The new Stackage Server

22 May 2015 Michael Snoyman

tl;dr Please check out

I made the first commit to the Stackage Server code base a little over a year ago. The goal was to provide a place to host package sets which both limited the number of packages from Hackage available, and modified packages where necessary. This server was to be populated by regular Stackage builds, targeted at multiple GHC versions, and consisted of both inclusive and exclusive sets. It also allowed interested individuals to create their own package sets.

If any of those details seem surprising today, they should. A lot has happened for the Stackage project in the past year, making details of what was initially planned irrelevant, and making other things (like hosting of package documentation) vital. We now have LTS Haskell. Instead of running with multiple GHC versions, we have Stackage Nightly which is targeted at a single GHC major version. To accomodate goals for GPS Haskell (which unfortunately never materialized), Stackage no longer makes corrections to upstream packages.

I could go into lots more detail on what is different in project requirements. Instead, I'll just summarize: I've been working on a simplified version of the Stackage Server codebase to address our goals better, more easily ensure high availability, and make the codebase easier to maintain. We also used this opportunity to test out a new hosting system our DevOps team put together. The result is running on, and will replace the official after a bit more testing (which I hope readers will help with).

The code

All of this code lives on the simpler branch of the stackage-server code base, and much to my joy, resulted in quite a bit less code. In fact, there's just about a 2000 line reduction. The rest of this post will get into how that happened.

No more custom package sets

One of the features I mentioned above was custom package sets. This fell out automatically from the initial way Stackage Server was written, so it was natural to let others create package sets of their own. However, since release, only one person actually used that feature. I discussed with him, and he agreed with the decision to deprecate and then remove that functionality.

So why get rid of it now? Two powerful reasons:

  • We already host a public mirror of all packages on S3. Since we no longer patch upstream packages, it's best if tooling is able to just refer to that high-reliability service.
  • We now have Git repositories for all of LTS Haskell and Stackage Nightly. Making these the sources of package sets means we don't have two (possibly conflicting) sources of data. That brings me to the second point

Upload code is gone

We had some complicated logic to allow users to upload package sets. It started off simple, but over time we added Haddock hosting and other metadata features, making the code more complex. Actually, it ended up having two parallel code paths for this. So instead, we now just upload information on the package sets to the Git repositories, and leave it up to a separate process (described below) to clone these repositories and make the data available to the server.

Haddocks on S3

After generating a snapshot, the Haddocks used to be tarred and compressed, and then uploaded as a compressed bundle to S3. Then, Stackage Server would receive a request for files, unpack them, and serve them. This presented some problems:

  • Users would have to wait for a first request to succeed during the unpacking
  • With enough snapshots being generated, we would eventually run out of disk space and need to clear our temp directory
  • Since we run our cluster in a high availabilty mode with multiple horizontally-scaled machines, one machine may have finished unpacking when another didn't, resulting in unstyled content (see issue #82).

Instead, we now just upload the files to S3 and redirect there from stackage-server (though we'll likely switch to reverse proxying to allow for nicer SSL urls). In fact, you can easily view these docs, at URLs such as or

These Haddocks are publicly available, and linkable from projects beyond Stackage Server. Each set of Haddocks is guaranteed to have consistent internal links to other compatible packages. And while some documentation doesn't generate due to known package bugs, the generation is otherwise reliable.

I've already offered access to these docs to Duncan for usage on Hackage, and hope that will improve the experience for users there.

Metadata SQLite database

Previously, information on snapshots was stored in a PostgreSQL database that was maintained by Stackage Server. This database also had package metadata, like author, homepage, and description. Now, we have a completely different process:

  • The all-cabal-metadata from the Commercial Haskell Special Interest Group provides an easily cloneable Git repo with package metadata, which is automatically updated by Travis.
  • We run a cron job on the stackage-build server that updates the lts-haskell, stackage-nightly, and all-cabal-metadata repos and generates a SQLite database from them with all of the data that Stackage Server needs. You can look at the Stackage.Database module for some ideas of what this consists of. That database gets uploaded to Amazon S3, and is actually publicly available if you want to poke at it
  • The live server downloads a new version of this file on a regular basis

I've considered spinning off the Stackage.Download code into its own repository so that others can take advantage of this functionality in different contexts if desired. Let me know if you're interested.

At this point, the PostgreSQL database is just used for non-critical functionality, such as social features (tags and likes).

Slightly nicer URLs

When referring to a snapshot, there are "official" short names (slugs), of the form lts-2.9 and nightly-2015-05-22. The URLs on the new server now reflect this perfectly, e.g.: We originally used hashes of the snapshot content for the original URLs, but that was fixed a while ago. Now that we only have to support these official snapshots, we can always (and exclusively) use these short names.

As a convenience, if you visit the following URLs, you get automatic redirects:

  • /nightly redirects to the most recent nightly
  • /lts to the latest LTS
  • /lts-X to the latest LTS in the X.* major version (e.g., today, /lts-2 redirects to /lts-2.9)

This also works for URLs under that hierarchy. For example, consider, which is an easy way to get set up with LTS in your project (by running wget

ECS-based hosting

While not a new feature of the server itself, the hosting cluster we're running this on is brand new. Amazon recently released EC2 Container Service, which is a service for running Docker containers. Since we're going to be using this for the new School of Haskell, it's nice to be giving it a serious usage now. We also make extensive use of Docker for customer projects, both for builds and hosting, so it's a natural extension for us.

This ECS cluster uses standard Amazon services like Elastic Load Balancer (ELB) and auto-scaling to provide for high availability in the case of machine failure. And while we have a lot of confidence in our ability to keep Stackage Server up and running regularly, it's nice that our most important user-facing content is provided by these external services:

This provides for a pleasant experience in both browsing the website and using Stackage in your build system.

A special thanks to Jason Boyer for providing this new hosting cluster, which the whole FP Complete team is looking forward to putting through its paces.

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