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Flows

A flow is a Python function decorated with @flow from prefect_compat (not prefect). When you invoke it, IronFlow creates a flow run in the control plane: the Rust engine records state transitions and append-only history for that run and its task runs.

Basics

  • Import: from prefect_compat import flow (see Prefect → IronFlow).
  • A flow coordinates task runs by calling task.submit(...), task.map(...), and wait(...) on futures—see Tasks.
  • You typically register a control plane (for example InMemoryControlPlane) with set_control_plane before executing the flow; see Quick start (demo flow).

Transition hooks (IronFlow extension)

Flows support transition_hooks: a sequence of TransitionHookSpec values built with on_transition(fn, from_state=..., to_state=...). Use None for from_state or to_state to match any state on that side.

Hooks run after a successful control-plane transition, in process, without holding the control-plane lock. They are not the same API names as Prefect’s on_running / on_failure hooks; map your logic to explicit edges (for example PENDING → RUNNING). For full semantics (including the batched start path and error handling), see Compatibility matrix.

Relevant exports: TransitionHookSpec, on_transition, TransitionContext from prefect_compat.