Commits on Mar 25, 2016

1. spec: add image tags …

   This patch introduces the concept of image tags.
   
   Since many ACEs (like rkt) wants to implement local caching I tried to
   define Image Tags considering this as a primary requirement.
   
   While an image is static since its labels are written in its manifest and
   changing them will change the image id, image tags are dynamic.
   
   In short words it adds another file that contains a map to convert a tag
   to another tag (alias) and/or expand a tag to a set of labels.
   It pratically adds a layer above current image discovery used to obtain
   the final set of labels to use to calculte `ac-discovery` URLs.
   
   Image Tags are defined to be per image name. This is needed to satisfy
   the need to locally cache them (since they should be cached and retrieved
   using a key and this key is the image name).
   
   This will be useful to fix different issues and enhancements.
   
   This patch:
   
   * Introduces Image Tags format.
   
   * Details the discovery process (since there's now an optional
   additional step)
   
   * Adds a "tag" inside image dependencies in image manifest.
   
   * Changes the app string format parsing function. It now defines the
   value after the ":" as a Tag instead of the version label.
   
   * Adds code for doing tag resolution and labels merging (that can be used
   by an ACE). As a fallback, if no image tags data is provided, the
   version label will be set from the tag value.
   
   * The image tags fetching and verification logic is left to the ACE
   (like done for an ACI).
   
   -- Docker compatibility
   
   Actually docker2aci converts a docker image (squashing it) to an ACI and
   sets the version label to the image tag. This is a fast but not
   consisten way to docker tags since they can be dynamically changed.
   
   With this proposal an idea will be to convert docker tags to an
   Image Tags file (using the docker APIs). Docker tags are per
   repository and a repository is mapped to an ACI image name. So this
   should match the proposal that dfines imagetags being per image.
   
   Docker images don't have a version label but can have a label
   representing the docker image ID. So an image tag can point to that
   "dockerimageid" label.
   
   -- rkt related work
   
   For rkt, the changes to implement to satisfy image tags should be:
   
   * Image Tags fetching and validation
   * Labels Merging (calling imageTags.MergeTag providing the starting
   * labels and tag value)
   * Caching of Tags Data per app name
   * Use of imageTags.MergeTag before calling GetACI.
   * Removal of "latest" column from the store since it was a hack around
   * current spec default "latest" version.
   
   Some open points:
   
   * Since there're an additional layer and an additional file there's the need to:
    * inspect Image Tags data (like the current `rkt image list` there
    should be something like `rkt imagetags list`)
    * fetch/update them (they are automatically fetched using `rkt
    --no-store fetch imagename`, but this will also fetch an image). To
    just fetch/update ImageTags something like `rkt imagetags --no-store fetch {IMAGENAME|file|URL}`
    should be added. Providing and image name will use discovery, while the other will
    just fetch it from the provided file/URL.
   
   * Since rkt also accept fetching an image from a file or URL, doing this
   won't carry Image Tags information. So a successive attemps to run an
   image by it's image string can use a different image than the one that
   will be run using an image discovery returning Image Tags data (due to
   the fallback of setting the version label value to tag if no image tag
   data is available). So a command like the above `rkt imagetags fetch` is
   needed also for this reason.

   

   sgotti committed Mar 25, 2016
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