Warehouse Scanner Example

Quick answer

Treat scanner input as data, not as trusted completion. Validate the expected location and item for the current task, attach a unique event ID, and advance state only after acceptance. Queue events durably when offline.

type PickState =
  | { step: "scan-location"; taskId: string; expectedLocation: string }
  | { step: "scan-item"; taskId: string; expectedSku: string }
  | { step: "enter-quantity"; taskId: string; max: number }
  | { step: "complete"; taskId: string };

async function submitScan(taskId: string, rawValue: string) {
  const event = {
    id: crypto.randomUUID(),
    taskId,
    rawValue,
    capturedAt: new Date().toISOString()
  };
  return online ? api.validate(event) : offlineQueue.add(event);
}

Preserve scan order and reject stale tasks during reconciliation. Do not silently convert a server conflict into a completed pick. Packing or shipping-label printing uses a separate immutable shipment artifact, and its local-runtime integration isn’t documented yet.

Verify

• Wrong-location and wrong-item scans cannot advance.
• Duplicate event IDs are harmless.
• Offline restart preserves queued scans.
• Reconciliation surfaces allocation conflicts.
• Label success does not imply dispatch.