I recently wrote up the Stackage
data flow. The primary intent was to assist the rest of the
Stackage curation team see how all the pieces fit together.
However, it may also be of general interest to the rest of the
community. In particular, some of the components used are not
widely known and may be beneficial for completely separate projects
(such as all-cabal-metadata).
Please check out the above linked copy of the file for the most
up-to-date content. For convenience, I'm copying in the current
content as of publication time below.
The Stackage project is really built on top of a number of
different subcomponents. This page covers how they fit together.
The Stackage data flow diagram gives a good bird's-eye view:
There are three inputs into the data flow:
Hackage is the
upstream repository of all available open source Haskell packages
that are part of our ecosystem. Hackage provides both cabal file
metadata (via the 00-index.tar file) and tarballs of the individual
build-constraints.yaml is the primary Stackage input file. This
is where package maintainers can add packages to the Stackage
package set. This also defines upper bounds, skipped tests, and a
few other pieces of metadata.
a Github repository containing static file content served from
reasons, we leverage Travis CI for running some processes. In
all-cabal-files clones all cabal files from Hackage's
00-index.tar file into a Git repository without any
all-cabal-hashes is mostly the same, but also includes
cryptographic hashes of the package tarballs for more secure
download (as leveraged by Stack. It is powered by all-cabal-hashes-tool
uses hackage-mirror to
populate the hackage.fpcomplete.com mirror of Hackage, which
provides S3-backed high availability hosting of all package
to query extra metadata from Hackage about packages and put them
into YAML files. As we'll see later, this avoids the need to make a
lot of costly calls to Hackage APIs
Travis does not currently provide a means of running jobs on a
regular basis. Therefore, we have a simple cron job on the Stackage
build server that triggers each of the above builds every 30
The heart of running Stackage builds is the stackage-curator
tool. We run this on a daily basis on the Stackage build server for
Stackage Nightly, and on a weekly basis for LTS Haskell. The build
process is highly
automated and leverages Docker quite a bit.
stackage-curator needs to know about the most recent versions of
all packages, their tarball contents, and some metadata, all of
which it gets from the Travis-generated sources mentioned in the
previous section. In addition, it needs to know about build
constraints, which can come from one of two places:
- When doing an LTS Haskell minor version bump (e.g., building
lts-5.13), it grabs the previous version (e.g., lts-5.12) and
converts the previous package set into constraints. For example, if
lts-5.12 contains the package foo-5.6.7, this will be converted
into the constraint
foo >= 5.6.7 && <
- When doing a Stackage Nightly build or LTS Haskell major
version bump (e.g., building lts-6.0), it grabs the latest version
of the build-constraints.yaml file.
By combining these constraints with the current package data,
stackage-curator can generate a build plan and check it. (As an
aside, this build plan generation and checking also occurs every
time you make a pull request to the stackage repo.) If there are
version bounds problems, one of the Stackage
curators will open up a Github issue and will add upper bounds,
temporarily block a package, or some other corrective action.
Once a valid build plan is found, stackage-curator will build
all packages, build docs, and run test suites. Assuming that all
succeeds, it generates some artifacts:
- Uploads the build plan as a YAML file to either stackage-nightly or
- Uploads the generated Haddock docs and a package index
(containing all used .cabal files) to haddock.stackage.org.
On the Stackage build server, we run the
stackage-server-cron executable regularly, which generates:
- A SQLite database containing information on snapshots, the
packages they contain, Hackage metadata about packages, and a bit
more. This database is uploaded to S3.
- A Hoogle database for each snapshot, which is also uploaded to
running stackage.org is a relatively simple Yesod web
application. It pulls data from the stackage-content repo, the
SQLite database, the Hoogle databases, and the build plans for
Stackage Nightly and LTS Haskell. It doesn't generate anything
important of its own except for a user interface.
Stack takes advantage of
many of the pieces listed above as well:
- It by default uses the all-cabal-hashes repo for getting
package metadata, and downloads package contents from the
hackage.fpcomplete.com mirror (using the hashes in the repo for
- There are some metadata files in stackage-content which contain
information on, for example, where to download GHC tarballs from to
stack setup work
- Stack downloads the raw build plans for Stackage Nightly and
LTS Haskell from the Github repo and uses them when deciding which
packages to build for a given stack.yaml file
Subscribe to our blog via email
Email subscriptions come from our Atom feed and are handled by Blogtrottr. You will only receive notifications of blog posts, and can unsubscribe any time.
Do you like this blog post and need help with Next Generation Software Engineering, Platform Engineering or Blockchain & Smart Contracts? Contact us.