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OCFL Storage Root Extensions

This page describes the specific OCFL extensions used in the storage root. As we saw during the initialization of the storage root, gocfl automatically creates configuration directories in the extensions/ folder.

1. Initial Extension (initial)

The Initial Extension is used in gocfl to determine which extension should be loaded first.

Content of extensions/initial/config.json:

{
   "extensionName": "initial",
   "extension": "NNNN-gocfl-extension-manager"
}

In our test42 example, it points to the NNNN-gocfl-extension-manager. This ensures that the basic management logic of gocfl is active before other extensions are processed.

2. gocfl Extension Manager (NNNN-gocfl-extension-manager)

This is a meta-extension of gocfl responsible solely for initializing the storage root (e.g., copying documentation files). Further details can be found in the Extension Manager documentation.

In the Extension Manager, the term StorageRootPath is used as an identifier for the extension type (in this case, the storage layout of the root). In the Go implementation of gocfl, this is defined in storagerootExtension.go.

Content of extensions/NNNN-gocfl-extension-manager/config.json:

{
   "extensionName": "NNNN-gocfl-extension-manager",
   "sort": {
      "StorageRootPath": [
         "0004-hashed-n-tuple-storage-layout"
      ]
   },
   "exclusion": {
      "StorageRootPath": [
         [
            "NNNN-direct-clean-path-layout",
            "0003-hash-and-id-n-tuple-storage-layout",
            "0004-hashed-n-tuple-storage-layout",
            "0002-flat-direct-storage-layout",
            "0006-flat-omit-prefix-storage-layout",
            "NNNN-pairtree-storage-layout",
            "NNNN-direct-path-layout"
         ]
      ]
   }
}

The Extension Manager also controls dependencies via the StorageRootPath type:

• sort: Specifies that the layout module 0004 is prioritized.
• exclusion: Prevents multiple (mutually exclusive) layout strategies from being activated simultaneously for the storage root.

3. Storage Layout (0004)

The ocfl_layout.json refers to the 0004-hashed-n-tuple-storage-layout extension. The associated configuration is located at: extensions/0004-hashed-n-tuple-storage-layout/config.json

Content of config.json:

{
   "extensionName": "0004-hashed-n-tuple-storage-layout",
   "digestAlgorithm": "sha256",
   "tupleSize": 3,
   "numberOfTuples": 3,
   "shortObjectRoot": false
}

This file (along with the information in ocfl_layout.json) controls how deeply the directory structure is nested and which hash algorithm is used for path formation. In the minimal version, the name of the extension is often sufficient as the specification provides default values.

Explanation of Parameters:

• extensionName: The official name of the extension (0004-hashed-n-tuple-storage-layout).
• digestAlgorithm: The hash algorithm (here sha256) applied to the object ID to generate the directory name. This ensures a balanced distribution of objects.
• tupleSize: Determines how many characters of the hash are used for each directory segment. With 3, folder names like 1f7 are created.
• numberOfTuples: Defines how many such segments are created sequentially as subdirectories (nesting depth). With 3, this results in a path like 1f7/d28/1ac/.
• shortObjectRoot: If false (default), the complete hash is used as the directory name for the object at the end of the path. If true, only the remaining part of the hash is used.

Default Values for Extension 0004:

If these parameters are not defined in config.json, the following defaults apply according to the specification:

• digestAlgorithm: sha256 (The algorithm used to calculate the hash value of the object ID).
• tupleSize: 3 (Number of characters per directory level).
• numberTuples: 3 (Number of directory levels created by hashing).
• shortObjectRoot: false (Determines whether only the remaining part of the hash is used at the end of the path).

This means an object is stored by default in a structure like [hash1-3]/[hash4-6]/[hash7-9]/[object-id].

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