Workflow saga orchestrator

Design a workflow/saga orchestrator that executes multi-step business transactions (e.g. travel booking: reserve flight, hotel, car) where some steps run concurrently and any failure must trigger compensating actions to undo completed steps. Constraints: ~5k workflows/sec, steps call external services with their own timeouts, the orchestrator may crash mid-workflow, and the same workflow must not double-execute a step. Describe state tracking, how concurrent branches are coordinated and joined, and the failure/compensation model.