How are Workflows Represented?
As we mentioned on the Background page, the general definition of a workflow is a process of tasks that may be dependent on one another. Let's revisit the bread recipe:
The high level set of tasks for baking bread are:
- Mix flour and water
- Mix in the yeast
- Fold the dough
- Proof the dough
Now, you must be wondering why we keep going back to this bread recipe. Simply put, it is something most people understand without getting lost in detail (which we will revisit later). Again, for now, let's ignore implementation; instead, let's just simply focus on the tasks to make bread. The steps are numbered in order of execution, meaning that you can not mix in the yeast until you've created the dough by mixing the flour and water. By extension, you cannot proof the dough until you fold the dough, but you must also mix the flour and and water. This recipe forms a linear dependency.
In Computer Science, we can map the idea of a recipe to an abstract data structure known as a graph. A graph is capable of capturing not only data, but also dependencies between different data entries.
What are Graphs?
A graph at its core is a discrete mathematics concept that defines a structure and is defined by both a set of vertices and a set of edges. Vertices describe nodes in a graph, whereas edges describe the connections between them. In terms of representing a workflow, we're primarily interested in a directed graph because edges can be traversed in only one direction. This limitation is useful as it enforces forward movement in processes. Let's revisit the bread recipe above, except this time looking at it from the perspective of a graph.
A Bread Recipe Graph
We previously established that the bread recipe steps are a linearization, or a linear chain of steps. We must complete the steps in sequential order in order to make bread. Here's how that looks as a graph:
graph TD; A(Mix flour + water)-->B(Mix in Yeast); B-->C(Fold dough); C-->D(Proof dough);
If we want to extend the graph to include the baking of the bread, this means
we must complete all the previous steps and then bake our prepared loaf. That
simply is represented as a new edge from
Proof dough to a new node called
Bake loaf and would look as follows:
graph TD; A(Mix flour + water)-->B(Mix in Yeast); B-->C(Fold dough); C-->D(Proof dough); D-->E(Bake loaf);
Now, you'll notice that the edges in the example above have arrows; these arrows represent the direction that these edges must be traversed. This property means that, when scanning through a graph, the ordering of events is preserved. For example, in the recipe graph above, you can not attempt to bake a loaf of bread before mixing the flour and the water because there are no edges pointing in that direction.
How does a Bread Recipe Help You?
So you might be wondering, how does a bread recipe help me? I develop software, run things on super computers -- what use is a graph about bread? Let's revisit the definition of workflow and intent. Let's use a typical workflow seen in high-performance computing as an example; simulation.
A Typical Simulation Workflow
The primary focus of high-performacne computing is to model physical phenomena and learn about them by constructing scenarios that represent controlled experiements. This paradigm often boils down to a workflow characterized by the following high-level process:
- Setup system inputs
- Simulate the system
- Post-process the results
Most simulation codes require some input files (parameter values, meshes, force fields) to run models of a system, meaning that there is a dependence between simulation and input setup. Further, you can not post process results you do not have; therefore post processing is dependent on simulation. If a step in this chain fails, it blocks the rest from being completed. This process is a linearization just like the process of baking bread.
Try it: Breakdown You Own Workflow into a Process
Exercise: Take a moment and think about one of your own workflows. It can be something in your daily routine or a technical item that requires multiple steps to complete (commands, or scripts, etc.). In much the same way that the recipe above is an abstract set of steps without implementation, try to break down your own workflow into an intent-based process. Some questions to think about as you come up with a process:
- What are the natural divisions of work that appear?
- Are there places where the process is unclear?
- Could you describe your process to another person without describing implementation?
- Is it easier to describe the goal of the work without resorting to implementation for detail?
Goal: The goal of this exercise is to start thinking with an intent-based mindset. While implementation is useful, explaining a workflow to others (especially those in other domains) by implementation is confusing and difficult. Boiling a process up to intent provides a conceptual substrate that allows others to more easily follow why a workflow is needed and then understand why each step in he process is needed.
Extra Credit: Describe your workflow to someone else. If the workflow was unclear, was the other person able to effectively ask questions or discuss with you?