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).
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 |
digestAlgorithm
tupleSize
numberOfTuples
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 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 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.
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):
digestAlgorithm.numberOfTuples tuples each containing tupleSize characters.This example demonstrates what the OCFL storage hierarchy looks like when using this extension’s default configuration.
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
}
| 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_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 [...]
This example demonstrates the effects of modifying the default parameters to use
a different digestAlgorithm, smaller tupleSize, and a larger
numberOfTuples.
{
"extensionName": "0003-hash-and-id-n-tuple-storage-layout",
"digestAlgorithm": "md5",
"tupleSize": 2,
"numberOfTuples": 15
}
| 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-$id | 08319766fb6c2935dd175b94267717e0 | 08/31/97/66/fb/6c/29/35/dd/17/5b/94/26/77/17/%2e%2ehor%2frib%3ale-%24id |
[storage_root]/
├── 0=ocfl_1.0
├── ocfl_layout.json
├── extensions/0003-hash-and-id-n-tuple-storage-layout/config.json
├── 08/
│ └── 31/
│ └── 97/
│ └── 66/
│ └── fb/
│ └── 6c/
│ └── 29/
│ └── 35/
│ └── dd/
│ └── 17/
│ └── 5b/
│ └── 94/
│ └── 26/
│ └── 77/
│ └── 17/
│ └── %2e%2ehor%2frib%3ale-%24id/
│ ├── 0=ocfl_object_1.0
│ ├── inventory.json
│ ├── inventory.json.sha512
│ └── v1 [...]
└── ff/
└── 75/
└── 53/
└── 44/
└── 92/
└── 48/
└── 5e/
└── ab/
└── b3/
└── 9f/
└── 86/
└── 35/
└── 67/
└── 28/
└── 88/
└── object-01/
├── 0=ocfl_object_1.0
├── inventory.json
├── inventory.json.sha512
└── v1 [...]
This example demonstrates what happens when tupleSize and numberOfTuples are
set to 0. This is an edge case and not a recommended configuration.
{
"extensionName": "0003-hash-and-id-n-tuple-storage-layout",
"digestAlgorithm": "sha256",
"tupleSize": 0,
"numberOfTuples": 0
}
| Object ID | Digest | Object Root Path |
|---|---|---|
| object-01 | 3c0ff4240c1e116dba14c7627f2319b58aa3d77606d0d90dfc6161608ac987d4 | object-id |
| ..hor/rib:le-$id | 487326d8c2a3c0b885e23da1469b4d6671fd4e76978924b4443e9e3c316cda6d | %2e%2ehor%2frib%3ale-%24id |
[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 [...]
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()