Developer Guide

This guide is intended for people who want to work on Spack itself. If you just want to develop packages, see the Packaging Guide.

It is assumed that you’ve read the Basic Usage and Packaging Guide sections, and that you’re familiar with the concepts discussed there. If you’re not, we recommend reading those first.


Spack is designed with three separate roles in mind:

  1. Users, who need to install software without knowing all the details about how it is built.
  2. Packagers who know how a particular software package is built and encode this information in package files.
  3. Developers who work on Spack, add new features, and try to make the jobs of packagers and users easier.

Users could be end users installing software in their home directory, or administrators installing software to a shared directory on a shared machine. Packagers could be administrators who want to automate software builds, or application developers who want to make their software more accessible to users.

As you might expect, there are many types of users with different levels of sophistication, and Spack is designed to accommodate both simple and complex use cases for packages. A user who only knows that he needs a certain package should be able to type something simple, like spack install <package name>, and get the package that he wants. If a user wants to ask for a specific version, use particular compilers, or build several versions with different configurations, then that should be possible with a minimal amount of additional specification.

This gets us to the two key concepts in Spack’s software design:

  1. Specs: expressions for describing builds of software, and
  2. Packages: Python modules that build software according to a spec.

A package is a template for building particular software, and a spec as a descriptor for one or more instances of that template. Users express the configuration they want using a spec, and a package turns the spec into a complete build.

The obvious difficulty with this design is that users under-specify what they want. To build a software package, the package object needs a complete specification. In Spack, if a spec describes only one instance of a package, then we say it is concrete. If a spec could describes many instances, (i.e. it is under-specified in one way or another), then we say it is abstract.

Spack’s job is to take an abstract spec from the user, find a concrete spec that satisfies the constraints, and hand the task of building the software off to the package object. The rest of this document describes all the pieces that come together to make that happen.

Directory Structure

So that you can familiarize yourself with the project, we’ll start with a high level view of Spack’s directory structure:

spack/                  <- installation root
      spack             <- main spack executable

      spack/            <- Spack config files.
                           Can be overridden by files in ~/.spack.

      spack/            <- build & stage directories
          repos/            <- contains package repositories
             builtin/       <- pkg repository that comes with Spack
                repo.yaml   <- descriptor for the builtin repository
                packages/   <- directories under here contain packages
          cache/        <- saves resources downloaded during installs

      spack/            <- packages are installed here

         docs/          <- source for this documentation
         env/           <- compiler wrappers for build environment

         external/      <- external libs included in Spack distro
         llnl/          <- some general-use libraries

         spack/         <- spack module; contains Python code
            cmd/        <- each file in here is a spack subcommand
            compilers/  <- compiler description files
            test/       <- unit test modules
            util/       <- common code

Spack is designed so that it could live within a standard UNIX directory hierarchy, so lib, var, and opt all contain a spack subdirectory in case Spack is installed alongside other software. Most of the interesting parts of Spack live in lib/spack.

Spack has one directory layout and there is no install process. Most Python programs don’t look like this (they use distutils,, etc.) but we wanted to make Spack very easy to use. The simple layout spares users from the need to install Spack into a Python environment. Many users don’t have write access to a Python installation, and installing an entire new instance of Python to bootstrap Spack would be very complicated. Users should not have to install a big, complicated package to use the thing that’s supposed to spare them from the details of big, complicated packages. The end result is that Spack works out of the box: clone it and add bin to your PATH and you’re ready to go.

Code Structure

This section gives an overview of the various Python modules in Spack, grouped by functionality.

Build environment

Handles creating temporary directories for builds.
This contains utility functions used by the compiler wrapper script, cc.
Classes that control the way an installation directory is laid out. Create more implementations of this to change the hierarchy and naming scheme in $spack_prefix/opt

Spack Subcommands

Each module in this package implements a Spack subcommand. See writing commands for details.

Unit tests

Implements Spack’s test suite. Add a module and put its name in the test suite in to add more unit tests.
This is a fake package hierarchy used to mock up packages for Spack’s test suite.

Other Modules

URL parsing, for deducing names and versions of packages from tarball URLs.
SpackError, the base class for Spack’s exception hierarchy.
Basic output functions for all of the messages Spack writes to the terminal.
Implements a color formatting syntax used by spack.tty.
In this package are a number of utility modules for the rest of Spack.

Spec objects

Package objects

Most spack commands look something like this:

  1. Parse an abstract spec (or specs) from the command line,
  2. Normalize the spec based on information in package files,
  3. Concretize the spec according to some customizable policies,
  4. Instantiate a package based on the spec, and
  5. Call methods (e.g., install()) on the package object.

The information in Package files is used at all stages in this process.

Conceptually, packages are overloaded. They contain:

Stage objects

Writing commands

Adding a new command to Spack is easy. Simply add a <name>.py file to lib/spack/spack/cmd/, where <name> is the name of the subcommand. At the bare minimum, two functions are required in this file:


Unless your command doesn’t accept any arguments, a setup_parser() function is required to define what arguments and flags your command takes. See the Argparse documentation for more details on how to add arguments.

Some commands have a set of subcommands, like spack compiler find or spack module lmod refresh. You can add subparsers to your parser to handle this. Check out spack edit --command compiler for an example of this.

A lot of commands take the same arguments and flags. These arguments should be defined in lib/spack/spack/cmd/common/ so that they don’t need to be redefined in multiple commands.


In order to run your command, Spack searches for a function with the same name as your command in <name>.py. This is the main method for your command, and can call other helper methods to handle common tasks.

Remember, before adding a new command, think to yourself whether or not this new command is actually necessary. Sometimes, the functionality you desire can be added to an existing command. Also remember to add unit tests for your command. If it isn’t used very frequently, changes to the rest of Spack can cause your command to break without sufficient unit tests to prevent this from happening.

Whenever you add/remove/rename a command or flags for an existing command, make sure to update Spack’s Bash tab completion script.

Unit tests

Unit testing

Developer commands

spack doc

spack unit-test

See the contributor guide section on spack unit-test.

spack python

spack python is a command that lets you import and debug things as if you were in a Spack interactive shell. Without any arguments, it is similar to a normal interactive Python shell, except you can import spack and any other Spack modules:

$ spack python
Spack version 0.10.0
Python 2.7.13, Linux x86_64
>>> from spack.version import Version
>>> a = Version('1.2.3')
>>> b = Version('1_2_3')
>>> a == b
>>> c = Version('1.2.3b')
>>> c > a

If you prefer using an IPython interpreter, given that IPython is installed you can specify the interpreter with -i:

$ spack python -i ipython
Python 3.8.3 (default, May 19 2020, 18:47:26)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.17.0 -- An enhanced Interactive Python. Type '?' for help.

Spack version 0.16.0
Python 3.8.3, Linux x86_64

In [1]:

With either interpreter you can run a single command:

$ spack python -c 'import distro; distro.linux_distribution()'
('Ubuntu', '18.04', 'Bionic Beaver')

$ spack python -i ipython -c 'import distro; distro.linux_distribution()'
Out[1]: ('Ubuntu', '18.04', 'Bionic Beaver')

or a file:

$ spack python ~/
$ spack python -i ipython ~/

just like you would with the normal python command.

spack url

A package containing a single URL can be used to download several different versions of the package. If you’ve ever wondered how this works, all of the magic is in spack.url. This module contains methods for extracting the name and version of a package from its URL. The name is used by spack create to guess the name of the package. By determining the version from the URL, Spack can replace it with other versions to determine where to download them from.

