extensions

OCFL Community Extension 0003: Hashed Truncated N-tuple Trees with Object ID Encapsulating Directory for OCFL Storage Hierarchies

Overview

This storage root extension describes how to safely map OCFL object identifiers of any length, containing any characters, to OCFL object root directories.

Using this extension, OCFL object identifiers are hashed and encoded as hex strings (all letters lower-case). These digests are then divided into N n-tuple segments, which are used to create nested paths under the OCFL storage root. Finally, the OCFL object identifier is percent-encoded to create a directory name for the OCFL object root (see “Encapsulation Directory” section below).

The n-tuple segments approach allows OCFL object identifiers to be evenly distributed across the storage hierarchy. The maximum number of files under any given directory is controlled by the number of characters in each n-tuple, and the tree depth is controlled by the number of n-tuple segments each digest is divided into. The encoded encapsulation directory name provides visibility into the object identifier from the file path (see “Encapsulation Directory” section below for details).

Encapsulation Directory

For basic OCFL object identifiers, the object identifier is used as the name of the encapsulation directory (ie. the object root directory).

Some object identifiers could contain characters that are not safe for directory names on all filesystems. Safe characters are defined as A-Z, a-z, 0-9, ‘-‘ and ‘_’. When an unsafe character is encountered in an object identifier, it is percent-encoded using the lower-case hex characters of its UTF-8 encoding.

Some object identifiers could also result in an encoded string that is longer than can be supported as a directory name. To handle that scenario, if the percent-encoded object identifier is longer than 100 characters, it is truncated to 100 characters, and then the digest of the original object identifier is appended to the encoded object identifier like this: <encoded-object-identifier-first-100-chars>-<digest>. Note: this means that it is no longer possible to determine the full object identifier from the encapsulation directory name - some characters have been removed, and even the first 100 characters of the encoded object identifier cannot be fully, reliably decoded, because the truncation may leave a partial encoding at the end of the 100 characters.

Object ID Encapsulation Directory Name
object-01 object-01
..Hor/rib:lè-$id %2e%2eHor%2frib%3al%c3%a8-%24id
abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghija abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij-5cc73e648fbcff136510e330871180922ddacf193b68fdeff855683a01464220

Parameters

Summary

Details

digestAlgorithm

digestAlgorithm is defaulted to sha256, and it MUST either contain a digest algorithm that’s officially supported by the OCFL specification or defined in a community extension. The specified algorithm is applied to OCFL object identifiers to produce hex encoded digest values that are then mapped to OCFL object root paths.

tupleSize

tupleSize determines the number of digest characters to include in each tuple. The tuples are used as directory names. The default value is 3, which means that each intermediate directory in the OCFL storage hierarchy could contain up to 4096 directories. Increasing this value increases the maximum number of sub-directories per directory.

If tupleSize is set to 0, then no tuples are created and numberOfTuples MUST also equal 0.

The product of tupleSize and numberOfTuples MUST be less than or equal to the number of characters in the hex encoded digest.

numberOfTuples

numberOfTuples determines how many tuples to create from the digest. The tuples are used as directory names, and each successive directory is nested within the previous. The default value is 3, which means that every OCFL object root will be 4 directories removed from the OCFL storage root, 3 tuple directories plus 1 encapsulation directory. Increasing this value increases the depth of the OCFL storage hierarchy.

If numberOfTuples is set to 0, then no tuples are created and tupleSize MUST also equal 0.

The product of numberOfTuples and tupleSize MUST be less than or equal to the number of characters in the hex encoded digest.

Procedure

The following is an outline of the steps to follow to map an OCFL object identifier to an OCFL object root path using this extension (also see the “Python Code” section):

  1. The OCFL object identifier is encoded as UTF-8 and hashed using the specified digestAlgorithm.
  2. The digest is encoded as a lower-case hex string.
  3. Starting at the beginning of the digest and working forwards, the digest is divided into numberOfTuples tuples each containing tupleSize characters.
  4. The tuples are joined, in order, using the filesystem path separator.
  5. The OCFL object identifier is percent-encoded to create the encapsulation directory name (see “Encapsulation Directory” section above for details).
  6. The encapsulation directory name is joined to the end of the path.

Examples

Example 1

This example demonstrates what the OCFL storage hierarchy looks like when using this extension’s default configuration.

