TL;DR: we propose to factor Hackage into a separate,
very simple service serving a stash of Haskell packages, with the
rest of Hackage built on top of that, in the name of availability,
reliability and extensibility.
One of the main strengths of Haskell is just how much it
encourages composable code. As programmers, we are goaded along a
good path for composability by the strictures of purity, which
forces us to be honest about the side effects we might use, but
first and foremost because first class functions and lazy
evaluation afford us the freedom to decompose solutions into
orthogonal components and recompose them elsewhere. In the words of John
Hughes, Haskell provides the necessary glue to build
composable programs, ultimately enabling robust code reuse. Perhaps
we ought to build our shared community infrastructure along the
same principles: freedom to build awesome new services by
assembling together existing ones, made possible by the
discipline to write these basic building blocks as
stateless, scalable, essentially pure services. Let's think about
how, taking packages hosting as an example, with a view towards
solving three concrete problems:
- Availability of package metadata and source code (currently
these are no longer available when hackage.haskell.org goes
cabal update download times.
- The difficulty of third party services and other community
services to interoperate with hackage.haskell.org and extend it
in any direction the community deems fit.
Today Haskell packages are sets of files with a distinguished
*.cabal file containing the package metadata. We host
these files on a central package repository called Hackage, a
community supported service. Hackage is a large service that has by
and large served the community well, and has done so since 2007.
The repository has grown tremendously, by now hosting no less than
5,600 packages. It implements many features, some of which include
package management. In particular, Hackage allows registered users
Upload a new package: either from the browser or via
Download an index of all packages available: this index
includes the full content of all
*.cabal files for all
packages and all versions.
Query the package database via a web interface: from
listing all packages available by category, to searching packages
by name. Hackage maintains additional metadata for each package not
stored in the package itself, such as download counts, package
availability in various popular Linux distributions. Perhaps in the
future this metadata will also include social features such as
number of "stars", à la Github.
Some of the above constitute the defining features of a
central package repository. Of course, Hackage is much more than
just that today - it is a portal for exploring what packages
are out there through a full blown web interface, running nightly
builds on all packages to make sure they compile and putting
together build reports, generating package API documentation and
providing access to the resulting HTML files, maintaining RSS feeds
for new package uploads, generating activity graphs, integration
with Hoogle and Hayoo, etc.
In the rest of this blog post, we'll explore why it's important
to tweeze out the package repository from the rest, and build the
Hackage portal on top of that. That is to say, talk separately
about Hackage-the-repository and Hackage-the-portal.
A tell-tale sign of a thriving development community is that a
number of services pop up independently to address the needs of
niche segments of the community or indeed the community as a whole.
Over time, these community resources together as a set of
resources form an ecosystem, or perhaps even a market, in much
the same way that the set of all Haskell packages form an
ecosystem. There is no central authority deciding which package
ought to be the unique consecrated package for e.g. manipulating
filesystem paths: on Hackage today there are at least 5, each
exploring different parts of the design space.
However, we do need common infrastructure in place, because we
do need consensus about what package names refer to what
code and where to find it. People often refer to Hackage as the
"wild west" of Haskell, due to its very permissive policies about
what content makes it on Hackage. But that's not to say that it's
an entirely chaotic free-for-all: package names are unique, only
designated maintainers can upload new versions of some given
package and version numbers are bound to a specific set of source
files and content for all time.
The core value of Hackage-the-repository then, is to establish
consensus about who maintains what package, what versions are
available and the metadata associated with each version. If Alice
has created a package called
foolib, then Bob can't
foolib for any of his own packages, he must
instead choose another name. There is therefore agreement across
the community about what
foolib means. Agreement
makes life much easier for users, tools and developers talking
about these packages.
What doesn't need consensus is anything outside of
package metadata and authorization: we may want multiple portals to
Haskell code, or indeed have some portals dedicated to particular
views (a particular subset of the full package set) of the
central repository. For example, stackage.org today is one such portal,
dedicated to LTS Haskell and Stackage Nightly, two popular views of
consistent package sets maintained by FP Complete. We fully
anticipate that others will over time contribute other views —
general-purpose or niche (e.g. specialized for a particular web
framework) — or indeed alternative portals — ranging from small,
fast and stable to experimental and replete with social features
aplenty. Think powerful new search functionality, querying reverse
dependencies, pre-built package sets for Windows, OS X and Linux,
package reviews, package voting ... you name it!
