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Commissions, Catalogs, and Continuity ohmy!

Managing data comes in three layers: « what » it is, « where » it is, and « why » it is.

  • The what is covered by the hash, in a content-addressible system. Formulas are already totally on top of this part.
  • The where is the URL to fetch things from. Again, totally covered already by Formula input/output config.
  • The why...

Well, the why is where things get fun :) In Repeatr, this is where Catalogs and Commissions come in.

Catalogs connect human-meaningful names to content-addressible hashes. Commissions are structurally almost identical for Formulas, but refer to the names defined by catalogs instead of directly pinning ware hashes, and thus can (well) commission an entire sequence of Formulas as the Catalogs publish new content versions under existing names.

Catalogs

Catalogs contain a bunch of names for Wares. Catalogs are composed of a one or more "tracks", and each track is an ordered list of "releases", going from newest to oldest.

Tracks can be used to name a series of releases for the purpose of smooth updating; e.g. the "4.x" series vs the "5.x" series would be two different tracks. Tracks bundled within the same Catalog share an author (and signing authority).

Here's a sketch of an example catalog structure. Note that it's perfectly normal for different tracks to refer to the same ware.

Catalog{
    "stable": Track{
        Releases[
            {ware:"a3vcj3"},
            {ware:"b56jql"},
        ]
    }
    "4.x": Track{
        Releases[
            {ware:"b56jql"},
        ]
    }
}

Commissions

Commissions have the same basic structural outline as Formulas -- inputs, actions, and outputs:

inputs:
    "/":
        catalog: "staid-ubuntu:14.04"
    "/app/whizbang/":
        catalog: "whizbangery:stable"
action:
    command: ["/app/whizbang/bin/buildr"]
outputs:
    "/task/output":
        catalog: "phase-2:unstable"

A Commission can be resolved against a library of Catalogs to produce a Formula. When the Commission refers to a catalog, or a specific track inside the catalog, the most recent releases in a track will automatically selected.

Since Catalogs retain their own historical versions, this process is reversible: given a Commission, a Formula, and the library of Catalogs (even if the catalogs are now more up-to-date than when the Formula was originally resolved), we can check if the Formula is a valid resolution of the Commission.