Practitioners of continuous integration often describe the process of automated software delivery as a pipeline: the source code enters the pipe, it is compiled, tested, packaged and released product comes out on the other end.
This methaphor evokes the notions of delivering, modularity and continuity. Teams of different backgrounds relate to this image even without understanding each transformation step.
But what is a software delivery pipeline? In this post, instead of a metaphor, I propose a precise mathematical model of it.
I reviewed five popular CI/CD systems where users model their software delivery process by defining a pipeline:
Let's see how the relevant user documentation describe the pipeline and its related concepts.
Task, action, step
The reviewed systems call the pipeline's unit of work task, action or step.
A step is the smallest building block of a pipeline. A step can either be a script or a task.
A step is an executable command.
A task is the smallest configurable unit. A task can be thought of as a function from inputs to outputs that can either succeed or fail.
Individual tasks that you combine as steps to create a job. Actions are the smallest portable building block of a workflow.
A build task is an action that needs to be performed. Usually, it is a single command.
The names differ but they all describe a similar concept: the unit of work is an executable script or command.
Concourse's task definition proposes a precise semantic model: a task is a function. We will build on this model later.
A job is an ensemble of tasks, actions or steps.
A job represents an execution boundary of a set of steps. All of the steps run together on the same agent.
Jobs are collections of steps.
Jobs are sequences steps to execute.
A defined task made up of steps. Each step in a job executes in the same runner.
A job consists of multiple tasks, each of which will be run in order.
At this concept the definitions start to diverge, still there are some common points:
- Actions, tasks or steps build up jobs.
- A job's components usually run sequentially.
- A job's components usually run on the same build agent, executor or runner.
The notable exceptions are:
- In Concourse it's possible to run a job's steps in parallel.
- In Concourse and GoCD there are no locality guarantees on where the job's tasks are run.
In some systems jobs can be grouped into a stage.
A stage is a logical boundary in the pipeline. Each stage contains one or more jobs.
A stage consists of multiple jobs, each of which can run independently of the others.
Azure Pipelines runs the stages sequentially by default, but arbitrary ordering can also be defined between them. This includes no ordering at all, meaning that stages can run concurrently.
CircleCI, Concourse and GitHub Actions don't have this concept.
We now are ready to define a pipeline, also called workflow.
A pipeline defines the continuous integration and deployment process for your app. It's made up of one or more stages.
Workflows define a list of jobs and their run order.
Pipelines are built around jobs and resources. They represent a dependency flow.
Workflows are made up of one or more jobs and can be scheduled or activated by an event.
A pipeline consists of multiple stages, each of which will be run in order.
Jobs or stages are grouped into pipelines. The definitions are again operational with an emphasis of execution order and dependencies.
What is a pipeline, really?
Now we've seen some of the concepts of the most popular CI/CD systems. Some systems have even more which I didn't cover here.
Do we need all these to model the software deliver process?
Task as a function
Let's revisit Concourse's task definition: A task can be thought of as a function from inputs to outputs that can either succeed or fail.
This is a great definition because it specifies what a task means and not what it does or how it does it. Developers can choose to implement a task as they deem most fitting but the user can think of it as a function no matter what.
Let's see some task examples:
- A compilation task takes a source code as input and produces a compiled binary as output.
- A test task takes the compiled binary as input and produces a test report as output.
- A release task takes the compiled binary and a test report. If the test report is acceptable (no tests fail, test coverage is OK) it releases the binary and returns a link to repository where the software can be downloaded.
I named the unit of work "task". As we've seen other systems prefer "step" or "action", which would be totally fine as well.
Let's write down formally Concourse's task definition.
type Task a b = a -> Maybe b
A Task is a function with two type parameters
b representing its
input and output types, respectively. To express possible failure, the output
is wrapped in Haskell's
Maybe type. In other languages this is called
I wrote down this definition in Haskell's syntax but this is not important. What matters is that our model, the Task's meaning, is mathematical function.
These are the type signatures of the tasks described previously in words:
-- input type output type build :: SourceCode -> Maybe CompiledBinary test :: CompiledBinary -> Maybe TestReport release :: CompiledBinary -> TestReport -> Maybe PackageURL
These are not the implementation of these tasks but their definition expressed as Haskell code.
Let's define a task to tests the incoming pull requests of our project.
This task takes the pull request's source code, builds the binary, runs the tests and returns the test report. The test report is for the reviewers to judge the quality of the proposed change.
In short, we want sequence the tasks
test. If we had an operator
with this type signature:
inSequence :: a -> Maybe b -- first task -> b -> Maybe c -- second task -> a -> Maybe c
we could express the pull request validating task as:
validatePullRequests :: SourceCode -> Maybe TestReport validatePullRequests = inSequence build test
Taskbecause it's a function with the right type signature
- The source code is fed to the first task,
- The resulting type of
buildfails the result of the whole task is failure
- Otherwise, feed the compiled binary to
I haven't shown you the definition of
inSequence, but you can verify that in
validatePullRequests the types match. You can also see that
inSequence looks almost like regular function composition except the output
types are wrapped in
Let's consider now two independent tasks:
unitTests :: SourceCode -> Maybe UnitTestReport integrationTests :: SourceCode -> Maybe IntegrationTestReport
These two test suites could be run in parallel, because they both only depend
We don't want to introduce a new concept, but we want the result of parallel
composition to be a
Task as well. We're after an operator with the following
inParallel :: (a -> Maybe b) -> (a -> Maybe c) -> (a -> Maybe (b, c))
The composite task yields the results of input tasks as a tuple. If any of
the two task fails, the result of the composite task is failure (represented by
inParallel we could write a task to run all tests:
runAllTests :: SourceCode -> Maybe (UnitTestReport, IntegrationTestReport) runAllTests = inParallel unitTests integrationTests
inParallel operator represents a "fan-out" structure in the pipeline
where independent transformation steps are applied on the same input.
In the previous sections we've defined a denotational model for CI/CD build tasks:
type Task a b = a -> Maybe b
which maps the Task concept to its meaning, a mathematical object. This serves not only as a mental model, but also it allows us introduce regular and powerful composition rules.
I've shown you
inParallel combinators. For reference,
without explanation, here are their definitions:
inSequence :: Task a b -> Task b c -> Task a c inSequence t1 t2 x = t1 x >>= t2 -- or equivalently = t1 >=> t2 inParallel :: Task a b -> Task a c -> Task a (b, c) inParallel t1 t2 x = liftA2 (,) (t1 x) (t2 x)
These combinators are expressed using the task's semantic model without operational terms or unnecessary limiting assumptions.
It turns out that
inParallel are not primitive operations.
Tasks and their composition rules can be defined using a more general
vocabulary of arrows. This suggests that
the semantic model is powerful enough to model any software delivery process.
Using this model, jobs, stages, workflows and pipelines are just
Today's popular CI/CD systems are built around the metaphor and not a rigorous
definition of a pipeline. I propose
Maybe-valued functions as a semantic
model for a build task. Using well-studied and precisely defined rules, tasks
can be composed to model the software delivery process.
In a future post I will present an experimental system which uses these principles to express continuous integration and continuous delivery pipelines.
Many thanks to the members of the Pix4D CI team for the inspirational discussions during coffee breaks.
I'm grateful to Conal Elliott for reviewing an early draft of this article and for providing valuable feedback.