The regular expressions in parse_name_offset and parse_version_offset are used to extract the name and version, but they aren’t perfect. In order to debug Spack’s URL parsing support, the spack url command can be used.

spack url parse

If you need to debug a single URL, you can use the following command:

$ spack url parse
==> Parsing URL:

==> Matched version regex  0: r'^[a-zA-Z+._-]+[._-]v?(\\d[\\d._-]*)$'
==> Matched  name   regex 10: r'^([A-Za-z\\d+\\._-]+)$'

==> Detected:
                                            ---- ~~~~~
    name:    ruby
    version: 2.2.0

==> Substituting version 9.9.9b:
                                            ---- ~~~~~~

You’ll notice that the name and version of this URL are correctly detected, and you can even see which regular expressions it was matched to. However, you’ll notice that when it substitutes the version number in, it doesn’t replace the 2.2 with 9.9 where we would expect 9.9.9b to live. This particular package may require a list_url or url_for_version function.

This command also accepts a --spider flag. If provided, Spack searches for other versions of the package and prints the matching URLs.

spack url list

This command lists every URL in every package in Spack. If given the --color and --extrapolation flags, it also colors the part of the string that it detected to be the name and version. The --incorrect-name and --incorrect-version flags can be used to print URLs that were not being parsed correctly.

spack url summary

This command attempts to parse every URL for every package in Spack and prints a summary of how many of them are being correctly parsed. It also prints a histogram showing which regular expressions are being matched and how frequently:

$ spack url summary
==> Generating a summary of URL parsing in Spack...

    Total URLs found:          4711
    Names correctly parsed:    4247/4711 (90.15%)
    Versions correctly parsed: 4368/4711 (92.72%)

==> Statistics on name regular expressions:

    Index   Right   Wrong   Total   Regular Expression
        0    1271     178    1449   r'github\\.com/[^/]+/([^/]+)'
        1       6       0       6   r'gitlab[^/]+/api/v4/projects/[^/]+%2F([^/]+)'
        2      23       3      26   r'gitlab[^/]+/(?!api/v4/projects)[^/]+/([^/]+)'
        3      18       5      23   r'bitbucket\\.org/[^/]+/([^/]+)'
        4       8       0       8   r'pypi\\.(?:python\\.org|io)/packages/source/[A-Za-z\\d]/([^/]+)'
        6       9       0       9   r'\\?f=([A-Za-z\\d+-]+)$'
        7      12       0      12   r'\\?package=([A-Za-z\\d+-]+)'
        8       2       0       2   r'\\?package=([A-Za-z\\d]+)'
        9       2       2       4   r'([^/]+)/download.php$'
       10    2896     268    3164   r'^([A-Za-z\\d+\\._-]+)$'

==> Statistics on version regular expressions:

