Specs and Multispecs

There are two ways in which the distgen command obtains values for template rendering: config and specs. These two differ in their purpose. While config values should provide template-agnostic values (e.g. facts about a Linux distro), specs provide template-specific values (e.g. information about version of software being built into a Docker image).

Spec values can be provided either through spec files or through multispec files.


Specs are simple key-value files that you can use in your templates. You pass them to the dg command via --spec <file> (can be specified multiple times).

Example spec:

version: 2.4

Example template:

This is documentation for version {{ spec.version }} of some software.

By using specs with different version in the example above, you could render the template for various software versions. While this is ok for simpler usecases, it might become impractical on bigger scale: imagine you want to render a Dockerfile for an image, that will be based on several different distributions and contain a combination of several versions of 2 different packages. This would mean you’d need lots of small spec files, each with couple of lines and you’d need to manually select and pass them to the dg command. This is why the multispec mechanism was added to distgen.


Multispec is a file that solves two problems:

  • Merges several different smaller spec files into a single file for better readability and convenience.
  • Puts smaller specs in logical groups and defines a “matrix” - a list of all combinations of distro config and other features to render templates for.

Here’s an example multispec file:

# This file specifies rendering "matrix" - the different combinations of values
# that the templates can be rendered for
version: 1

# "specs" contains named "spec groups"
  # "distroinfo" is a mandatory "spec group"
  # - each of its members must contain "distros" list
  # - it can also contain any extra values
        - fedora-26-x86_64
        - fedora-25-x86_64
      vendor: "Fedora Project"
      authoritative_source_url: "some.url.fedoraproject.org"
      distro_specific_help: "Some Fedora specific help"
        - centos-7-x86_64
      vendor: "CentOS"
      authoritative_source_url: "some.url.centos.org"
      distro_specific_help: "Some CentOS specific help"
  # apart from "distroinfo", you can specify as many arbitrary spec groups as you want
  # - any of the members of these spec groups can contain arbitrary values
      version: "2.2"
      version: "2.4"

# in the "matrix" section, you can define an action that is
# applied only to specified distro and version combinations
    - distros:
        - fedora-26-x86_64
      version: "2.2"

    - distros:
        - centos-7-x86_64
      version: "2.4"
        extra_pkgs: ['foo', 'bar']

A multispec has 3 attributes (see below for the explanation of mechanics behind this file):

  • version (mandatory) - The version of the multispec file, currently there’s only version 1.
  • specs (mandatory) - contains list of groups (distroinfo and version in the example above). Each group contains named specs - these are exactly like the specs that you would otherwise write into separate files and pass to distgen via --spec.
    • The distroinfo group is mandatory and each of its members must contain the distros list. These are names of the distro configs shipped with distgen.
    • The specs groups implicitly define a rendering matrix, which is the cartesian product of all groups except distroinfo. The distroinfo group is an exception, as its members distros list are used in the cartesian product.
  • matrix (optional) - currently, this attribute can only contain two members.
    • The exclude attribute contains a list of combinations excluded from the matrix. The distroinfo members must be referred to via distro list.
    • The combination_extras member contains a list of combinations and extras, mapping of key-value pairs, which are only added to this combination and can be used in your templates.

Hence the above example produces a following rendering matrix:

  • distroinfo: fedora (for fedora-25-x86_64 distro), version: "2.2"
  • distroinfo: fedora (for fedora-25-x86_64 distro), version: "2.4"
  • distroinfo: fedora (for fedora-26-x86_64 distro), version: "2.4"
  • distroinfo: fedora (for centos-7-x86_64 distro), version: "2.2"
  • distroinfo: fedora (for centos-7-x86_64 distro), version: "2.4"

Note that version: "2.2" is excluded for fedora-26-x86_64.

Using Multispecs

Let’s consider the example above. We could use it like this:

$ dg --template docker.tpl \
     --spec common.yaml \
     --multispec multispec.yaml \
     --multispec-selector version=2.4 \
     --distro fedora-26-x86_64.yaml \
> Dockerfile

On calling this command, distgen will:

  • Take values from common.yaml for base of the result values used for rendering the template.
  • It will then add values from multispec.yaml:
    • The --distro fedora-26-x86_64 argument will automatically select the distroinfo.fedora section of multispec and add it to result values.
    • The --multispec-selector version=2.4 will make the version."2.4" section of multispec added to the result values.
  • Render the template providing the result of operations above accessible under spec.* values.

Notes on Multispec Usage

  • There can be as many groups as you want, not just distroinfo and version. This also means that you need to use --multispec-selector multiple times on commandline.
  • The --multispec-selector must be used for all groups except distroinfo. A proper section to be used from distroinfo is implicitly specified by passing the --distro argument.
  • Only a combination of specs belonging to groups can be used when using multispec. In the example above, you can’t use fedora-22_i686, since it’s not listed in any distroinfo section.
  • Combinations explicitly listed in matrix.exclude cannot be used.
  • You can use dg --multispec <path> --multispec-combinations to print out all available combinations of distros and selectors based on the given multispec file.

Combining Specs and Multispecs

As shown in the example above, it is perfectly possible to combine specs and multispec. In this case, the specs will be used as a base and values from multispec will be added on top of that (overwriting values if their names conflict).