Content Linking (spec)

[rosy1280]

The following notes pertain to the Object Forking Use Case, specifically Use Case #44. While Object Forking is a primary focus, it is encompassed by the broader concept of content linking. This functionality is enabled by content-addressable storage and establishes a relationship between a "parent" (the original object) and a "child" (the object derived from the parent).

[rosy1280]

Storage Root

• In a storage root that supports file inheritance, a flag MUST be placed in the ocfl_layout.json file.
• An object can only inherit files from other objects in the same storage root.

OCFL Object

• The OCFL accommodates the relationship between a parent (original) object and a child (derived) object. This is achieved by embedding pointers in the child object's inventory.json file. The referenced content MUST be located in the links section of the inventory.json file for one or more parent objects.
• Each entry in the links block MUST be a digest key and a parent object identifier. The digest MUST be found in either the manifest or tombstones block of the parent object.

"links": {
    "7dcc35...c31": "ark:/67890/fgh123",
    "df83e1...a3e": "ark:/67890/fgh123",
    "ffccf6...62e": "ark:/67890/fgh123"
  },

• The version state block MUST list the logical path for the child object. This allows users to change the file name and path from those in the parent object's state.
• An object can inherit files from multiple parent objects; it's not limited to the set of files of a single version from a single parent object.
• Linked files MUST NOT be included in the fixity block. Additional fixity may be found in the parent object's fixity block.
• The child object MUST use the same digestAlgorithm as all parent objects.

A full inventory.json example of file inheritance:

{
  "digestAlgorithm": "sha512",
  "head": "v3",
  "id": "ark:/12345/bcd987",
  "manifest": {
    "4d27c8...b53": [ "v2/content/foo/bar.xml" ]
  },
  "links": {
    "7dcc35...c31": "ark:/67890/fgh123",
    "df83e1...a3e": "ark:/67890/fgh123",
    "ffccf6...62e": "ark:/67890/fgh123"
  },
  "type": "https://ocfl.io/1.1/spec/#inventory",
  "versions": {
    "v1": {
      "created": "2018-01-01T01:01:01Z",
      "message": "Initial import. bar.xml, bigdata.dat and image.tiff are inherited from a parent object.",
      "state": {
        "7dcc35...c31": [ "foo/bar.xml" ],
        "df83e1...a3e": [ "bigdata.dat" ],
        "ffccf6...62e": [ "image.tiff" ]
      },
      "user": {
        "address": "mailto:alice@example.com",
        "name": "Alice"
      }
    },
    "v2": {
      "created": "2018-02-02T02:02:02Z",
      "message": "Fix bar.xml replacing import with a local edit, remove image.tiff",
      "state": {
        "4d27c8...b53": [ "foo/bar.xml" ],
        "df83e1...a3e": [ "bigdata.dat" ]
      },
      "user": {
        "address": "mailto:bob@example.com",
        "name": "Bob"
      }
    },
    "v3": {