Module:Guidelines

Find more information about creating a modular operating system on the Modularity Website.

Disclaimer

Note that this document is just a draft. These aren't official, approved Fedora guidelines.

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: 1
 data:
     (...)

The version is an integer and denotes the version of the metadata format the rest of the document is written in. As of now, only one officialy released version of the format exists, version 1.

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 name, update stream, version, context and architecture

Every module artifact SHOULD define its name, update stream, version, context and architecture.

 name: example
 stream: another-example
 version: 20161109235500
 context: c0ffee43
 arch: armv7hl

Note, however, that while technically possible, module packagers MUST NOT define these values manually but should rather expect the Module Build Service to do it for them, using the module's dist-git repository name as the module name, the dist-git repository branch name as the stream name, the particular commit timestamp as the version, a hash of the previous, plus runtime dependencies as the context flag and finally the main content hardware architecture compatibility as the arch. This simplifies module rebuilds and moving modules between branches or repositories.

name and stream may contain any lowercase or uppercase letters, numbers, hyphens, underscores, dots or plus signs. version is a positive integer. context is a short hexadecimal hash value distinguishing module artifacts with the same name, stream and version but built against different sets of dependencies. arch may only include one of our supported hardware architectures; note this is not $basearch.

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. However, this is not yet implemented.

Module dependencies

Modules MAY depend on other modules. These module relationships are listed in the depepdencies section. Dependencies are expressed using module names and their stream names.

 dependencies:
     buildrequires:
         platform: master
     requires:
         platform: master

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; this will typically be the platform module, at minimum
• 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; this too will typically be the platform module, at minimum

Either or both of these sections may be omitted, if necessary.

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 profiles

The module author MAY define lists of packages that would be installed by default, and a minimum, when the module is enabled and the particular profile is selected. Whether the packages actually get installed depends on the user's configuration. It is possible to define a profile that doesn't install any packages.

Profile names are arbitrary strings. There is currently one special-purpose profile name defined — default. More special-purpose profile names might be defined in the future.

The default profile lists packages that would be installed unless the user's configuration dictates otherwise.

In the case of RPM content, the profile package lists reference binary RPM package names.

 profiles:
     default:
         rpms:
             - myapplication
             - myapplication-plugins
     minimal:
         description: An example minimal profile installing only the myapplication package.
         rpms:
             - myapplication

Different use cases
If the primary API component(s) of a module serve different use cases with a different set of subpackages, you should describe these use cases as different install profiles of the module.

Module API

Module API are components, symbols, files or abstract features the module explicitly declares to be its supported interface. Everything else is considered an internal detail and shouldn't be relied on by any other module.

Every module SHOULD define its public API.

 api:
     rpms:
         - mypackage
         - mylibrary
         - mylibrary-devel

Currently the only supported type of API are binary RPM packages, that is the list of RPMs that are guaranteed to a) be present in the module, and b) not break their interfaces such as binaries their provide or their ABI.

Module filters

Module filters define lists of components or other content that should not be part of the resulting, composed module deliverable. They can be used to only ship a limited subset of generated RPM packages, for instance.

 filter:
     rpms:
         - mypackage-plugins

Currently the only supported type of filter are binary RPM packages.

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.

 components:
     rpms:
         foo:
             rationale: The key component of this module.
             buildorder: 100
             repository: git://git.example.com/foo.git
             ref: branch-tag-or-commit-hash
             cache: http://www.example.com/lookasidecache/
             arches:
                 - i686
                 - x86_64
             multilib:
                 - x86_64
         dependency-of-foo:
             rationale: Needed for foo.
             buildorder: 50
             repository: git://git.example.com/dependency-of-foo.git
             ref: master
             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. It should end with a period.
• buildorder - marks the component as a member of a specific build group; components are scheduled to be built in batches according to their buildorder tags, from the lowest to the highest; built components are tagged back into the buildroot before the next batch is built; several components can belong to the same build group by specifying the same buildorder value; build order within build groups is undefined; optional, integer, may be negative and defaults to zero if not specified.
• 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.
• ref - the repository reference (a branch or tag name or a commit hash) that should be built and included in this module; recommended. If not defined, the current HEAD or equivalent is used. ref is always populated by the exact commit hash used by the Module Build System during build.
• 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.

