Metadata workflow#

Intrinsic metadata#

Indexing intrinsic metadata requires extracting information from the lowest levels of the Merkle DAG (directories, files, and content blobs) and associate them to the highest ones (origins). In order to deduplicate the work between origins, we split this work between multiple indexers, which coordinate with each other and save their results at each step in the indexer storage.

Indexer architecture#

Sequence diagram#

Data flow and storage#

Origin-Head Indexer#

First, the Origin-Head indexer gets called externally, with an origin as argument (or multiple origins, that are handled sequentially). For now, its tasks are scheduled manually via recurring Scheduler tasks; but in the near future, the journal will be used to do that.

It first looks up the last snapshot and determines what the main branch of origin is (the “Head branch”) and what revision it points to (the “Head”). Intrinsic metadata for that origin will be extracted from that revision.

It schedules a Directory Metadata Indexer task for the root directory of that revision.

Directory and Content Metadata Indexers#

These two indexers do the hard part of the work. The Directory Metadata Indexer fetches the root directory associated with a revision, then extracts the metadata from that directory.

To do so, it lists files in that directory, and looks for known names, such as codemeta.json, package.json, or pom.xml. If there are any, it runs the Content Metadata Indexer on them, which in turn fetches their contents and runs them through extraction dictionaries/mappings. See below for details.

Their results are saved in a database (the indexer storage), associated with the content and directory hashes.

Origin Metadata Indexer#

The job of this indexer is very simple: it takes an origin identifier and uses the Origin-Head and Directory indexers to get metadata from the head directory of an origin, and copies the metadata of the former to a new table, to associate it with the latter.

The reason for this is to be able to perform searches on metadata, and efficiently find out which origins matched the pattern. Running that search on the directory_metadata table would require either a reverse lookup from directories to origins, which is costly.

Translation from ecosystem-specific metadata to CodeMeta#

Intrinsic metadata is extracted from files provided with a project’s source code, and translated using CodeMeta’s crosswalk table; which is vendored in swh/indexer/data/codemeta/codemeta.csv. Ecosystems not yet included in Codemeta’s crosswalk have their own swh/indexer/data/*.csv file, with one row for each CodeMeta property, even when not supported by the ecosystem.

All input formats supported so far are straightforward dictionaries (eg. JSON) or can be accessed as such (eg. XML); and the first part of the translation is to map their keys to a term in the CodeMeta vocabulary. This is done by parsing the crosswalk table’s CSV file and using it as a map between these two vocabularies; and this does not require any format-specific code in the indexers.

The second part is to normalize values. As language-specific metadata files each have their way(s) of formatting these values, we need to turn them into the data type required by CodeMeta. This normalization makes up for most of the code of swh.indexer.metadata_dictionary.

Extrinsic metadata#

Indexer architecture#

The extrinsic metadata indexer works very differently from the intrinsic metadata indexers we saw above. While the latter extract metadata from software artefacts (files and directories) which are already a core part of the archive, the former extracts such data from API calls pulled from forges and package managers, or pushed via the SWORD deposit.

In order to preserve original information verbatim, the Software Heritage itself stores the result of these calls, independently of indexers, in their own archive as described in the Extrinsic metadata specification. In this section, we assume this information is already present in the archive, but in the “raw extrinsic metadata” form, which needs to be translated to a common vocabulary to be useful, as with intrinsic metadata.

The common vocabulary we chose is JSON-LD, with both CodeMeta and ForgeFed’s vocabulary (including ActivityStream’s vocabulary)

Instead of the four-step architecture above, the extrinsic-metadata indexer is standalone: it reads “raw extrinsic metadata” from the swh-journal, and produces new indexed entries in the database as they come.

The caveat is that, while intrinsic metadata are always unambiguously authoritative (they are contained by their own origin repository, therefore they were added by the origin’s “owners”), extrinsic metadata can be authored by third-parties. Support for third-party authorities is currently not implemented for this reason; so extrinsic metadata is only indexed when provided by the same forge/package-repository as the origin the metadata is about. Metadata on non-origin objects (typically, directories), is also ignored for this reason, for now.

