Durable Functions and Orchestration

Standard Azure Functions are stateless — each invocation is independent with no built-in way to coordinate multiple steps, wait for external events, or manage long-running workflows. Durable Functions is an extension that adds stateful orchestration capabilities, enabling you to write complex workflows as code. This lesson covers orchestrator functions, activity functions, patterns, and best practices.

---

What are Durable Functions?

Durable Functions is an extension of Azure Functions that lets you write stateful functions in a serverless environment. The extension manages state, checkpoints, and restarts automatically using an Azure Storage backend.

The framework introduces three new function types:

Function Type	Purpose
Orchestrator	Defines the workflow — calls activities, waits for events, manages control flow
Activity	A single unit of work within the workflow — the actual business logic
Entity	A stateful object that can be read and updated (actor model)
Client	Starts orchestrations or sends events to running instances

---

How It Works: Replay Architecture

Durable Functions uses an event-sourcing and replay architecture: