PEP 658 – Serve Distribution Metadata in the Simple Repository API
- Author:
- Tzu-ping Chung <uranusjr at gmail.com>
- Sponsor:
- Brett Cannon <brett at python.org>
- PEP-Delegate:
- Donald Stufft <donald at stufft.io>
- Discussions-To:
- Discourse thread
- Status:
- Accepted
- Type:
- Standards Track
- Topic:
- Packaging
- Created:
- 10-May-2021
- Post-History:
- 10-May-2021
- Resolution:
- Discourse post
Abstract
This PEP proposes adding an anchor tag to expose the METADATA file
from distributions in the PEP 503 “simple” repository API. A
data-dist-info-metadata attribute is introduced to indicate that
the file from a given distribution can be independently fetched.
Motivation
Package management workflows made popular by recent tooling increase the need to inspect distribution metadata without intending to install the distribution, and download multiple distributions of a project to choose from based on their metadata. This means they end up discarding much downloaded data, which is inefficient and results in a bad user experience.
Rationale
Tools have been exploring methods to reduce the download size by partially downloading wheels with HTTP range requests. This, however, adds additional run-time requirements to the repository server. It also still adds additional overhead, since a separate request is needed to fetch the wheel’s file listing to find the correct offset to fetch the metadata file. It is therefore desired to make the server extract the metadata file in advance, and serve it as an independent file to avoid the need to perform additional requests and ZIP inspection.
The metadata file defined by the Core Metadata Specification
[core-metadata] will be served directly by repositories since it
contains the necessary information for common use cases. The metadata
must only be served for standards-compliant distributions such as
wheels [wheel] and sdists [sdist], and must be identical to the
distribution’s canonical metadata file, such as a wheel’s METADATA
file in the .dist-info directory [dist-info].
An HTML attribute on the distribution file’s anchor link is needed to indicate whether a client is able to choose the separately served metadata file. The attribute is also used to provide the metadata content’s hash for client-side verification. The attribute’s absence indicates that a separate metadata entry is not available for the distribution, either because of the distribution’s content, or lack of repository support.
Specification
In a simple repository’s project page, each anchor tag pointing to a
distribution MAY have a data-dist-info-metadata attribute. The
presence of the attribute indicates the distribution represented by
the anchor tag MUST contain a Core Metadata file that will not be
modified when the distribution is processed and/or installed.
If a data-dist-info-metadata attribute is present, the repository
MUST serve the distribution’s Core Metadata file alongside the
distribution with a .metadata appended to the distribution’s file
name. For example, the Core Metadata of a distribution served at
/files/distribution-1.0-py3.none.any.whl would be located at
/files/distribution-1.0-py3.none.any.whl.metadata. This is similar
to how PEP 503 specifies the GPG signature file’s location.
The repository SHOULD provide the hash of the Core Metadata file
as the data-dist-info-metadata attribute’s value using the syntax
<hashname>=<hashvalue>, where <hashname> is the lower cased
name of the hash function used, and <hashvalue> is the hex encoded
digest. The repository MAY use true as the attribute’s value
if a hash is unavailable.
Backwards Compatibility
If an anchor tag lacks the data-dist-info-metadata attribute,
tools are expected to revert to their current behaviour of downloading
the distribution to inspect the metadata.
Older tools not supporting the new data-dist-info-metadata
attribute are expected to ignore the attribute and maintain their
current behaviour of downloading the distribution to inspect the
metadata. This is similar to how prior data- attribute additions
expect existing tools to operate.
Rejected Ideas
Put metadata content on the project page
Since tools generally only need dependency information from a
distribution in addition to what’s already available on the project
page, it was proposed that repositories may directly include the
information on the project page, like the data-requires-python
attribute specified in PEP 503.
This approach was abandoned since a distribution may contain arbitrarily long lists of dependencies (including required and optional), and it is unclear whether including the information for every distribution in a project would result in net savings since the information for most distributions generally ends up unneeded. By serving the metadata separately, performance can be better estimated since data usage will be more proportional to the number of distributions inspected.
Expose more files in the distribution
It was proposed to provide the entire .dist-info directory as a
separate part, instead of only the metadata file. However, searving
multiple files in one entity through HTTP requires re-archiving them
separately after they are extracted from the original distribution
by the repository server, and there are no current use cases for files
other than METADATA when the distribution itself is not going to
be installed.
It should also be noted that the approach taken here does not preclude other files from being introduced in the future, whether we want to serve them together or individually.
Explicitly specify the metadata file’s URL on the project page
An early version of this draft proposed putting the metadata file’s
URL in the data-dist-info-metadata attribute. But people feel it
is better for discoverability to require the repository to serve the
metadata file at a determined location instead. The current approach
also has an additional benefit of making the project page smaller.
References
Copyright
This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.
Source: https://github.com/python/peps/blob/main/pep-0658.rst
Last modified: 2022-06-14 21:22:20 GMT