@centient/dag

Generic directed-acyclic-graph scheduling core. Pure computation, zero runtime dependencies. Operates over DAGNode<TId> — an id plus a dependsOn edge list — and makes no assumptions about node payloads: callers keep their domain data out of band (e.g. a Map<TId, Payload> keyed by node id).

Provides the primitives a scheduler needs:

• Adjacency building — in/out edges + in-degree, with missing-dependency validation.
• Cycle detection — DFS three-colour marking that returns the ordered cycle path.
• Topological sort — Kahn's algorithm with deterministic tie-breaking.
• Wave computation — parallelizable layers (longest-path level assignment).
• Failure-cascade propagation — downstream nodes that become unreachable when a node fails.

Installation

npm install @centient/dag

Or as a workspace dependency in the monorepo:

pnpm add @centient/dag --workspace

Quick Start

import {
  topologicalSort,
  computeWaves,
  propagateFailure,
  detectCycle,
  type DAGNode,
} from "@centient/dag";

// TId narrows to your own string-literal union — fully type-checked.
type Stage = "build" | "test" | "lint" | "deploy";

const nodes: DAGNode<Stage>[] = [
  { id: "build", dependsOn: [] },
  { id: "test", dependsOn: ["build"] },
  { id: "lint", dependsOn: ["build"] },
  { id: "deploy", dependsOn: ["test", "lint"] },
];

topologicalSort(nodes);
// → ["build", "lint", "test", "deploy"]

computeWaves(nodes);
// → [["build"], ["lint", "test"], ["deploy"]]
//   each inner array can run concurrently

propagateFailure(nodes, "build");
// → { failed: ["build"], cascaded: ["deploy", "lint", "test"] }

Determinism

Every operation that emits node IDs is reproducible. Ties (independent nodes, peers within a wave) break with a single comparator — ascending Unicode codepoint order by default — so output never depends on Map/Set insertion order. Pass a custom IdComparator<TId> to any function to override the order. There is no Date.now() or Math.random() anywhere in the package.

const descending = (a: string, b: string) => (a < b ? 1 : a > b ? -1 : 0);
topologicalSort(nodes, descending);

API

Graph algorithms

Failure cascade

Errors

• DAGCycleError — thrown by topologicalSort / computeWaves on a cyclic graph. Carries cycle: ReadonlyArray<TId> (the ordered, closed path) and a human-readable message (DAG cycle detected: a -> b -> a).
• DAGMissingNodeError — thrown when a dependsOn entry (or a cascade/query target) references a node not declared in the graph. Carries missingId and the referencing nodeId (when applicable). The core refuses to silently treat an undeclared dependency as a phantom node.

Wave semantics

computeWaves assigns each node to wave 1 + max(wave of its dependencies) — the longest-path distance from any root. Wave 0 holds dependency-free nodes. Every node inside a wave is mutually independent, so a scheduler may dispatch a whole wave concurrently and only advance to wave N+1 once wave N settles.

Failure cascade

When a node fails, everything that transitively depends on it can no longer run (its inputs will never be produced). propagateFailure returns that downstream set in cascaded so a scheduler can mark those nodes skipped/blocked instead of dispatching them. A node that is itself in the failed set is never double-counted as cascaded.

Provenance & divergences

This package was harvested from two copies of the same engine and cross-checked against the original:

• Original: platform/crucible packages/crucible/src/orchestrator/dag.ts (+ its test suite).
• Extracted copy: pipeline-sdk packages/orchestrator/src/dag.ts (header: "100% generic — extracted as-is from crucible").

Where the two disagreed, the original's behaviour was chosen unless the copy fixed a demonstrable bug. Recorded divergences:

1. Cycle detection returns the cycle path, not a node set. The crucible original uses DFS three-colour marking and reconstructs the real ordered cycle (a -> b -> a). The pipeline-sdk copy reused Kahn's algorithm and returned only the unordered set of unvisited (cycle-participating) nodes, losing the path. We kept the original's path-returning behaviour — it is both the original and what the consuming spec requires ("the cycle path in the error").

2. Missing dependencies are a hard error, not a silent phantom node. Both seeds tolerated a dependsOn id with no declared node (treating it as an implicit zero-edge node). This core throws DAGMissingNodeError instead, per the no-silent-degradation principle. Orchestrator-level schema validation (which previously caught this upstream) lives in the consumer, not here.

3. Orchestrator-specific surface was deliberately dropped. DAGDefinition validation (kebab-case ids, chainMode, conflictStrategy, stageTimeoutMinutes), the DAGEngine class, ready-queue resolution, and chain extraction are pipeline/orchestrator concerns, not generic DAG scheduling — they stay in the consumer to preserve this package's zero-dep, payload-agnostic property. The failure classifiers (failure-classifier.ts in both seeds) classify failures by exit code / stderr and are likewise domain logic; this package owns failure cascade propagation through the graph, not failure classification.

License

MIT