Find more information about creating a modular operating system on the Modularity Website.
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.
What goes into a module?
Before we start digging into the technical details of defining a new module, we need to know what goes there.
You should include all the dependencies needed for your module to run on the Platform module, with few exceptions:
- If you have something shared in your set, with the same lifecycle as Platform, you should try to add it to Platform first. By filing a ticket with https://github.com/fedora-modularity/hp/issues
- If your module needs a common runtime (like Perl or Python) that are already modularized, you should use these as a dependencies rather than bundling them to your module.
- If you see that your module needs a common package (like net-tools), you shouldn’t bundle them either. They should be split into individual modules.
- A set of small “binaries” should be composed in to a shared module if they would be used by other modules
To make sure your module doesn’t conflict with other modules, it can’t contain the same packages as other modules. Your module can either depend on these other modules, or such packages can be split into new modules.
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 Service Levels and End of Life
Every module SHOULD define its EOL date. Defined in the ISO 8601 format (YYYY-MM-DD), the date marks the day when the module stops receiving updates.
eol: 2077-10-23
Module packagers MUST NOT define the EOL in the modulemd but should define it in other infrastructure tooling. The eol property is filled in by the buildsystem, using data from PDC.
The modulemd format doesn't support service levels yet.
The work in Changes/ArbitraryBranching will enable packagers to select independent SLs 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 input or spec file contained therein.
Packagers will have to choose from a set of pre-defined SLs maintained by Release Engineering.
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 dependencies 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:
buildrequiresfor listing build dependencies of the module, i.e. modules that define the buildroot for building the module's components; this will typically be theplatformmodule, at minimumrequiresfor 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 theplatformmodule, at minimum
Either or both of these sections may be omitted, if necessary.
Currently only fully specific name / stream pairs are supported. Future versions of modulemd will support stream expansion, allowing packagers to depend on any active stream, with black & whitelisting features.
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
Profiles may list components provided by modules the parent module depends on.
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.
Generic packages with standard names and installing files into standard locations should be always part of the module API.
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. The packages listed contain everything that makes up the API of the module, as well as any build and runtime dependencies of them that aren't satisfied by modules listed as buildrequires or requires in the dependencies section.
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 building 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- therepositoryreference (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.refis 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. ifx86_64is 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 sectionbuildorder- see the description in the RPM content sectionrepository- see the description in the RPM content sectionref- 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 and build dependency
Yet another flavour of the minimal module, containing one RPM package with SRPM name foo and another with SRPM name bar which is required for building and running foo. This module doesn't define any dependencies on other modules 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.
buildorder: 2
bar:
rationale: >-
An example build and runtime dependency of foo.
buildorder: 1
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 filters were 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.
buildorder: 2
ref: common-release-branch
common-plugins:
rationale: Extensions for common-package.
buildorder: 3
ref: common-release-branch
common-builddep:
rationale: A build dependency of common-package.
buildorder: 1