Fedora Packaging Guidelines for Modules

Disclaimer

Note this document is just a draft. These aren't any official, approved Fedora guidelines. Neither the modulemd format we're working with here is finalized.

Overview

The goal of this document is to describe how to create valid module files, document purposes of all the data fields in them, hint best practices and demonstrate some examples.

Each module is defined by a single YAML file and comprises of a number of key-value pairs describing the module's properties and components it contains. Not everything needs to (or even should) be filled in by the module packager; some of the fields get populated later during the module build or distribution phase. The module file format is commonly known as modulemd.

The original format specification can be found in the modulemd repository.

Required fields

Document header and the data section

Every modulemd file MUST contain a modulemd document header which consists of the document type tag and the document format version, and a data section holding the module data.

 document: modulemd
 version: 0
 data:
     (...)

The version is an integer and is not currently used for anything; this will change later in the modulemd development phase.

Module name

Every module MUST define its name. The format isn't strictly defined yet but will most likely follow the format of SRPM names.

 name: example

Module version and release

Every module MUST also specify its version and release. The format isn't strictly defined yet but will most likely follow RPM versioning conventions. See Packaging:NamingGuidelines#Package_Versioning for details.

 version: 1.23
 release: 1

The version field SHOULD be the same as the main module component version, where applicable. For example a webserver module that contains the httpd-2.4.18 webserver as its key component should use 2.4.18 for its version. If this cannot be done, the module maintainer may choose their own versioning scheme.

The release field is incremented when the module is updated in a way that doesn't warrant a version field bump or the module needs to be rebuilt. (Note: The fedora dist tag should not be included in the release field.)

Module summary and description

Every module MUST include human-readable short summary and description. Both should be written in US English.

 summary: An example module
 description: >
     An example long description of an example module, written just
     to demonstrate the purpose of this field.

The summary is a one sentence concise description of the module and SHOULD NOT end in a period.

The description expands on this and SHOULD end in a period. Description SHOULD NOT contain installation instructions or configuration manuals.

Module licensing

Every module MUST contain a license section and declare a list of the module's licenses. Note these aren't the module's components' licenses.

 license:
     module:
         - MIT

Fedora content, such as SPEC files or patches not included upstream, uses the MIT license by default, unless the component packager declares otherwise. Therefore MIT might be a reasonable default for most module authors as well.

See Fedora_Packaging_Guidelines_for_Modules#Module_content_licensing to see how to declare components' licenses.

Optional fields

Module content licensing

If the module includes some RPM or non-RPM content, the packager MAY also define a list of content licenses.

 license:
     module:
         - MIT
     content:
         - GPL+
         - BSD

Not every module includes packages and therefore doesn't necessarily have to include this field.

Furthermore, the content licenses list should ideally be automatically filled by module build tools rather than the module author.

Module dependencies

Modules MAY depend on other modules. These module relationships are listed in the depepdencies section.

Note: This section may change significantly in the future.

 dependencies:
     buildrequires:
         example-build-dependency: 1.23-1
     requires:
         example-runtime-dependency: 45-1

So far modulemd supports two kinds of dependencies:

buildrequires for listing build dependencies of the module, i.e. modules that define the buildroot for building the module's components
requires for listing runtime dependencies of the module, i.e. modules that need to be available on the target system for this module to work properly

Both these sections contain key-value pairs where keys are the required module names and values the minimum required version in the version-release format.

Either or both of these sections may be omitted.

Extensible module metadata block

Modules MAY also contain an extensible metadata block, a list of vendor-defined key-value pairs.

 xmd:
     user-defined-key: 42
     another-user-defined-key:
         - the first value of the list
         - the second value of the list

Module references

Modules MAY define links referencing various upstream resources, such as community website, project documentation or upstream bug tracker.

 references:
     community: http://www.example.com/
     documentation: http://www.example.com/docs/1.23/
     tracker: http://www.example.com/bugs/

Module components

Modules MAY, and most modules do contain a components section defining the module's content.

 components:
     (...)

RPM content

Module RPM content is defined in the rpms subsection of components and typically consists of one or more packages described by their SRPM names and additional extra key-value pairs, some required and some optional, associated with them.

