Distribution interface refactoring plan

[stevvooe]

Over the next few weeks, we need to make several changes to the distribution interfaces to support their role as a client to the registry.

The following is a rough list of the goals we'd like to accomplish:

• Propagate context.Context through storage driver method calls (Add golang/x/net/context.Context to storage driver method calls #264)
• Incorporate changes from WIP: distribution interface improvements #193 in stages
  • Refactor distribution.Layer to distribution.Blob from WIP: distribution interface improvements #193. (Refactor Blob Service API #519)
  • Decouple manifest service from concrete manifest struct (Implementation of the Manifest Service API refactor. #1268)
  • Separate TagService from manifest service
• Incorporate blob store interface into backend
• Provide more flexible service instantiation (not actionable from description)

Other issues may be filed to further explain details of these items.

[xiekeyang]

@stevvooe
On sub-issue of "Separate TagService from manifest service"
According to your proposal in this issue and "TODO" comment in registry.go, I assume you want service to be refactored as,
ManifestService

type ManifestService interface {
    // Exists returns true if the manifest exists.
    Exists(dgst digest.Digest) (bool, error)

    // Get retrieves the identified by the digest, if it exists.
    Get(dgst digest.Digest) (*manifest.SignedManifest, error)

    // Delete removes the manifest, if it exists.
    Delete(dgst digest.Digest) error

    // Put creates or updates the manifest.
    Put(manifest *manifest.SignedManifest) error
}

tagService

type tagService interface {
    // TODO(stevvooe): The methods after this message should be moved to a
    // discrete TagService, per active proposals.

    // Tags lists the tags under the named repository.
    Tags() ([]string, error)

    // ExistsByTag returns true if the manifest exists.
    ExistsByTag(tag string) (bool, error)

    // GetByTag retrieves the named manifest, if it exists.
    GetByTag(tag string) (*manifest.SignedManifest, error)
}

And remove tagStore field from manifestStore. Am I correct?
Thanks a lot!

[stevvooe]

@xiekeyang Yes, this is the plan. However, this plan is somewhat ordered. We first must do the distribution.Blob refactor then we can undertake the other parts. I am currently working on this.

[xiekeyang]

@stevvooe
Refactor Blob Service API #519 is done. Then, do you have the plan to start refactoring what we talk above (separation tag service to manifest service)? Thanks a lot!

[stevvooe]

@xiekeyang Yes, that is the plan. We may want to decouple the manifest structure first but the ordering is not important. I'm starting this work later this week.

[xiekeyang]

@stevvooe Are there any help-wanted kind of job on refactor issues? Thanks.