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Package URL specification v1.0.X

The Package URL core specification defines a versioned and formalized format, syntax, and rules used to represent and validate purl.

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.

See <PURL-TYPES.rst>_ for known type definitions.

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

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 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/[email protected]?arch=i386&distro=jessie
pkg:gem/[email protected]
pkg:github/package-url/purl-spec@244fd47e07d1004f0aed9c
pkg:golang/google.golang.org/genproto#googleapis/api/annotations
pkg:maven/org.apache.xmlgraphics/[email protected]?packaging=sources
pkg:npm/[email protected]
pkg:nuget/[email protected]
pkg:pypi/[email protected]
pkg:rpm/fedora/[email protected]?arch=i386&distro=fedora-25

A purl is a URL

  • A purl is a valid URL and URI that conforms to the URL definitions or specifications at:
  • This is a valid URL because it is a locator even though it has no Authority URL component: each type has a default repository location when defined.
  • The purl components are mapped to these URL components:
    • purl scheme: this is a URL scheme with a constant value: pkg
    • purl type, namespace, name and version components: these are collectively mapped to a URL path
    • purl qualifiers: this maps to a URL query
    • purl subpath: this is a URL fragment
    • In a purl there is no support for a URL Authority (e.g. NO username, password, host and port components).
  • Special URL schemes as defined in https://url.spec.whatwg.org/ such as file://, https://, http:// and ftp:// are NOT valid purl types. They are valid URL or URI schemes but they are not purl. They may be used to reference URLs in separate attributes outside of a purl or in a purl qualifier.
  • Version control system (VCS) URLs such git://, svn://, hg:// or as defined in Python pip or SPDX download locations are NOT valid purl types. They are valid URL or URI schemes but they are not purl. They are a closely related, compact and uniform way to reference vcs URLs. They may be used as references in separate attributes outside of a purl or in a purl qualifier.

Rules for each purl component

A purl string is an ASCII URL string composed of seven components.

Some components are allowed to use other characters beyond ASCII: these components must then be UTF-8-encoded strings and percent-encoded as defined in the "Character encoding" section.

The rules for each component are:

  • scheme:

    • The scheme is a constant with the value "pkg"

    • Since a purl never contains a URL Authority, its scheme must not be suffixed with double slash as in 'pkg://' and should use instead 'pkg:'. Otherwise this would be an invalid URI per rfc3986 at https://tools.ietf.org/html/rfc3986#section-3.3:

      If a URI does not contain an authority component, then the path
cannot begin with two slash characters ("//").

      It is therefore incorrect to use such '://' scheme suffix as the URL would no longer be valid otherwise. In its canonical form, a purl must NOT use such '://' scheme suffix but only ':' as a scheme suffix.

    • purl parsers must accept URLs such as 'pkg://' and must ignore the '//'.

    • purl builders must not create invalid URLs with such double slash '//'.

    • The scheme is followed by a ':' separator

    • For example these two purls are strictly equivalent and the first is in canonical form. The second purl with a '//' is an acceptable purl but is an invalid URI/URL per rfc3986:

      pkg:gem/[email protected]
pkg://gem/[email protected]

  • type:

    • The package type is composed only of ASCII letters and numbers, '.', '+' and '-' (period, plus, and dash)
    • The type cannot start with a number
    • The type cannot contains spaces
    • The type must NOT be percent-encoded
    • The type is case insensitive. The canonical form is lowercase

  • namespace:

    • The optional namespace contains zero or more segments, separated by slash '/'
    • Leading and trailing slashes '/' are not significant and should be stripped in the canonical form. They are not part of the namespace
    • Each namespace segment must be a percent-encoded string
    • When percent-decoded, a segment:
      • must not contain a '/'
      • must not be empty
    • A URL host or Authority must NOT be used as a namespace. Use instead a repository_url qualifier. Note however that for some types, the namespace may look like a host.

  • name:

    • The name is prefixed by a '/' separator when the namespace is not empty
    • This '/' is not part of the name
    • A name must be a percent-encoded string

  • version:

    • The version is prefixed by a '@' separator when not empty
    • This '@' is not part of the version
    • A version must be a percent-encoded string
    • A version is a plain and opaque string. Some package types use versioning conventions such as SemVer for NPMs or NEVRA conventions for RPMS. A type may define a procedure to compare and sort versions, but there is no reliable and uniform way to do such comparison consistently.

