" so no client will attempt to use it. When an image is ready for general consumption, it'll have phased-percentage set to "100" or simply not set at all (same result).

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