Previously, I distilled the essence of a software delivery pipeline and argued that a transformation step that builds or tests a piece of code can be viewed as a function. Functions compose according to well-defined mathematical rules and they are a suitable model for defining arbitrary pipelines. Instead of talking about build tasks, jobs, stages and workflows, the build pipeline could be a function that takes the source code as argument and returns build artifacts.

In this article I describe, Kevlar, an experimental build automation tool that tries to build on these concepts.

Programming in YAML

For almost two years I was part of a team building a build automation system for Pix4D, a medium-sized organization with:

We wanted a system to build our software products on all the major desktop and mobile platforms. A few years back when the project started the scene of managed build automation tools was less exiting than today: TravisCI was a a major player, GitLab was on the rise, GitHub Actions and CircleCI didn't exist.

For various reasons we ruled out the available hosted options and we deployed Concourse CI in our data centers. Concourse served us well: today Pix4D's continuous integration system builds more libraries and products than ever. Concourse uses YAML for describing its pipelines which model the software delivery process. And YAML started to sprout everywhere.

Although YAML is used to configure virtually all build automation systems its limitations become apparent when you write a pipeline for any reasonably complex software project. Using only numbers, strings, lists and associative arrays pipeline definitions are verbose and repetitive.

To allow for reusing pipeline code build automation systems introduced ad-hoc concepts and workarounds. Some examples are:

Many systems also provide control flow operators such as conditionals, loops and other special constructs to express, for example, matrix builds. Ultimately the pipeline configuration becomes an implicitly functional programming language using YAML's syntax.

In programming we reduce repetition using methods, functions and procedures. We organize our code in modules and packages for sharing and reuse. Let's try to use these concepts to express software delivery pipelines.

Enter Kevlar

I found programming in YAML frustrating and I wanted to explore how a build pipeline would look like in a general-purpose programming language. Project Kevlar was born.

In Kevlar you express the build pipelines using Haskell functions. A step that builds Kevlar itself looks like this:

build repo = do
  src <- clone repo
  img <-
    [Need src ""]
    [ Need src "",
      Image img,

This function takes the source repository as a parameter and describes the following actions:

The Need argument of the functions express data dependencies explicitly. We need the cloned repository to build the Docker image because the Dockerfile is found there. Similarly, because the build script is running in a container it needs both the source code checked out and the container's image built.

The syntax may be unusual, but it's just regular Haskell code. This could have been easily written in YAML with equal clarity, so why bother with Haskell? The advantages of using real programming language constructs appear when we build more complicated workflows. As pipelines grow we can factor out common into helper functions or shared modules and packages.

Let's continue building Kevlar's own pipeline by adding a publish step:

publish repo binary = do
  src <- clone repo
  img <-
    [Need src ""]
    [ Need binary "build",
      Need src "",
      Image img,

The publish function takes the address of the source repository and the build binary. The steps are similar to that of the build function:

With the build and publish functions we can succinctly define Kevlar's build-and-release pipeline:

buildAndPublish src = build src >>= publish src

This function, which composes build and publish using the monadic bind operator, actually works it does what you'd expect: the pipeline builds and publishes the built binaries running both steps in dedicated Docker containers.

We may refer to the functions build and publish as "steps" and the function buildAndPublish as the final "pipeline". In Kevlar they are all represented as functions returning a Task value.

The function buildAndPublish defines the following graph of dependencies:

           ↗ build image  → compile ↘
clone repo                            publish
           ↘        build image     ↗

This figure reveals interesting optimization opportunities:

The next sections expand on these ideas in detail.

Automatic parallelism

Instead of naively executing each steps as they appear in the source code, Kevlar uses as much parallelism as possible. The user defines data dependencies using the Need parameter and parallelism is automatic. This idea is not new: for example build systems like make and ninja track dependencies among build steps and schedule as many of them as they can on the available processors.

Initially I built Kevlar on top of Shake, a library for creating build systems. Shake is a well-designed and performant library but it was a poor fit for Kevlar. Shake takes the source code and builds your program's binary as fast as possible. Build rules and dependencies between tasks rely on file names and file patterns. I needed a library to express general data dependencies in the pipeline code without using the file system.

I switched Kevlar to use Haxl to make parallelism automatic. Haxl relies on some algebraic properties of the program to identify data dependencies between tasks and schedules many independent tasks concurrently. The magic of Haxl is contained within its own codebase and I only had to implement a custom data source. Even better, the user, in this case the pipeline author, doesn't need to be aware of any of this and the pipeline remains a regular Haskell function.

Today's popular build automation systems require the user to explicitly choose between sequential or parallel execution when designing the pipeline. Concepts like tasks, steps, jobs are introduced with an emphasis of execution order instead of capturing the meaning of the transformation steps in the software delivery process.

In Kevlar the user is only concerned with data dependencies. The system makes sure that the pipeline's task run in the right order as fast as possible.

Incremental work

Avoiding extra work is crucial for good performance. The output of a given task should be reused as the pipeline executes: the output might be available from earlier steps or from earlier executions.

Reusing a result is desired if the pipeline fans out: we compute the result once and copy it to the downstream tasks. We've seen an example of this in the previous section where the repository's local copy was passed to start building the two container images independently.

Reusing results from earlier executions is harder. Tasks may return many kinds of outputs: single files, directories, container images. These all need to be persisted somewhere to be reused during the next pipeline execution. I couldn't find a satisfactory solution to this in Kevlar.


Frustrated by the verbose YAML configuration used by popular build automation tools I wrote Kevlar, an experimental system, where the pipeline configuration is expressed in a functional programming language.

Pipelines are functions which define data dependencies and not execution order. With some help from great libraries parallelism is automatic and duplicate work is avoided.

Working on Kevlar made me appreciate even more the power of pure functions: values, functions and their combinations are powerful modeling tools. When thinking functionally you ask what things are instead of what they do and this leads to interesting discoveries and simple, solid designs.

I also realized that I'm rediscovering the basic principles of Nix and Nix is so much better than Kevlar ever wanted to be. If the ideas in this article resonate with you I recommend to try building your next software delivery pipeline with Nix.


I dedicate this post to the memory of my friend and colleague Salah Missri who tragically passed away earlier this year. Salah was the biggest fan of Kevlar. He patiently listened to me ranting about continuous integration systems and encouraged me keep working on Kevlar until it reaches world domination.