Parameters

It is not necessary to specify any parameters to use the default configuration. However, if you were to do so, it would look like the following:

{
    "extensionName": "0003-hash-and-id-n-tuple-storage-layout",
    "digestAlgorithm": "sha256",
    "tupleSize": 3,
    "numberOfTuples": 3
}

Mappings

Object ID Digest Object Root Path
object-01 3c0ff4240c1e116dba14c7627f2319b58aa3d77606d0d90dfc6161608ac987d4 3c0/ff4/240/object-01
..hor/rib:le-$id 487326d8c2a3c0b885e23da1469b4d6671fd4e76978924b4443e9e3c316cda6d 487/326/d8c/%2e%2ehor%2frib%3ale-%24id

Storage Hierarchy

[storage_root]/
├── 0=ocfl_1.0
├── ocfl_layout.json
├── extensions/0003-hash-and-id-n-tuple-storage-layout/config.json
├── 3c0/
│   └── ff4/
│       └── 240/
│           └── object-01/
│               ├── 0=ocfl_object_1.0
│               ├── inventory.json
│               ├── inventory.json.sha512
│               └── v1 [...]
└── 487/
    └── 326/
        └── d8c/
            └── %2e%2ehor%2frib%3ale-%24id/
                ├── 0=ocfl_object_1.0
                ├── inventory.json
                ├── inventory.json.sha512
                └── v1 [...]

Example 2

This example demonstrates the effects of modifying the default parameters to use a different digestAlgorithm, smaller tupleSize, and a larger numberOfTuples.

Parameters

{
    "extensionName": "0003-hash-and-id-n-tuple-storage-layout",
    "digestAlgorithm": "md5",
    "tupleSize": 2,
    "numberOfTuples": 15
}

Mappings

Object ID Digest Object Root Path
object-01 ff75534492485eabb39f86356728884e ff/75/53/44/92/48/5e/ab/b3/9f/86/35/67/28/88/object-01
..hor/rib:le-$id 08319766fb6c2935dd175b94267717e0 08/31/97/66/fb/6c/29/35/dd/17/5b/94/26/77/17/%2e%2ehor%2frib%3ale-%24id

Storage Hierarchy

[storage_root]/
├── 0=ocfl_1.0
├── ocfl_layout.json
├── extensions/0003-hash-and-id-n-tuple-storage-layout/config.json
├── 08/
│   └── 31/
│       └── 97/
│           └── 66/
│               └── fb/
│                   └── 6c/
│                       └── 29/
│                           └── 35/
│                               └── dd/
│                                   └── 17/
│                                       └── 5b/
│                                           └── 94/
│                                               └── 26/
│                                                   └── 77/
│                                                       └── 17/
│                                                           └── %2e%2ehor%2frib%3ale-%24id/
│                                                               ├── 0=ocfl_object_1.0
│                                                               ├── inventory.json
│                                                               ├── inventory.json.sha512
│                                                               └── v1 [...]
└── ff/
    └── 75/
        └── 53/
            └── 44/
                └── 92/
                    └── 48/
                        └── 5e/
                            └── ab/
                                └── b3/
                                    └── 9f/
                                        └── 86/
                                            └── 35/
                                                └── 67/
                                                    └── 28/
                                                        └── 88/
                                                            └── object-01/
                                                                ├── 0=ocfl_object_1.0
                                                                ├── inventory.json
                                                                ├── inventory.json.sha512
                                                                └── v1 [...]

Example 3

This example demonstrates what happens when tupleSize and numberOfTuples are set to 0. This is an edge case and not a recommended configuration.

Parameters

{
    "extensionName": "0003-hash-and-id-n-tuple-storage-layout",
    "digestAlgorithm": "sha256",
    "tupleSize": 0,
    "numberOfTuples": 0
}

Mappings

Object ID Digest Object Root Path
object-01 3c0ff4240c1e116dba14c7627f2319b58aa3d77606d0d90dfc6161608ac987d4 object-id
..hor/rib:le-$id 487326d8c2a3c0b885e23da1469b4d6671fd4e76978924b4443e9e3c316cda6d %2e%2ehor%2frib%3ale-%24id

Storage Hierarchy

[storage_root]/
├── 0=ocfl_1.0
├── ocfl_layout.json
├── extensions/0003-hash-and-id-n-tuple-storage-layout/config.json
├── object-id/
│   ├── 0=ocfl_object_1.0
│   ├── inventory.json
│   ├── inventory.json.sha512
│   └── v1 [...]
└── %2e%2ehor%2frib%3ale-%24id/
    ├── 0=ocfl_object_1.0
    ├── inventory.json
    ├── inventory.json.sha512
    └── v1 [...]

Python Code

Here is some python code that implements the algorithm:

import codecs
import hashlib
import os
import re


def _percent_encode(c):
    c_bytes = c.encode('utf8')
    s = ''
    for b in c_bytes:
        s += '%' + codecs.encode(bytes([b]), encoding='hex_codec').decode('utf8')
    return s


def _get_encapsulation_directory(object_id, digest):
    d = ''
    for c in object_id:
        if re.match(r'[A-Za-z0-9-_]{1}', c):
            d += c
        else:
            d += _percent_encode(c)
    if len(d) > 100:
        return f'{d[:100]}-{digest}'
    return d


def ocfl_path(object_id, algorithm='sha256', tuple_size=3, number_of_tuples=3):
    object_id_utf8 = object_id.encode('utf8')
    if algorithm == 'md5':
        digest = hashlib.md5(object_id_utf8).hexdigest()
    elif algorithm == 'sha256':
        digest = hashlib.sha256(object_id_utf8).hexdigest()
    elif algorithm == 'sha512':
        digest = hashlib.sha512(object_id_utf8).hexdigest()
    digest = digest.lower()
    path = ''
    for i in range(number_of_tuples):
        part = digest[i*tuple_size:i*tuple_size+tuple_size]
        path = os.path.join(path, part)
    encapsulation_directory = _get_encapsulation_directory(object_id, digest=digest)
    path = os.path.join(path, encapsulation_directory)
    return path


def _check_path(object_id, correct_path, algorithm='sha256', tuple_size=3, number_of_tuples=3):
    p = ocfl_path(object_id, algorithm=algorithm, tuple_size=tuple_size, number_of_tuples=number_of_tuples)
    assert p == correct_path, f'{p} != {correct_path}'
    print(f'  "{object_id}" {algorithm} => {p}')


def run_tests():
    print('running tests...')
    assert _percent_encode('.') == '%2e'
    assert _percent_encode('ç') == '%c3%a7'
    _check_path(object_id='object-01', correct_path='3c0/ff4/240/object-01')
    _check_path(object_id='object-01', correct_path='ff7/553/449/object-01', algorithm='md5')
    _check_path(object_id='object-01', correct_path='ff755/34492/object-01', algorithm='md5', tuple_size=5, number_of_tuples=2)
    _check_path(object_id='object-01', correct_path='object-01', algorithm='md5', tuple_size=0, number_of_tuples=0)
    _check_path(object_id='object-01', correct_path='ff/75/53/44/92/48/5e/ab/b3/9f/86/35/67/28/88/object-01', algorithm='md5', tuple_size=2, number_of_tuples=15)
    _check_path(object_id='..hor/rib:le-$id', correct_path='487/326/d8c/%2e%2ehor%2frib%3ale-%24id')
    _check_path(object_id='..hor/rib:le-$id', correct_path='083/197/66f/%2e%2ehor%2frib%3ale-%24id', algorithm='md5') #08319766fb6c2935dd175b94267717e0
    _check_path(object_id='..Hor/rib:lè-$id', correct_path='373/529/21a/%2e%2eHor%2frib%3al%c3%a8-%24id')
    long_object_id = 'abcdefghij' * 26
    long_object_id_digest = '55b432806f4e270da0cf23815ed338742179002153cd8d896f23b3e2d8a14359'
    _check_path(object_id=long_object_id, correct_path=f'55b/432/806/abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij-{long_object_id_digest}')
    long_object_id_101 = 'abcdefghij' * 10 + 'a'
    long_object_id_101_digest = '5cc73e648fbcff136510e330871180922ddacf193b68fdeff855683a01464220'
    _check_path(object_id=long_object_id_101, correct_path=f'5cc/73e/648/abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij-{long_object_id_101_digest}')


if __name__ == '__main__':
    run_tests()