  • qualifiers:

    • The qualifiers string is prefixed by a '?' separator when not empty
    • This '?' is not part of the qualifiers
    • This is a query string composed of zero or more key=value pairs each separated by a '&' ampersand. A key and value are separated by the equal '=' character
    • These '&' are not part of the key=value pairs.
    • key must be unique within the keys of the qualifiers string
    • value cannot be an empty string: a key=value pair with an empty value is the same as no key/value at all for this key
    • For each pair of key = value:
      • The key must be composed only of ASCII letters and numbers, '.', '-' and '_' (period, dash and underscore)
      • A key cannot start with a number
      • A key must NOT be percent-encoded
      • A key is case insensitive. The canonical form is lowercase
      • A key cannot contains spaces
      • A value must be a percent-encoded string
      • The '=' separator is neither part of the key nor of the value

  • subpath:

    • The subpath string is prefixed by a '#' separator when not empty
    • This '#' is not part of the subpath
    • The subpath contains zero or more segments, separated by slash '/'
    • Leading and trailing slashes '/' are not significant and should be stripped in the canonical form
    • Each subpath segment must be a percent-encoded string
    • When percent-decoded, a segment:
      • must not contain a '/'
      • must not be any of '..' or '.'
      • must not be empty
    • The subpath must be interpreted as relative to the root of the package

Character encoding

For clarity and simplicity a purl is always an ASCII string. To ensure that there is no ambiguity when parsing a purl, separator characters and non-ASCII characters must be UTF-encoded and then percent-encoded as defined at:

https://en.wikipedia.org/wiki/Percent-encoding

Use these rules for percent-encoding and decoding purl components:

  • the type must NOT be encoded and must NOT contain separators
  • the '#', '?', '@' and ':' characters must NOT be encoded when used as separators. They may need to be encoded elsewhere
  • the ':' scheme and type separator does not need to and must NOT be encoded. It is unambiguous unencoded everywhere
  • the '/' used as type/namespace/name and subpath segments separator does not need to and must NOT be percent-encoded. It is unambiguous unencoded everywhere
  • the '@' version separator must be encoded as %40 elsewhere
  • the '?' qualifiers separator must be encoded as %3F elsewhere
  • the '=' qualifiers key/value separator must NOT be encoded
  • the '#' subpath separator must be encoded as %23 elsewhere
  • All non-ASCII characters must be encoded as UTF-8 and then percent-encoded

It is OK to percent-encode purl components otherwise except for the type. Parsers and builders must always percent-decode and percent-encode purl components and component segments as explained in the "How to parse" and "How to build" sections.

How to build purl string from its components

Building a purl ASCII string works from left to right, from type to subpath.

Note: some extra type-specific normalizations are required. See the "Known types section" for details.

To build a purl string from its components:

  • Start a purl string with the "pkg:" scheme as a lowercase ASCII string
  • Append the type string to the purl as a lowercase ASCII string
    • Append '/' to the purl
  • If the namespace is not empty:
    • Strip the namespace from leading and trailing '/'
    • Split on '/' as segments
    • Apply type-specific normalization to each segment if needed
    • UTF-8-encode each segment if needed in your programming language
    • Percent-encode each segment
    • Join the segments with '/'
    • Append this to the purl
    • Append '/' to the purl
    • Strip the name from leading and trailing '/'
    • Apply type-specific normalization to the name if needed
    • UTF-8-encode the name if needed in your programming language
    • Append the percent-encoded name to the purl
  • If the namespace is empty:
    • Apply type-specific normalization to the name if needed
    • UTF-8-encode the name if needed in your programming language
    • Append the percent-encoded name to the purl
  • If the version is not empty:
    • Append '@' to the purl
    • UTF-8-encode the version if needed in your programming language
    • Append the percent-encoded version to the purl
  • If the qualifiers are not empty and not composed only of key/value pairs where the value is empty:
    • Append '?' to the purl
    • Build a list from all key/value pair:
      • discard any pair where the value is empty.
      • UTF-8-encode each value if needed in your programming language
      • If the key is checksums and this is a list of checksums join this list with a ',' to create this qualifier value
      • create a string by joining the lowercased key, the equal '=' sign and the percent-encoded value to create a qualifier
    • sort this list of qualifier strings lexicographically
    • join this list of qualifier strings with a '&' ampersand
    • Append this string to the purl
  • If the subpath is not empty and not composed only of empty, '.' and '..' segments:
    • Append '#' to the purl
    • Strip the subpath from leading and trailing '/'
    • Split this on '/' as segments
    • Discard empty, '.' and '..' segments
    • Percent-encode each segment
    • UTF-8-encode each segment if needed in your programming language
    • Join the segments with '/'
    • Append this to the purl

How to parse a purl string in its components

Parsing a purl ASCII string into its components works from right to left, from subpath to type.