Besides packages the rpms section MAY also define the following keys:

the dependencies switch which controls whether RPM-level dependencies should be automatically included, or bundled, in the module if none of the modules required at runtime would provide these dependencies; this defaults to False and the value cannot be overridden for production module builds in Fedora.
the api list of RPM binary package names that are considered the stable module API, i.e. supported RPM components of this module that shouldn't break or disappear without a notice; binary RPM packages not listed here are considered internal details and other modules SHOULD NOT depend on them or even expect them to be shipped. Modules with RPM content SHOULD define some RPM module API.

 components:
     rpms:
         dependencies: False
         api:
             - foo
             - foo-devel
         packages:
             foo:
                 rationale: The key component of this module
                 commit: abcdef1
                 repository: git://git.example.com/foo.git
                 cache: http://www.example.com/lookasidecache/
                 arches:
                     - i686
                     - x86_64
                 multilib:
                     - x86_64

The following key-value pairs extend the SRPM name:

rationale - every component MUST declare why it was added to the module; this is currently a free form string and might change in the future.
commit - the git hash or other commit ID in the repository that should be built and included in this module; recommended. If not defined, the current HEAD or equivalent is used and the hash or other commit ID is recorded in the module by module build tools. So, for example, if we're looking at the commit for the Fedora httpd package, the git hash should come from the official Fedora httpd package repo.
repository - specifies git or other VCS repository to use as the component's source; in Fedora, dist-git is used and this option cannot be overridden.
cache - points to RPM lookaside cache; in Fedora this option cannot be overriden.
arches - a list of architectures this component should be built for; defaults to all available architectures.
multilib - a list of architectures where this component should be available as multilib, e.g. if x86_64 is listed, x86_64 repositories will also include i686 builds. Defaults to no multilib.

Non-RPM content

Non-RPM content isn't currently supported.

Examples

Minimal module

A minimal module example-1.23-1, containing no packages, having no dependencies whatsoever and defining only the minimal set of required metadata.

 document: modulemd
 version: 0
 data:
     name: example
     version: 1.23
     release: 1
     summary: An example summary
     description: >
         An example description.
     license:
         module:
             - MIT

Minimal module with RPM content

A minimal module, example-1.23-1, containing one RPM package with SRPM name foo. This module doesn't define any dependencies or optional metadata.

 document: modulemd
 version: 0
 data:
     name: example
     version: 1.23
     release: 1
     summary: An example summary
     description: >
         An example description.
     license:
         module:
             - MIT
     components:
         rpms:
             packages:
                 foo:
                     rationale: An example RPM component

Minimal module with dependencies only (stack)

A minimal module, example-stack-1.23-1, containing no packages or any optional metadata besides dependencies. Modules of this type are referred to as stacks.

 document: modulemd
 version: 0
 data:
     name: example-stack
     version: 1.23
     release: 1
     summary: An example summary
     description: >
         An example description.
     license:
         module:
             - MIT
     dependencies:
         requires:
             another-module: 0
             one-more: 22

Common Fedora module

A typical Fedora module defines all the mandatory metadata plus some useful references, has build and runtime dependencies and contains one or more packages built from specific commits in dist-git. It relies on Fedora build tools to extract licensing information from the included component and populate the data→license→content list.

 document: modulemd
 version: 0
 data:
     name: common-module
     version: 5.67
     release: 8
     summary: An example of a common Fedora module
     description:  This module demonstrates what most Fedora modules look like.
     license:
         module: [ MIT ]
     dependencies:
         buildrequires:
             base-runtime: 25
             common-build-tools: 0
         requires:
             base-runtime: 25
     references:
         community: http://www.example.com/common-package
         documentation: http://www.example.com/common-package/docs/5.67/
     components:
         rpms:
             api:
                 - common-package
                 - common-package-devel
                 - common-plugins
             packages:
                 common-package:
                     rationale: The key component of this module
                     commit: fd245a0
                 common-plugins:
                     rationale: Extensions for common-package
                     commit: ff09556

Complete module definition

See the modulemd specification.

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