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:

• Grafeas uses a scheme, namespace, name and version in a URL-like string. https://github.com/Grafeas/Grafeas
• Here.com OSRK uses a package manager, name and version field and a colon- separated URL-like string https://github.com/heremaps/oss-review-toolkit
• JFrog XRay uses a scheme, namespace, name and version in a URL-like string https://www.jfrog.com/confluence/display/XRAY/Xray+REST+API#XrayRESTAPI-ComponentIdentifiers
• Libraries.io uses a platform, name and version https://libraries.io/
• OpenShift fabric8 analytics uses ecosystem, name and version https://github.com/fabric8-analytics/
• ScanCode and AboutCode.org use a type, name and version https://github.com/nexB/scancode-toolkit
• SPDX has an appendix for external repository references and uses a type and a locator with a type-specific syntax for component separators in a URL-like string https://spdx.github.io/spdx-spec/latest/package-information/
• versioneye uses a type, name and version https://github.com/versioneye/
• Sonatype Lifecycle uses a format id followed by format specific coordinates. https://links.sonatype.com/products/nxiq/doc/component-identifier

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 programming 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 definition 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 form a hierarchy from the most significant component on the left to the least significant component 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

• .NET: https://github.com/package-url/packageurl-dotnet
• Elixir: https://github.com/maennchen/purl
• Go: https://github.com/package-url/packageurl-go
• Java: https://github.com/package-url/packageurl-java, https://github.com/sonatype/package-url-java
• JavaScript: https://github.com/package-url/packageurl-js
• Perl: https://github.com/giterlizzi/perl-URI-PackageURL
• PHP: https://github.com/package-url/packageurl-php
• Python: https://github.com/package-url/packageurl-python
• Ruby: https://github.com/package-url/packageurl-ruby
• Rust: https://github.com/package-url/packageurl.rs
• Swift: https://github.com/package-url/packageurl-swift

Users, adopters and links (alphabetical order)

• CycloneDX: A lightweight software bill-of-material (SBOM) specification
• GitHub Dependency Submission API: allows third-party tools to submit dependency data to GitHub for inclusion in a repository's dependency graph.
• OWASP Dependency-Track: Open source component analysis platform
• OSS Index: A free catalog of Open Source Components and scanning tools to help developers identify vulnerable components
• OSV Schema and OSV.dev: Open Source Vulnerability Schema and distributed vulnerability database
• Scancode Toolkit: Reports purl from parsed package manifests using https://github.com/package-url/packageurl-python
• Sonatype Nexus Lifecycle: Enterprise grade Open Source component management
• SPDX: A data exchange standard for human-readable and machine-processable software bill-of-materials (SBOM)

License

This document is licensed under the MIT license