# Core Concepts (/docs/concepts)
Ductor clears work through one lifecycle — **priced → routed → executed →
settled → proven**. This section is the engine vocabulary underneath that
lifecycle: the concepts here are how a
actually gets executed and routed. The public primitives — Work, Route,
Settlement, and the — are defined in the
[glossary](/docs/reference/glossary); the pages below are the mechanisms that
carry them.
Under the hood Ductor is two engines sharing one substrate. A **Go DAG workflow
engine** runs durable, multi-step processes (the *executed* stage); a **routing
engine** turns an incoming event into a concrete assignment (the *routed*
stage). Both sit on **Postgres** (the source of truth) plus **Redis/Dragonfly**
(wakeups and hot counters), and both are driven by the same shape: a
single-writer **coordinator** — the "tick" engine — owns all state transitions,
while **stateless workers** execute the side effects in parallel and only ever
append results.
Read this section before the deeper
[Architecture](/docs/architecture), [Strategies](/docs/strategies),
[Workflows](/docs/management/workflows), [AI](/docs/ai), and
[SDK](/docs/sdks) sections — each of those assumes the model laid out here.
## The concepts [#the-concepts]
The single-writer coordinator + append-only workers that keep run state correct under concurrency.
Steps, edges, scatter/gather, sub-workflows, waits, and ContinueAsNew — the vocabulary for durable work.
How a routable event flows through registered stages to a concrete assignment.
How step tasks and coordinator wakeups are delivered fairly across tenants.
The `record_version` primitive that makes every tick commit race-free.
How durable append plus dedup keys turn at-least-once dispatch into effectively-once work.
Suppress duplicate inbound work, then feed outcomes back into learning and billing.
The trigger plane plus Ductor's three distinct event systems — sourcing, bus, and journal.
How a step reaches the provider catalog with credentials encrypted at rest.
How every run, decision, and counter is scoped to a tenant.
The quota and plan checks that gate the hot path.