Assuming the metadata was provided by such an authority, it is then passed to metadata mappings; identified by a mimetype (or custom format name) they declared rather than filenames.

Implementation status#

Supported intrinsic metadata#

The following sources of intrinsic metadata are supported:

Supported extrinsic metadata#

The following sources of extrinsic metadata are supported:

Supported JSON-LD properties#

The following terms may be found in the output of the metadata translation (other than the codemeta mapping, which is the identity function, and therefore supports all properties):
	composer, pkg-info
	cff, gemspec, gitea, github, npm, pubspec
	gitea, github
	gitea, github
	cff, composer, gemspec, gitea, github, maven, npm, nuget, pkg-info, pubspec
	gemspec, pkg-info
	cff, maven
	cff, composer, github, npm, nuget, pkg-info, pubspec
	cff, composer, gemspec, github, npm, nuget, pkg-info, pubspec
	composer, gemspec, gitea, github, maven, npm, nuget, pkg-info, pubspec
	gitea, github
	composer, gitea, npm, nuget, pkg-info, pubspec
	cff, composer, gemspec, maven, npm, nuget, pkg-info
	npm, pubspec
	gitea, github
	gitea, github
	gitea, github
	gitea, github
	gitea, github


The rest of this page is made of two tutorials: one to index intrinsic metadata (ie. from a file in a VCS or in a tarball), and one to index extrinsic metadata (ie. obtained via external means, such as GitHub’s or GitLab’s APIs).

Adding support for additional ecosystem-specific intrinsic metadata#

This section will guide you through adding code to the metadata indexer to detect and translate new metadata formats.

First, you should start by picking one of the CodeMeta crosswalks. Then create a new file in swh-indexer/swh/indexer/metadata_dictionary/, that will contain your code, and create a new class that inherits from helper classes, with some documentation about your indexer:

from .base import DictMapping, SingleFileIntrinsicMapping
from swh.indexer.codemeta import CROSSWALK_TABLE

class MyMapping(DictMapping, SingleFileIntrinsicMapping):
        """Dedicated class for ..."""
        name = 'my-mapping'
        filename = b'the-filename'
        mapping = CROSSWALK_TABLE['Name of the CodeMeta crosswalk']

And reference it from swh.indexer.metadata_dictionary.INTRINSIC_MAPPINGS.

Then, add a string_fields attribute, that is the list of all keys whose values are simple text values. For instance, to translate Python PKG-INFO, it’s:

string_fields = ['name', 'version', 'description', 'summary',
                 'author', 'author-email']

These values will be automatically added to the above list of supported terms.

Last step to get your code working: add a translate method that will take a single byte string as argument, turn it into a Python dictionary, whose keys are the ones of the input document, and pass it to _translate_dict.

For instance, if the input document is in JSON, it can be as simple as:

def translate(self, raw_content):
    raw_content = raw_content.decode()  # bytes to str
    content_dict = json.loads(raw_content)  # str to dict
    return self._translate_dict(content_dict)  # convert to CodeMeta

_translate_dict will do the heavy work of reading the crosswalk table for each of string_fields, read the corresponding value in the content_dict, and build a CodeMeta dictionary with the corresponding names from the crosswalk table.

One last thing to run your code: add it to the list in swh-indexer/swh/indexer/metadata_dictionary/, so the rest of the code is aware of it.

Now, you can run it:

python3 -m swh.indexer.metadata_dictionary MyMapping path/to/input/file

and it will (hopefully) returns a CodeMeta object.

If it works, well done!

You can now improve your translation code further, by adding methods that will do more advanced conversion. For example, if there is a field named license containing an SPDX identifier, you must convert it to an URI, like this:

def normalize_license(self, s):
    if isinstance(s, str):
        return rdflib.URIRef("" + s)

This method will automatically get called by _translate_dict when it finds a license field in content_dict.

Adding support for additional ecosystem-specific extrinsic metadata#

[this section is a work in progress]