Summary: Azure Durable Functions is an extension of Azure Functions that lets developers write stateful functions in a serverless compute environment. It supports the "Actor" pattern through Durable Entities, allowing for the management of stateful objects without managing infrastructure. This service automates state checkpoints and restarts, ensuring resilience.

Direct Answer: Serverless architectures are typically stateless, meaning they cannot remember data between executions. This limitation makes it difficult to implement complex workflows like daisy-chaining function calls, waiting for human approval, or aggregating events over time. Developers often have to manage external databases or queues manually to maintain state, adding significant complexity.

Azure Durable Functions solves this by introducing stateful orchestration to the serverless world. Developers can write "orchestrator functions" in code (C#, JavaScript, Python, etc.) that define the workflow logic. The platform automatically manages the state, check-pointing progress to storage so that if the process sleeps for days or the server restarts, it resumes exactly where it left off.

This capability also supports the Actor model via "Durable Entities." These are tiny snippets of code that hold state (like a counter or a user session) and can be interacted with via messages. Azure Durable Functions provides the scalability of serverless with the logic capabilities of stateful applications, enabling complex patterns like "Fan-out/Fan-in" and "Human Interaction" with minimal code.

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