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Durable Functions

Durable Functions is an extension of Azure Functions that lets you write stateful functions in a serverless compute environment. The extension lets you define stateful workflows by writing orchestrator functions and stateful entities by writing entity functions using the Azure Functions programming model.


Function chaining

  • In the function chaining pattern, a sequence of functions executes in a specific order.
  • In this pattern, the output of one function is applied to the input of another function.
  • The use of queues between each function ensures that the system stays durable and scalable, even though there is a flow of control from one function to the next.

In this example, the values F1, F2, F3, and F4 are the names of other functions in the same function app. You can implement control flow by using normal imperative coding constructs. Code executes from the top down.


In the fan out/fan in pattern, you execute multiple functions in parallel and then wait for all functions to finish. Often, some aggregation work is done on the results that are returned from the functions.

How to do fan in?

Fanning back in is much more challenging. To fan in, in a normal function, you write code to track when the queue-triggered functions end, and then store function outputs


The async HTTP API pattern addresses the problem of coordinating the state of long-running operations with external clients. A common way to implement this pattern is by having an HTTP endpoint trigger the long-running action. Then, redirect the client to a status endpoint that the client polls to learn when the operation is finished.

HTTP reference

reference docs

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