The surprise, reading a workflow engine from the inside, is how little of it is about graphs. Topological sorting is a first-year algorithm and it takes maybe fifty lines. Almost everything else is bookkeeping: who holds the lock, what counts as a fatal error, what happens when a process dies without saying so. That is the honest description of the category. The DAG is the part you would write in an afternoon. The rest is the part you would get wrong for a year. None of this is required ...