A conceptual framework for durable execution proposes three canonical function forms mapped along a behavior-state continuum: stateless functions (one-shot deterministic logic), sessions (interactive long-running workflows with execution-scoped identity), and actors (persistent stateful objects with unbounded lifetime). Each form is defined by properties including addressability, identity type, lifetime, and communication model. The post maps these forms to real durable execution engines — Temporal, Restate, DBOS, and Resonate — showing how each engine's primitives (workflows, activities, virtual objects, handlers) correspond to one or more of the three forms. Mixed-form durable function trees are also explored, where a session can orchestrate stateless functions and actors together.