    Index   Right   Wrong   Total   Regular Expression
        0    3002      53    3055   r'^[a-zA-Z+._-]+[._-]v?(\\d[\\d._-]*)$'
        1     972      13     985   r'^v?(\\d[\\d._-]*)$'
        2      11      17      28   r'^[a-zA-Z+]*(\\d[\\da-zA-Z]*)$'
        3       8      14      22   r'^[a-zA-Z+-]*(\\d[\\da-zA-Z-]*)$'
        4       5     103     108   r'^[a-zA-Z+_]*(\\d[\\da-zA-Z_]*)$'
        5      50      21      71   r'^[a-zA-Z+.]*(\\d[\\da-zA-Z.]*)$'
        6     177       7     184   r'^[a-zA-Z\\d+-]+-v?(\\d[\\da-zA-Z.]*)$'
        7       1       0       1   r'^[a-zA-Z\\d+-]+-v?(\\d[\\da-zA-Z_]*)$'
        8      24       1      25   r'^[a-zA-Z\\d+_]+_v?(\\d[\\da-zA-Z.]*)$'
        9       0       1       1   r'^[a-zA-Z\\d+_]+\\.v?(\\d[\\da-zA-Z.]*)$'
       11      32      44      76   r'^(?:[a-zA-Z\\d+-]+-)?v?(\\d[\\da-zA-Z.-]*)$'
       12       3       0       3   r'^[a-zA-Z+]+v?(\\d[\\da-zA-Z.-]*)$'
       13       4       2       6   r'^[a-zA-Z\\d+_]+-v?(\\d[\\da-zA-Z.]*)$'
       14      20       4      24   r'^[a-zA-Z\\d+.]+_v?(\\d[\\da-zA-Z.-]*)$'
       15       1       0       1   r'^[a-zA-Z\\d+-]+-v?(\\d[\\da-zA-Z._]*)$'
       16       1       1       2   r'^[a-zA-Z\\d+._]+-v?(\\d[\\da-zA-Z.]*)$'
       17       5       0       5   r'^[a-zA-Z+-]+(\\d[\\da-zA-Z._]*)$'
       18       1       2       3   r'^[a-zA-Z\\d+_-]+-v?(\\d[\\da-zA-Z.]*)$'
       19       0       1       1   r'bzr(\\d[\\da-zA-Z._-]*)$'
       20      10       1      11   r'[?&](?:sha|ref|version)=[a-zA-Z\\d+-]*[_-]?v?(\\d[\\da-zA-Z._-]*)$'
       21      21       0      21   r'[?&](?:filename|f|get)=[a-zA-Z\\d+-]+[_-]v?(\\d[\\da-zA-Z.]*)'
       22       9       1      10   r'github\\.com/[^/]+/[^/]+/releases/download/[a-zA-Z+._-]*v?(\\d[\\da-zA-Z._-]*)/'
       23      11       9      20   r'(\\d[\\da-zA-Z._-]*)/[^/]+$'

This command is essential for anyone adding or changing the regular expressions that parse names and versions. By running this command before and after the change, you can make sure that your regular expression fixes more packages than it breaks.


Spack has some limited built-in support for profiling, and can report statistics using standard Python timing tools. To use this feature, supply --profile to Spack on the command line, before any subcommands.

spack --profile

spack --profile output looks like this:

$ spack --profile graph hdf5
o  hdf5
| o  openmpi
| |\
| | |\
| | | |\
| | | o |  libevent
| | | o |  openssl
| |_|/| | 
|/| | | | 
| | | | o  hwloc
| | |_|/| 
| |/| | | 
| | | | |\
| | | | o |  libxml2
| |_|_|/| | 
|/| |_|/| | 
| |/| | | | 
| | | | |\ \
o | | | | | |  zlib
 / / / / / /
| | | o | |  xz
| | |  / /
| | | | o  libpciaccess

The bottom of the output shows the top most time consuming functions, slowest on top. The profiling support is from Python’s built-in tool, cProfile.


This section documents Spack’s release process. It is intended for project maintainers, as the tasks described here require maintainer privileges on the Spack repository. For others, we hope this section at least provides some insight into how the Spack project works.

Release branches

There are currently two types of Spack releases: major releases (0.13.0, 0.14.0, etc.) and point releases (0.13.1, 0.13.2, 0.13.3, etc.). Here is a diagram of how Spack release branches work:

o    branch: develop  (latest version)
o    merge v0.14.1 into develop
| o  branch: releases/v0.14, tag: v0.14.1
o |  merge v0.14.0 into develop
| o  tag: v0.14.0
o    merge v0.13.2 into develop
| o  branch: releases/v0.13, tag: v0.13.2
o |  merge v0.13.1 into develop
| o  tag: v0.13.1
o |  merge v0.13.0 into develop
| o  tag: v0.13.0
o |
| o

The develop branch has the latest contributions, and nearly all pull requests target develop.

Each Spack release series also has a corresponding branch, e.g. releases/v0.14 has 0.14.x versions of Spack, and releases/v0.13 has 0.13.x versions. A major release is the first tagged version on a release branch. Minor releases are back-ported from develop onto release branches. This is typically done by cherry-picking bugfix commits off of develop.

To avoid version churn for users of a release series, minor releases should not make changes that would change the concretization of packages. They should generally only contain fixes to the Spack core.

Both major and minor releases are tagged. After each release, we merge the release branch back into develop so that the version bump and any other release-specific changes are visible in the mainline. As a convenience, we also tag the latest release as releases/latest, so that users can easily check it out to get the latest stable version. See Updating releases/latest and develop for more details.

Scheduling work for releases

We schedule work for releases by creating GitHub projects. At any time, there may be several open release projects. For example, here are two releases (from some past version of the page linked above):


Here, there’s one release in progress for 0.15.1 and another for 0.16.0. Each of these releases has a project board containing issues and pull requests. GitHub shows a status bar with completed work in green, work in progress in purple, and work not started yet in gray, so it’s fairly easy to see progress.

Spack’s project boards are not firm commitments, and we move work between releases frequently. If we need to make a release and some tasks are not yet done, we will simply move them to next minor or major release, rather than delaying the release to complete them.

For more on using GitHub project boards, see GitHub’s documentation.

Making Major Releases

Assuming you’ve already created a project board and completed the work for a major release, the steps to make the release are as follows:

  1. Create two new project boards:

    • One for the next major release
    • One for the next point release
  2. Move any tasks that aren’t done yet to one of the new project boards. Small bugfixes should go to the next point release. Major features, refactors, and changes that could affect concretization should go in the next major release.

  3. Create a branch for the release, based on develop:

    $ git checkout -b releases/v0.15 develop

    For a version vX.Y.Z, the branch’s name should be releases/vX.Y. That is, you should create a releases/vX.Y branch if you are preparing the X.Y.0 release.

  4. Bump the version in lib/spack/spack/ See this example from 0.13.0

  5. Update with major highlights in bullet form. Use proper markdown formatting, like this example from 0.15.0.

  6. Push the release branch to GitHub.

  7. Make sure CI passes on the release branch, including:

    If CI is not passing, submit pull requests to develop as normal and keep rebasing the release branch on develop until CI passes.

  8. Follow the steps in Publishing a release on GitHub.

  9. Follow the steps in Updating releases/latest and develop.

  10. Follow the steps in Announcing a release.

Making Point Releases

This assumes you’ve already created a project board for a point release and completed the work to be done for the release. To make a point release:

  1. Create one new project board for the next point release.

  2. Move any cards that aren’t done yet to the next project board.

  3. Check out the release branch (it should already exist). For the X.Y.Z release, the release branch is called releases/vX.Y. For v0.15.1, you would check out releases/v0.15:

    $ git checkout releases/v0.15
  4. Cherry-pick each pull request in the Done column of the release project onto the release branch.

    This is usually fairly simple since we squash the commits from the vast majority of pull requests, which means there is only one commit per pull request to cherry-pick. For example, this pull request has three commits, but the were squashed into a single commit on merge. You can see the commit that was created here:


    You can easily cherry pick it like this (assuming you already have the release branch checked out):

    $ git cherry-pick 7e46da7

    For pull requests that were rebased, you’ll need to cherry-pick each rebased commit individually. There have not been any rebased PRs like this in recent point releases.


    It is important to cherry-pick commits in the order they happened, otherwise you can get conflicts while cherry-picking. When cherry-picking onto a point release, look at the merge date, not the number of the pull request or the date it was opened.

    Sometimes you may still get merge conflicts even if you have cherry-picked all the commits in order. This generally means there is some other intervening pull request that the one you’re trying to pick depends on. In these cases, you’ll need to make a judgment call:

    1. If the dependency is small, you might just cherry-pick it, too. If you do this, add it to the release board.
    2. If it is large, then you may decide that this fix is not worth including in a point release, in which case you should remove it from the release project.
    3. You can always decide to manually back-port the fix to the release branch if neither of the above options makes sense, but this can require a lot of work. It’s seldom the right choice.
  5. Bump the version in lib/spack/spack/ See this example from 0.14.1.

