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Context

We build and release software by massively consuming and producing software packages such as NPMs, RPMs, Rubygems, etc.

Each package manager, platform, type or ecosystem has its own conventions and protocols to identify, locate and provision software packages.

Problem

When tools, APIs and databases process or store multiple package types, it is difficult to reference the same software package across tools in a uniform way.

For example, these tools, specifications and API use relatively similar approaches to identify and locate software packages, each with subtle differences in syntax, naming and conventions:

Solution

A purl or package URL is an attempt to standardize existing approaches to reliably identify and locate software packages.

A purl is a URL string used to identify and locate a software package in a mostly universal and uniform way across programing languages, package managers, packaging conventions, tools, APIs and databases.

Such a package URL is useful to reliably reference the same software package using a simple and expressive syntax and conventions based on familiar URLs.

Check also this short purl presentation (with video) at FOSDEM 2018 https://fosdem.org/2018/schedule/event/purl/ for an overview.

purl

purl stands for package URL.

A purl is a URL composed of seven components:

scheme:type/namespace/name@version?qualifiers#subpath

Components are separated by a specific character for unambiguous parsing.

The defintion for each components is:

  • scheme: this is the URL scheme with the constant value of "pkg". One of the primary reason for this single scheme is to facilitate the future official registration of the "pkg" scheme for package URLs. Required.
  • type: the package "type" or package "protocol" such as maven, npm, nuget, gem, pypi, etc. Required.
  • namespace: some name prefix such as a Maven groupid, a Docker image owner, a GitHub user or organization. Optional and type-specific.
  • name: the name of the package. Required.
  • version: the version of the package. Optional.
  • qualifiers: extra qualifying data for a package such as an OS, architecture, a distro, etc. Optional and type-specific.
  • subpath: extra subpath within a package, relative to the package root. Optional.

Components are designed such that they for a hierarchy from the most significant on the left to the least significant components on the right.

A purl must NOT contain a URL Authority i.e. there is no support for username, password, host and port components. A namespace segment may sometimes look like a host but its interpretation is specific to a type.

Some purl examples

pkg:bitbucket/birkenfeld/pygments-main@244fd47e07d1014f0aed9c

pkg:deb/debian/curl@7.50.3-1?arch=i386&distro=jessie

pkg:docker/cassandra@sha256:244fd47e07d1004f0aed9c
pkg:docker/customer/dockerimage@sha256:244fd47e07d1004f0aed9c?repository_url=gcr.io

pkg:gem/jruby-launcher@1.1.2?platform=java
pkg:gem/ruby-advisory-db-check@0.12.4

pkg:github/package-url/purl-spec@244fd47e07d1004f0aed9c

pkg:golang/google.golang.org/genproto#googleapis/api/annotations

pkg:maven/org.apache.xmlgraphics/batik-anim@1.9.1?packaging=sources
pkg:maven/org.apache.xmlgraphics/batik-anim@1.9.1?repository_url=repo.spring.io/release

pkg:npm/%40angular/animation@12.3.1
pkg:npm/foobar@12.3.1

pkg:nuget/EnterpriseLibrary.Common@6.0.1304

pkg:pypi/django@1.11.1

pkg:rpm/fedora/curl@7.50.3-1.fc25?arch=i386&distro=fedora-25
pkg:rpm/opensuse/curl@7.56.1-1.1.?arch=i386&distro=opensuse-tumbleweed

(NB: some checksums are truncated for brevity)

Specification details

The purl specification consists of a core syntax definition and independent type definitions:

  • Package URL core: Defines a versioned and formalized format, syntax, and rules used to represent and validate purl.
  • Type definitions: Defines purl types (e.g. maven, npm, cargo, rpm, etc) independent of the core specification. Definitions also include types reserved for future use.

Known implementations

Users, adopters and links

License

This document is licensed under the MIT license

About

A minimal specification for purl aka. a package "mostly universal" URL, join the discussion at https://gitter.im/package-url/Lobby

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