Concept-Check: Orchestration

This arc installed the seam that composes everything you built before it: the pipelines became nodes in a typed DAG, execution order became a computed property (Kahn's sort with a deterministic, sorted ready-set), and the executor learned two things a bare sort cannot do — skip a still-fresh source, and gate a node whose upstream failed. Above it all sits the dependency-inversion property that is the real payoff of the course: the executor depends on etl-core only and schedules dyn Source, so a fourth source drops in with zero orchestrator changes.

This check pins down the parts that are easy to get almost right — why the ready-set is a BTreeSet and not a queue, how cycle detection falls out of a length comparison, the difference between a blocked node and a failed one, and what cargo tree proves about the crate boundary.

Two of these are Tracing questions: read the program, decide whether it compiles, and if it does, predict its exact output before revealing the answer. Predicting is the point.