Note: some extra type-specific normalizations are required. See the "Known types section" for details.

To parse a purl string in its components:

  • Split the purl string once from right on '#'
    • The left side is the remainder
    • Strip the right side from leading and trailing '/'
    • Split this on '/'
    • Discard any empty string segment from that split
    • Discard any '.' or '..' segment from that split
    • Percent-decode each segment
    • UTF-8-decode each segment if needed in your programming language
    • Join segments back with a '/'
    • This is the subpath
  • Split the remainder once from right on '?'
    • The left side is the remainder
    • The right side is the qualifiers string
    • Split the qualifiers on '&'. Each part is a key=value pair
    • For each pair, split the key=value once from left on '=':
      • The key is the lowercase left side
      • The value is the percent-decoded right side
      • UTF-8-decode the value if needed in your programming language
      • Discard any key/value pairs where the value is empty
      • If the key is checksums, split the value on ',' to create a list of checksums
    • This list of key/value is the qualifiers object
  • Split the remainder once from left on ':'
    • The left side lowercased is the scheme
    • The right side is the remainder
  • Strip the remainder from leading and trailing '/'
    • Split this once from left on '/'
    • The left side lowercased is the type
    • The right side is the remainder
  • Split the remainder once from right on '@'
    • The left side is the remainder
    • Percent-decode the right side. This is the version.
    • UTF-8-decode the version if needed in your programming language
    • This is the version
  • Split the remainder once from right on '/'
    • The left side is the remainder
    • Percent-decode the right side. This is the name
    • UTF-8-decode this name if needed in your programming language
    • Apply type-specific normalization to the name if needed
    • This is the name
  • Split the remainder on '/'
    • Discard any empty segment from that split
    • Percent-decode each segment
    • UTF-8-decode the each segment if needed in your programming language
    • Apply type-specific normalization to each segment if needed
    • Join segments back with a '/'
    • This is the namespace

Known purl types

There are several known purl package type definitions tracked in the separate PURL-TYPES.rst document.

Known qualifiers key/value pairs

Note: Do not abuse qualifiers: it can be tempting to use many qualifier keys but their usage should be limited to the bare minimum for proper package identification to ensure that a purl stays compact and readable in most cases.

Additional, separate external attributes stored outside of a purl are the preferred mechanism to convey extra long and optional information such as a download URL, vcs URL or checksums in an API, database or web form.

With this warning, the known key and value defined here are valid for use in all package types:

  • repository_url is an extra URL for an alternative, non-default package repository or registry. When a package does not come from the default public package repository for its type a purl may be qualified with this extra URL. The default repository or registry of a type is documented in the "Known purl types" section.

  • download_url is an extra URL for a direct package web download URL to optionally qualify a purl.

  • vcs_url is an extra URL for a package version control system URL to optionally qualify a purl. The syntax for this URL should be as defined in Python pip or the SPDX specification. See https://github.com/spdx/spdx-spec/blob/cfa1b9d08903/chapters/3-package-information.md#37-package-download-location

    • TODO: incorporate the details from SPDX here.

  • file_name is an extra file name of a package archive.

  • checksum is a qualifier for one or more checksums stored as a comma-separated list. Each item in the value is in form of lowercase_algorithm:hex_encoded_lowercase_value such as sha1:ad9503c3e994a4f611a4892f2e67ac82df727086. For example (with checksums truncated for brevity)

    checksum=sha1:ad9503c3e994a4f,sha256:41bf9088b3a1e6c1ef1d

Tests

To support the language-neutral testing of purl implementations, a test suite is provided as JSON document named test-suite-data.json. This JSON document contains an array of objects. Each object represents a test with these key/value pairs some of which may not be normalized:

  • purl: a purl string.
  • canonical: the same purl string in canonical, normalized form
  • type: the type corresponding to this purl.
  • namespace: the namespace corresponding to this purl.
  • name: the name corresponding to this purl.
  • version: the version corresponding to this purl.
  • qualifiers: the qualifiers corresponding to this purl as an object of {key: value} qualifier pairs.
  • subpath: the subpath corresponding to this purl.
  • is_invalid: a boolean flag set to true if the test should report an error

To test purl parsing and building, a tool can use this test suite and for every listed test object, run these tests:

  • parsing the test canonical purl then re-building a purl from these parsed components should return the test canonical purl
  • parsing the test purl should return the components parsed from the test canonical purl
  • parsing the test purl then re-building a purl from these parsed components should return the test canonical purl
  • building a purl from the test components should return the test canonical purl

License

This document is licensed under the MIT license