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diff --git a/docs/extrinsic-metadata-specification.rst b/docs/extrinsic-metadata-specification.rst
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+.. _extrinsic-metadata-specification:
+
+Extrinsic metadata specification
+================================
+
+:term:`Extrinsic metadata` is information about software that is not part
+of the source code itself but still closely related to the software.
+Typical sources for extrinsic metadata are: the hosting place of a
+repository, which can offer metadata via its web view or API; external
+registries like collaborative curation initiatives; and out-of-band
+information available at source code archival time.
+
+Since they are not part of the source code, a dedicated mechanism to fetch
+and store them is needed.
+
+This specification assumes the reader is familiar with Software Heritage's
+:ref:`architecture` and :ref:`data-model`.
+
+
+Metadata sources
+----------------
+
+Authorities
+^^^^^^^^^^^
+
+Metadata authorities are entities that provide metadata about an
+:term:`origin`. Metadata authorities include: code hosting places,
+:term:`deposit` submitters, and registries (eg. Wikidata).
+
+An authority is uniquely defined by these properties:
+
+* its type, representing the software/database from which metadata is
+ extracted (eg. `gitlab`, `wikidata`, `hal`).
+
+* its URL, which unambiguously identifies an instance of the authority type.
+
+Examples:
+
+=============== =================================
+type url
+=============== =================================
+deposit https://hal.archives-ouvertes.fr/
+deposit https://hal.inria.fr/
+deposit https://software.intel.com/
+gitlab https://gitlab.com/
+gitlab https://gitlab.inria.fr/
+gitlab https://0xacab.org/
+github https://github.com/
+wikidata https://www.wikidata.org/
+swmath https://swmath.org/
+ascl.net http://ascl.net/
+=============== =================================
+
+Metadata fetchers
+^^^^^^^^^^^^^^^^^
+
+Metadata fetchers are software components used to fetch metadata from
+a metadata authority, and ingest them into the Software Heritage archive.
+
+A metadata fetcher is uniquely defined by these properties:
+
+* its type
+* its version
+
+Examples:
+
+* :term:`loaders <loader>`, which may either discover metadata as a
+ side-effect of loading source code, or be dedicated to fetching metadata.
+
+* :term:`listers <lister>`, which may discover metadata as a side-effect
+ of discovering origins.
+
+* :term:`deposit` submitters, which push metadata to SWH from a
+ third-party; usually at the same time as a :term:`software artifact`
+
+* crawlers, which fetch metadata from an authority in a way that is
+ none of the above (eg. by querying a specific API of the origin's forge).
+
+
+Storage API
+~~~~~~~~~~~
+
+Authorities and metadata fetchers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The :term:`storage` API offers these endpoints to manipulate metadata
+authorities and metadata fetchers:
+
+* ``metadata_authority_add(type, url, metadata)``
+ which adds a new metadata authority to the storage.
+
+* ``metadata_authority_get(type, url)``
+ which looks up a known authority (there is at most one) and if it is
+ known, returns a dictionary with keys ``type``, ``url``, and ``metadata``.
+
+* ``metadata_fetcher_add(name, version, metadata)``
+ which adds a new metadata fetcher to the storage.
+
+* ``metadata_fetcher_get(name, version)``
+ which looks up a known fetcher (there is at most one) and if it is
+ known, returns a dictionary with keys ``name``, ``version``, and
+ ``metadata``.
+
+These `metadata` fields contain JSON-encodable dictionaries
+with information about the authority/fetcher, in a format specific to each
+authority/fetcher.
+With authority, the `metadata` field is reserved for information describing
+and qualifying the authority.
+With fetchers, the `metadata` field is reserved for configuration metadata
+and other technical usage.
+
+Origin metadata storage
+-----------------------
+
+Extrinsic metadata are stored in SWH's :term:`storage database`.
+The storage API offers three endpoints to manipulate origin metadata:
+
+* Adding metadata::
+
+ origin_metadata_add(origin_url, discovery_date,
+ authority, fetcher,
+ metadata)
+
+ which adds a new `metadata` byte string obtained from a given authority
+ and associated to the origin.
+ `authority` must be a dict containing keys `type` and `url`, and
+ `fetcher` a dict containing keys `name` and `version`.
+ The authority and fetcher must be known to the storage before using this
+ endpoint.
+
+* Getting latest metadata::
+
+ origin_metadata_get_latest(origin_url, authority)
+
+ where `authority` must be a dict containing keys `type` and `url`,
+ which returns a dictionary corresponding to the latest metadata entry
+ added from this origin, in the format::
+
+ {
+ 'authority': {'type': ..., 'url': ...},
+ 'fetcher': {'name': ..., 'version': ...},
+ 'discovery_date': ...,
+ 'metadata': b'...'
+ }
+
+
+* Getting all metadata::
+
+ origin_metadata_get(origin_url,
+ authority,
+ after, limit)
+
+ which returns a list of dictionaries, one for each metadata item
+ deposited, corresponding to the given origin and obtained from the
+ specified authority.
+ `authority` must be a dict containing keys `type` and `url`.
+
+ Each of these dictionaries is in the following format::
+
+ {
+ 'authority': {'type': ..., 'url': ...},
+ 'fetcher': {'name': ..., 'version': ...},
+ 'discovery_date': ...,
+ 'metadata': b'...'
+ }
+
+The parameters ``after`` and ``limit`` are used for pagination based on the
+order defined by the ``discovery_date``.
+
+``metadata`` is a bytes array (eventually encoded using Base64).
+Its format is specific to each authority; and is treated as an opaque value
+by the storage.
+Unifying these various formats into a common language is outside the scope
+of this specification.
diff --git a/docs/index.rst b/docs/index.rst
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -30,6 +30,11 @@
* :ref:`archive-copies`
+Specifications
+--------------
+
+* :ref:`extrinsic-metadata-specification`
+
Reference Documentation
-----------------------

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