diff --git a/docs/persistent-identifiers.rst b/docs/persistent-identifiers.rst index 588e5e0..105f58c 100644 --- a/docs/persistent-identifiers.rst +++ b/docs/persistent-identifiers.rst @@ -1,408 +1,427 @@ .. _persistent-identifiers: .. _swhids: ================================================= SoftWare Heritage persistent IDentifiers (SWHIDs) ================================================= **version 1.6, last modified 2021-04-30** .. contents:: :local: :depth: 2 Overview ======== You can point to objects present in the `Software Heritage `_ `archive `_ by the means of **SoftWare Heritage persistent IDentifiers**, or **SWHIDs** for short, 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 URL-based `resolvers`_ for SWHIDs are also available. A SWHID consists of two separate parts, a mandatory *core identifier* that can point to any software artifact (or "object") available in the Software Heritage archive, and an optional list of *qualifiers* that allows to specify the context where the object is meant to be seen and point to a subpart of the object itself. Objects come in different types: * contents * directories * revisions * releases * snapshots Each object is identified by an intrinsic, type-specific object identifier that is embedded in its SWHID as described below. The intrinsic identifiers embedded in SWHIDs are strong cryptographic hashes computed on the entire set of object properties. Together, these identifiers form a `Merkle structure `_, specifically a Merkle `DAG `_. See the :ref:`Software Heritage data model ` for an overview of object types and how they are linked together. See :py:mod:`swh.model.git_objects` for details on how the intrinsic identifiers embedded in SWHIDs are computed. The optional qualifiers are of two kinds: * **context qualifiers:** carry information about the context where a given object is meant to be seen. This is particularly important, as the same object can be reached in the Merkle graph following different *paths* starting from different nodes (or *anchors*), and it may have been retrieved from different *origins*, that may evolve between different *visits* * **fragment qualifiers:** allow to pinpoint specific subparts of an object .. _swhids-syntax: Syntax ====== Syntactically, SWHIDs are generated by the ```` entry point in the following grammar: .. code-block:: bnf ::= [ ] ; ::= "swh" ":" ":" ":" ; ::= "1" ; ::= "snp" (* snapshot *) | "rel" (* release *) | "rev" (* revision *) | "dir" (* directory *) | "cnt" (* content *) ; ::= 40 * ; (* intrinsic object id, as hex-encoded SHA1 *) ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ; ::= | "a" | "b" | "c" | "d" | "e" | "f" ; := ";" [ ] ; ::= | ; ::= | | | ; ::= "origin" "=" ; ::= "visit" "=" ; ::= "anchor" "=" ; ::= "path" "=" ; ::= "lines" "=" ["-" ] ; ::= + ; ::= (* RFC 3987 IRI *) ::= (* RFC 3987 absolute path *) Where: - ```` is an ```` from `RFC 3987`_, and - ```` is a `RFC 3987`_ IRI in either case all occurrences of ``;`` (and ``%``, as required by the RFC) have been percent-encoded (as ``%3B`` and ``%25`` respectively). Other characters *can* be percent-encoded, e.g., to improve readability and/or embeddability of SWHID in other contexts. .. _RFC 3987: https://tools.ietf.org/html/rfc3987 .. _swhids-semantics: Semantics ========= .. _swhids-core: Core identifiers ---------------- ``:`` is used as separator between the logical parts of core identifiers. The ``swh`` prefix makes explicit that these identifiers are related to *SoftWare Heritage*. ``1`` (````) 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 SWHIDs that conform to previous versions of the scheme. A SWHID points to a single object, whose type is explicitly captured by ````: * ``snp`` 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 ````, 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 SHA1 hashes of manifests computed as per :py:func:`swh.model.git_objects.snapshot_git_object` * for **releases**, as per :py:func:`swh.model.git_objects.release_git_object` that produces the same result as a git release hash * for **revisions**, as per :py:func:`swh.model.git_objects.revision_git_object` that produces the same result as a git commit hash * for **directories**, per :py:func:`swh.model.git_objects.directory_git_object` that produces the same result as a git tree hash * for **contents**, the intrinsic identifier is the ``sha1_git`` hash returned by :py:meth:`swh.hashutil.MultiHash.digest`, 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. .. _swhids-qualifiers: Qualifiers ---------- ``;`` is used as separator between the core identifier and the optional qualifiers, as well as between qualifiers. Each qualifier is specified as a key/value pair, using ``=`` as a separator. The following *context qualifiers* are available: * **origin:** the *software origin* where an object has been found or observed in the wild, as an URI; * **visit:** the core identifier of a *snapshot* corresponding to a specific *visit* of a repository containing the designated object; * **anchor:** a *designated node* in the Merkle DAG relative to which a *path to the object* is specified, as the core identifier of a directory, a revision, a release or a snapshot; * **path:** the *absolute file path*, from the *root directory* associated to the *anchor node*, to the object; when the anchor denotes a directory or a revision, and almost always when it's a release, the root directory is uniquely determined; when the anchor denotes a snapshot, the root directory is the one pointed to by ``HEAD`` (possibly indirectly), and undefined if such a reference is missing; The following *fragment qualifier* is available: * **lines:** *line number(s)* of interest, usually within a content object We recommend to equip identifiers meant to be shared with as many qualifiers as possible. While qualifiers may be listed in any order, it is good practice to present them in the order given above, i.e., ``origin``, ``visit``, ``anchor``, ``path``, ``lines``. Redundant information should be omitted: for example, if the *visit* is present, and the *path* is relative to the snapshot indicated there, then the *anchor* qualifier is superfluous; similarly, if the *path* is empty, it may be omitted. Interoperability ================ URI scheme ---------- The ``swh`` URI scheme is registered at IANA for SWHIDs. The present documents constitutes the scheme specification for such URI scheme. Git compatibility ----------------- SWHIDs for contents, directories, revisions, and releases are, at present, compatible with the `Git `_ way of `computing identifiers `_ for its objects. The ```` part of a SWHID for a content object is the Git blob identifier of any file with the same content; for a revision it is the Git commit identifier for the same revision, etc. This is not the case for snapshot identifiers, as Git does not 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). Automatically fixing invalid SWHIDs ----------------------------------- User interfaces may fix invalid SWHIDs, by lower-casing the ```` part of a SWHID, if it contains upper-case letters because of user errors or limitations in software displaying SWHIDs. However, implementations displaying or generating SWHIDs should not rely on this behavior, and must display or generate only valid SWHIDs when technically possible. User interfaces should show an error when such an automatic fix occurs, so users have a chance to fix their SWHID before pasting it to an other interface that does not perform the same corrections. This also makes it easier to understand issues when a case-sensitive qualifier has its casing altered. Examples ======== Core identifiers ---------------- * ``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 Identifiers with qualifiers --------------------------- * The following :swh_web:`SWHID ` denotes the lines 9 to 15 of a file content that can be found at absolute path ``/Examples/SimpleFarm/simplefarm.ml`` from the root directory of the revision ``swh:1:rev:2db189928c94d62a3b4757b3eec68f0a4d4113f0`` that is contained in the snapshot ``swh:1:snp:d7f1b9eb7ccb596c2622c4780febaa02549830f9`` taken from the origin ``https://gitorious.org/ocamlp3l/ocamlp3l_cvs.git``:: swh:1:cnt:4d99d2d18326621ccdd70f5ea66c2e2ac236ad8b; origin=https://gitorious.org/ocamlp3l/ocamlp3l_cvs.git; visit=swh:1:snp:d7f1b9eb7ccb596c2622c4780febaa02549830f9; anchor=swh:1:rev:2db189928c94d62a3b4757b3eec68f0a4d4113f0; path=/Examples/SimpleFarm/simplefarm.ml; lines=9-15 * Here is an example of a :swh_web:`SWHID ` with a file path that requires percent-escaping:: swh:1:cnt:f10371aa7b8ccabca8479196d6cd640676fd4a04; origin=https://github.com/web-platform-tests/wpt; visit=swh:1:snp:b37d435721bbd450624165f334724e3585346499; anchor=swh:1:rev:259d0612af038d14f2cd889a14a3adb6c9e96d96; path=/html/semantics/document-metadata/the-meta-element/pragma-directives/attr-meta-http-equiv-refresh/support/x%3Burl=foo/ Implementation ============== Computing --------- An important property of any SWHID is that its core identifier is *intrinsic*: it can be *computed from the object itself*, without having to rely on any third party. An implementation of SWHID that allows to do so locally is the `swh identify `_ tool, available from the `swh.model `_ Python package under the GPL license. This package can be installed via the ``pip`` package manager with the one liner ``pip3 install swh.model[cli]`` on any machine with Python (at least version 3.7) and ``pip`` installed (on a Debian or Ubuntu system a simple ``apt install python3 python3-pip`` will suffice, see `the general instructions `_ for other platforms). SWHIDs are also automatically computed by Software Heritage for all archived objects as part of its archival activity, and can be looked up via the project :swh_web:`Web interface <>`. This has various practical implications: * when a software artifact is obtained from Software Heritage by resolving a SWHID, it is straightforward to verify that it is exactly the intended one: just compute the core identifier from the artefact itself, and check that it is the same as the core identifier part of the SHWID * the core identifier of a software artifact can be computed *before* its archival on Software Heritage +Choosing what type of SWHID to use +---------------------------------- + +``swh:1:dir:`` SWHIDs are the most robust SWHIDs, as they can be recomputed from +the simplest objects (a directory structure on a filesystem), even when all +metadata is lost, without relying on the Software Heritage archive. + +Therefore, we advise implementers and users to prefer this type of SWHIDs +over ``swh:1:rev:`` and ``swh:1:rel:`` to reference a source code artifacts. + +However, since keeping the metadata is also important, you should add an anchor +qualifier to ``swh:1:dir:`` SWHIDs whenever possible, so the metadata stored +in the Software Heritage archive can be retrieved when needed. + +This means, for example, that you should prefer +``swh:1:dir:a8eded6a2d062c998ba2dcc3dcb0ce68a4e15a58;anchor=swh:1:rel:22ece559cc7cc2364edc5e5593d63ae8bd229f9f`` +over ``swh:1:rel:22ece559cc7cc2364edc5e5593d63ae8bd229f9f``. + + Resolvers --------- Software Heritage resolver ~~~~~~~~~~~~~~~~~~~~~~~~~~ SWHIDs can be resolved using the Software Heritage :swh_web:`Web interface <>`. In particular, the **root endpoint** ``/`` can be given a SWHID and will lead to the browsing page of the corresponding object, like this: ``https://archive.softwareheritage.org/``. A **dedicated** ``/resolve`` **endpoint** of the Software Heritage :swh_web:`Web API ` is also available to programmatically resolve SWHIDs; see: :http:get:`/api/1/resolve/(swhid)/`. Examples: * :swh_web:`` * :swh_web:`` * :swh_web:`` * :swh_web:`` * :swh_web:`` * :swh_web:`` * :swh_web:`` Third-party resolvers ~~~~~~~~~~~~~~~~~~~~~ The following **third party resolvers** support SWHID resolution: * `Identifiers.org `_; see: ``_ (registry identifier `MIR:00000655 `_). * `Name-to-Thing (N2T) `_ Note that resolution via Identifiers.org currently only supports *core identifiers* due to `syntactic incompatibilities with qualifiers `_. Examples: * ``_ * ``_ * ``_ * ``_ * ``_ * ``_ * ``_ References ========== * Roberto Di Cosmo, Morane Gruenpeter, Stefano Zacchiroli. `Identifiers for Digital Objects: the Case of Software Source Code Preservation `_. In Proceedings of `iPRES 2018 `_: 15th International Conference on Digital Preservation, Boston, MA, USA, September 2018, 9 pages. * Roberto Di Cosmo, Morane Gruenpeter, Stefano Zacchiroli. `Referencing Source Code Artifacts: a Separate Concern in Software Citation `_. In Computing in Science and Engineering, volume 22, issue 2, pages 33-43. ISSN 1521-9615, IEEE. March 2020.