persistent-identifiers.rst

Persistent identifiers

You can point to objects present in the Software Heritage archive by the means of persistent identifiers that are guaranteed to remain stable (persistent) over time. Their syntax, meaning, and usage is described below. Note that they are identifiers and not URLs, even though an URL-based resolver for Software Heritage persistent identifiers is also provided.

A persistent identifier can point to any software artifact (or "object") available in the Software Heritage archive. Objects come in different types, and most notably:

• contents
• directories
• revisions
• releases
• snapshots

Each object is identified by an intrinsic, type-specific object identifier that is embedded in its persistent identifier as described below. Object identifiers are strong cryptographic hashes computed on the entire set of object properties to form a Merkle structure.

See :ref:`data-model` for an overview of object types and how they are linked together. See :py:mod:`swh.model.identifiers` for details on how intrinsic object identifiers are computed.

Syntax

Syntactically, persistent identifiers are generated by the <identifier> entry point of the grammar:

<identifier> ::= "swh" ":" <scheme_version> ":" <object_type> ":" <object_id> ;
<scheme_version> ::= "1" ;
<object_type> ::=
    "snp"  (* snapshot *)
  | "rel"  (* release *)
  | "rev"  (* revision *)
  | "dir"  (* directory *)
  | "cnt"  (* content *)
  ;
<object_id> ::= 40 * <hex_digit> ;  (* intrinsic object id, as hex-encoded SHA1 *)
<hex_digit> ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
              | "a" | "b" | "c" | "d" | "e" | "f" ;

Semantics

: is used as separator between the logical parts of identifiers. The swh prefix makes explicit that these identifiers are related to SoftWare Heritage. 1 (<scheme_version>) is the current version of this identifier scheme; future editions will use higher version numbers, possibly breaking backward compatibility (but without breaking the resolvability of identifiers that conform to previous versions of the scheme).

A persistent identifier points to a single object, whose type is explicitly captured by <object_type>:

• snp identifiers points to snapshots,
• rel to releases,
• rev to revisions,
• dir to directories,
• cnt to contents.

The actual object pointed to is identified by the intrinsic identifier <object_id>, which is a hex-encoded (using lowercase ASCII characters) SHA1 computed on the content and metadata of the object itself, as follows:

• for snapshots, intrinsic identifiers are computed as per :py:func:`swh.model.identifiers.snapshot_identifier`
• for releases, as per :py:func:`swh.model.identifiers.release_identifier`
• for revisions, as per :py:func:`swh.model.identifiers.revision_identifier`
• for directories, as per :py:func:`swh.model.identifiers.directory_identifier`
• for contents, the intrinsic identifier is the sha1_git hash of the multiple hashes returned by :py:func:`swh.model.identifiers.content_identifier`, i.e., the SHA1 of a byte sequence obtained by juxtaposing the ASCII string "blob" (without quotes), a space, the length of the content as decimal digits, a NULL byte, and the actual content of the file.

Git compatibility

Intrinsic object identifiers for contents, directories, revisions, and releases are, at present, compatible with the Git way of computing identifiers for its objects. A Software Heritage content identifier will be identical to a Git blob identifier of any file with the same content, a Software Heritage revision identifier will be identical to the corresponding Git commit identifier, etc. This is not the case for snapshot identifiers as Git doesn't have a corresponding object type.

Note that Git compatibility is incidental and is not guaranteed to be maintained in future versions of this scheme (or Git).

Examples

• swh:1:cnt:94a9ed024d3859793618152ea559a168bbcbb5e2 points to the content of a file containing the full text of the GPL3 license
• swh:1:dir:d198bc9d7a6bcf6db04f476d29314f157507d505 points to a directory containing the source code of the Darktable photography application as it was at some point on 4 May 2017
• swh:1:rev:309cf2674ee7a0749978cf8265ab91a60aea0f7d points to a commit in the development history of Darktable, dated 16 January 2017, that added undo/redo supports for masks
• swh:1:rel:22ece559cc7cc2364edc5e5593d63ae8bd229f9f points to Darktable release 2.3.0, dated 24 December 2016
• swh:1:snp:c7c108084bc0bf3d81436bf980b46e98bd338453 points to a snapshot of the entire Darktable Git repository taken on 4 May 2017 from GitHub

Resolution

Persistent identifiers can be resolved using the Software Heritage Web application (see :py:mod:`swh.web`).

In particular, the /browse/ endpoint can be given a persistent identifier and will lead to the browsing page of the corresponding object, like this: https://archive.softwareheritage.org/browse/<identifier>. For example:

• https://archive.softwareheritage.org/browse/swh:1:cnt:94a9ed024d3859793618152ea559a168bbcbb5e2
• https://archive.softwareheritage.org/browse/swh:1:dir:d198bc9d7a6bcf6db04f476d29314f157507d505
• https://archive.softwareheritage.org/browse/swh:1:rev:309cf2674ee7a0749978cf8265ab91a60aea0f7d
• https://archive.softwareheritage.org/browse/swh:1:rel:22ece559cc7cc2364edc5e5593d63ae8bd229f9f
• https://archive.softwareheritage.org/browse/swh:1:snp:c7c108084bc0bf3d81436bf980b46e98bd338453