Module content

Modules may include other modules. This is similar to dependencies (both build- and run-time) but differs in a few key points:

• included modules are distributed with the parent module as one deliverable, no matter the format
• included modules are built in the buildroot defined by the parent module, recursively

Dependencies and module inclusions can be freely combined. Deciding on which is more fitting for your module varies from application to application.

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

 components:
     modules:
         my-favourite-module:
             rationale: An example of an included module.
             buildorder: 20
             repository: git://git.example.com/my-favourite-module.git
             ref: 12ab34cd5

The following key-value pairs extend the module-style components:

• rationale - see the description in the RPM content section
• buildorder - see the description in the RPM content section
• repository - see the description in the RPM content section
• ref - see the description in the RPM content section

Other content

No other content is currently supported.

Examples

Minimal module

A minimal module distributed as example-master-20161109172409, stored in the modules/example dist-git repository and its master branch, built on November 9, 2016, at 17:24:09 UTC, containing no packages, having no dependencies whatsoever and defining only the minimal set of required metadata.

 document: modulemd
 version: 1
 data:
     summary: An example summary
     description: And an example description.
     license:
         module:
             - MIT

Minimal module with RPM content

Another flavour of the abovementioned module, containing one RPM package with SRPM name foo. This module doesn't define any dependencies or optional metadata.

 document: modulemd
 version: 1
 data:
     summary: An example summary
     description: And an example description.
     license:
         module:
             - MIT
     components:
         rpms:
             foo:
                 rationale: An example RPM component.

Minimal module with RPM content but with the -docs subpackage excluded

Yet another flavour of the minimal module, containing one RPM package with SRPM name foo. A build of 'foo' creates binary packages 'foo-1.0-1' and the subpackage 'foo-doc-1.0-1'. Both would get included in the module for any architecture if no filter were be used. This module doesn't define any dependencies or optional metadata.

 document: modulemd
 version: 1
 data:
     summary: An example summary
     description: And an example description.
     license:
         module:
             - MIT
     filter:
         rpms:
             - foo-docs
     components:
         rpms:
             foo:
                 rationale: An example RPM component.

Minimal module with dependencies only (a variant of stack)

Another minimal module, containing no packages or any optional metadata besides dependencies. Modules of this type are, together with modules that include other modules, referred to as stacks.

 document: modulemd
 version: 1
 data:
     summary: An example summary
     description: And an example description.
     license:
         module:
             - MIT
     dependencies:
         requires:
             platform: master
             a-framework-module: and-its-stream

Minimal module which includes another (another variant of stack)

Yet another minimal module, containing no optional metadata besides a single included module in the components section. Modules of this type are, together with modules that only depend on other modules, referred to as stacks.

 document: modulemd
 version: 1
 data:
     summary: An example summary
     description: And an example description.
     license:
         module:
             - MIT
     content:
         modules:
             a-framework-module:
                 rationale: Bundled for various reasons.

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 refs in dist-git. It relies on the Module Build Service to extract the name, stream and version properties from the VCS data and to fill in the licensing information from the included components and populate the data→license→content list.

 document: modulemd
 version: 1
 data:
     summary: An example of a common Fedora module
     description: This module demonstrates what most Fedora modules look like.
     license:
         module: [ MIT ]
     dependencies:
         buildrequires:
             platform: master
             extra-build-environment: master
         requires:
             platform: master
     references:
         community: http://www.example.com/common-package
         documentation: http://www.example.com/common-package/docs/5.67/
     profiles:
         default:
             rpms:
                 - common-package
                 - common-plugins
         development:
             rpms:
                 - common-package
                 - common-package-devel
                 - common-plugins
     api:
         rpms:
             - common-package
             - common-package-devel
             - common-plugins
     components:
         rpms:
             common-package:
                 rationale: The key component of this module.
                 ref: common-release-branch
             common-plugins:
                 rationale: Extensions for common-package.
                 buildorder: 1
                 ref: common-release-branch

Complete module definition

See the modulemd specification.

SLs and EOLs

While not a part of the modulemd specification yet, modules will eventually carry a Service Level (SL) value and an End of Life (EOL) value.

The work in Changes/ArbitraryBranching will enable packagers to select independent SLAs and EOLs for both their rpm branches as well as their module branches. Both of these values are associated with the branch in a dist-git repo, but not with the modulemd or spec file contained therein.

Packagers will have to choose from a set of pre-defined SLs maintained by Release Engineering. More info coming soon!