  6. Update with a list of bugfixes. This is typically just a summary of the commits you cherry-picked onto the release branch. See the changelog from 0.14.1.

  7. Push the release branch to GitHub.

  8. Make sure CI passes on the release branch, including: * Regular unit tests * Build tests * The E4S pipeline at

    If CI does not pass, you’ll need to figure out why, and make changes to the release branch until it does. You can make more commits, modify or remove cherry-picked commits, or cherry-pick more from develop to make this happen.

  9. Follow the steps in Publishing a release on GitHub.

  10. Follow the steps in Updating releases/latest and develop.

  11. Follow the steps in Announcing a release.

Publishing a release on GitHub

  1. Go to and click Draft a new release. Set the following:

    • Tag version should start with v and contain all three parts of the version, .g. v0.15.1. This is the name of the tag that will be created.
    • Target should be the releases/vX.Y branch (e.g., releases/v0.15).
    • Release title should be vX.Y.Z (To match the tag, e.g., v0.15.1).
    • For the text, paste the latest release markdown from your

    You can save the draft and keep coming back to this as you prepare the release.

  2. When you are done, click Publish release.

  3. Immediately after publishing, go back to and download the auto-generated .tar.gz file for the release. It’s the Source code (tar.gz) link.

  4. Click Edit on the release you just did and attach the downloaded release tarball as a binary. This does two things:

    1. Makes sure that the hash of our releases doesn’t change over time. GitHub sometimes annoyingly changes they way they generate tarballs, and then hashes can change if you rely on the auto-generated tarball links.
    2. Gets us download counts on releases visible through the GitHub API. GitHub tracks downloads of artifacts, but not the source links. See the releases page and search for download_count to see this.
  5. Go to and activate the release tag. This builds the documentation and makes the released version selectable in the versions menu.

Updating releases/latest and develop

If the new release is the highest Spack release yet, you should also tag it as releases/latest. For example, suppose the highest release is currently 0.15.3:

  • If you are releasing 0.15.4 or 0.16.0, then you should tag it with releases/latest, as these are higher than 0.15.3.
  • If you are making a new release of an older major version of Spack, e.g. 0.14.4, then you should not tag it as releases/latest (as there are newer major versions).

To tag releases/latest, do this:

$ git checkout releases/vX.Y     # vX.Y is the new release's branch
$ git tag --force releases/latest
$ git push --tags

The --force argument makes git overwrite the existing releases/latest tag with the new one.

We also merge each release that we tag as releases/latest into develop. Make sure to do this with a merge commit:

$ git checkout develop
$ git merge --no-ff vX.Y.Z  # vX.Y.Z is the new release's tag
$ git push

We merge back to develop because it:

  • updates the version and on develop.
  • ensures that your release tag is reachable from the head of develop

We must use a real merge commit (via the --no-ff option) because it ensures that the release tag is reachable from the tip of develop. This is necessary for spack -V to work properly – it uses git describe --tags to find the last reachable tag in the repository and reports how far we are from it. For example:

$ spack -V

This says that we are at commit b80d5e74e5, which is 1,486 commits ahead of the 0.14.2 release.

We put this step last in the process because it’s best to do it only once the release is complete and tagged. If you do it before you’ve tagged the release and later decide you want to tag some later commit, you’ll need to merge again.

Announcing a release

We announce releases in all of the major Spack communication channels. Publishing the release takes care of GitHub. The remaining channels are Twitter, Slack, and the mailing list. Here are the steps:

  1. Make a tweet to announce the release. It should link to the release’s page on GitHub. You can base it on this example tweet.
  2. Ping @channel in #general on Slack ( with a link to the tweet. The tweet will be shown inline so that you do not have to retype your release announcement.
  3. Email the Spack mailing list to let them know about the release. As with the tweet, you likely want to link to the release’s page on GitHub. It’s also helpful to include some information directly in the email. You can base yours on this example email.

Once you’ve announced the release, congratulations, you’re done! You’ve finished making the release!