Finally, by carving out the central package repository into its
own simple and reliable service, we limit the impact of bugs on
both availability and reliably, and thus preserve on one of our
most valuable assets: the code that we together as a community have
written. Complex solutions invariably affect reliability. Keeping
the core infrastructure small and making it easy to build on top is
how we manage that.
The next section details one way to carve the central package
repository, to illustrate my point. Alternative designs are
possible of course - I merely wish to seek agreement that a modular
architecture with at its core a set of very small and simple
services as our community commons would be beneficial to the
down the central hub to its bare essence
Before we proceed, let's first introduce a little bit of
- A persistent
data structure is an data structure that is never destructively
updated: when modified, all previous versions of the data structure
are still available.
Data.Map from the
containers package is persistent in this sense, as are
lists and most other data structures in Haskell.
- A service is stateless if its response is a function of
the state of other services and the content of the request.
Stateless services are trivially scaled horizontally - limited only
by the scalability of the services they depend on.
- A persistent service is a service that maintains its
only state as a persistent data structure. Most resources served by
a persistent service are immutable. Persistent services share many
of the same properties as stateless services: keeping complexity
down and scaling them is easy because concurrent access and
modification of a persistent data structure requires little to no
coordination (think locks, critical sections, etc).
A central hub for all open source Haskell packages might look
something like this:
A persistent read-only directory of metadata for all versions of
all packages (i.e. the content of the
Only the upload service may modify this directory, and even then,
only in a persistent way.
A persistent read-only directory of the packages themselves,
that is to say the content of the archives produced by
An upload service, for uploading new revisions of the metadata
directory. This service maintains no state of its own, therefore
multiple upload services can be spawned if necessary.
An authentication service, granting access tokens to users for
adding a new package or modifying the metadata for their own
existing packages via the upload service(s).
The metadata and package directories together form a central
repository of all open source Haskell packages. Just as is the case
with Hackage today, anyone is allowed to upload any package they
like via the upload service. We might call these directories
collectively The Haskell Stash. End-user command-line tools,
cabal-install, need only interact with the
Stash to get the latest list of packages and versions. If the
upload or authentication services go down, existing packages can
still be downloaded without any issue.
Availability is a crucial property for such a core piece
of infrastructure: users from around the world rely on it today to
locate the dependencies necessary for building and deploying
Haskell code. The strategy for maintaining high availability can be
worked out independently for each service. A tried and tested
approach is to avoid reinventing as much of the wheel as we can,
reusing existing protocols and infrastructure where possible. I
envision the following implementation:
Serve the metadata directory as a simple Git repository. A Git
repository is persistent (objects are immutable and live forever),
easy to add new content to, easy to backup, easy to mirror and easy
to mine for insights on how packages changes over time. Advanced
features such as package candidates fall out nearly for free.
Rather than serving in its entirety a whole new static tarball of
all package metadata (totalling close to 9MB of compressed data) as
we do today, we can leverage the existing Git wire protocol to
transfer new versions to end users much more efficiently. In
short, a faster
The point here is very much not to use Git as a favoured
version control system (VCS), fine as it may be for that purpose,
at the expense of any other such tool. Git is at its core an
efficient persistent object store first and foremost, with a VCS
layered on top. The idea is to not reinvent our own object store.
It features a simple disk format that has remained incredibly
stable over the years. Hosting all our metadata as a simple Git
repository means we can leverage any number of existing Git hosting
providers to serve our community content with high uptime
Serve package source archives (produced by
sdist) via S3, a de facto standard API for file
storage, supported by a large array of cloud providers. These
archives can be large, but unlike package metadata, their content
is fixed for all time. Uploading a new version of a package means
uploading a new source archive with a different name. Serving our
package content via a standard API means we can have that content
hosted on a reliable cloud platform. In short, better uptime and
higher chance that
cabal install will not randomly
The Haskell Stash is a repository in which to store our
community's shared code assets in as simple, highly available and
composable a manner as possible. Reduced to its bare essence,
easily consumable by all manner of downstream services, most
notably, Hackage itself, packdeps.haskellers.com,
hdiff.luite.com, stackage.org, etc. It is by enabling
people to extend core infrastructure in arbitrary directions that
we can hope to build a thriving community that meets not just the
needs of those that happened to seed it, but that furthermore
embraces new uses, new needs, new people.
Provided community interest in this approach, the next steps
- implement the Haskell Stash;
- implement support for the Haskell Stash in Hackage Server;
- in the interim, if needed, mirror Hackage content in the
In the next post in this series, we'll explore ways to apply the
same principles of composability to our command-line tooling, in
the interest of making our tools more hackable, more powerful and
ship with fewer bugs.
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