Create your first durable function in Python

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 for you.

In this article, you learn how to use the Visual Studio Code Azure Functions extension to locally create and test a "hello world" durable function. This function will orchestrate and chains together calls to other functions. You can then publish the function code to Azure.

Screenshot of the running durable function in Azure.

Prerequisites

To complete this tutorial:

If you don't have an Azure subscription, create an Azure free account before you begin.

Create your local project

In this section, you use Visual Studio Code to create a local Azure Functions project.

  1. In Visual Studio Code, press F1 (or Ctrl/Cmd+Shift+P) to open the command palette. In the command palette, search for and select Azure Functions: Create New Project....

    Screenshot of Create function window.

  2. Choose an empty folder location for your project and choose Select.

  3. Follow the prompts and provide the following information:

    Prompt Value Description
    Select a language for your function app project Python Create a local Python Functions project.
    Select a version Azure Functions v4 You only see this option when the Core Tools aren't already installed. In this case, Core Tools are installed the first time you run the app.
    Python version Python 3.6, 3.7, or 3.8 Visual Studio Code will create a virtual environment with the version you select.
    Select a template for your project's first function Skip for now
    Select how you would like to open your project Open in current window Reopens Visual Studio Code in the folder you selected.

Visual Studio Code installs the Azure Functions Core Tools if needed. It also creates a function app project in a folder. This project contains the host.json and local.settings.json configuration files.

A requirements.txt file is also created in the root folder. It specifies the Python packages required to run your function app.

Install azure-functions-durable from PyPI

When you've created the project, the Azure Functions Visual Studio Code extension automatically creates a virtual environment with your selected Python version. You then need to activate the virtual environment in a terminal and install some dependencies required by Azure Functions and Durable Functions.

  1. Open the requirements.txt in the editor and change its content to the following code:

    azure-functions
    azure-functions-durable
    
  2. Open the editor's integrated terminal in the current folder (Ctrl+Shift+`).

  3. In the integrated terminal, activate the virtual environment in the current folder, depending on your operating system:

    source .venv/bin/activate
    
  4. In the integrated terminal where the virtual environment is activated, use pip to install the packages you defined.

    python -m pip install -r requirements.txt
    

Create your functions

A basic Durable Functions app contains three functions:

  • Orchestrator function: Describes a workflow that orchestrates other functions.
  • Activity function: It's called by the orchestrator function, performs work, and optionally returns a value.
  • Client function: It's a regular Azure Function that starts an orchestrator function. This example uses an HTTP triggered function.

Orchestrator function

You use a template to create the durable function code in your project.

  1. In the command palette, search for and select Azure Functions: Create Function....

  2. Follow the prompts and provide the following information:

    Prompt Value Description
    Select a template for your function Durable Functions orchestrator Create a Durable Functions orchestration
    Provide a function name HelloOrchestrator Name of your durable function

You've added an orchestrator to coordinate activity functions. Open HelloOrchestrator/__init__.py to see the orchestrator function. Each call to context.call_activity invokes an activity function named Hello.

Next, you'll add the referenced Hello activity function.

Activity function

  1. In the command palette, search for and select Azure Functions: Create Function....

  2. Follow the prompts and provide the following information:

    Prompt Value Description
    Select a template for your function Durable Functions activity Create an activity function
    Provide a function name Hello Name of your activity function

You've added the Hello activity function that is invoked by the orchestrator. Open Hello/__init__.py to see that it takes a name as input and returns a greeting. An activity function is where you'll perform actions such as making a database call or performing a computation.

Finally, you'll add an HTTP triggered function that starts the orchestration.

Client function (HTTP starter)

  1. In the command palette, search for and select Azure Functions: Create Function....

  2. Follow the prompts and provide the following information:

    Prompt Value Description
    Select a template for your function Durable Functions HTTP starter Create an HTTP starter function
    Provide a function name DurableFunctionsHttpStart Name of your client function
    Authorization level Anonymous For demo purposes, allow the function to be called without authentication

You've added an HTTP triggered function that starts an orchestration. Open DurableFunctionsHttpStart/__init__.py to see that it uses client.start_new to start a new orchestration. Then it uses client.create_check_status_response to return an HTTP response containing URLs that can be used to monitor and manage the new orchestration.

You now have a Durable Functions app that can be run locally and deployed to Azure.

Test the function locally

Azure Functions Core Tools lets you run an Azure Functions project on your local development computer. If you don't have it installed, you're prompted to install these tools the first time you start a function from Visual Studio Code.

  1. To test your function, set a breakpoint in the Hello activity function code (Hello/__init__.py). Press F5 or select Debug: Start Debugging from the command palette to start the function app project. Output from Core Tools is displayed in the Terminal panel.

    Note

    For more information on debugging, see Durable Functions Diagnostics.

  2. Durable Functions require an Azure storage account to run. When Visual Studio Code prompts you to select a storage account, select Select storage account.

    Screenshot of how to create a storage account.

  3. Follow the prompts and provide the following information to create a new storage account in Azure:

    Prompt Value Description
    Select subscription name of your subscription Select your Azure subscription
    Select a storage account Create a new storage account
    Enter the name of the new storage account unique name Name of the storage account to create
    Select a resource group unique name Name of the resource group to create
    Select a location region Select a region close to you
  4. In the Terminal panel, copy the URL endpoint of your HTTP-triggered function.

    Screenshot of Azure local output.

  5. Use your browser, or a tool like Postman or cURL, send an HTTP request to the URL endpoint. Replace the last segment with the name of the orchestrator function (HelloOrchestrator). The URL must be similar to http://localhost:7071/api/orchestrators/HelloOrchestrator.

    The response is the initial result from the HTTP function letting you know the durable orchestration has started successfully. It isn't yet the end result of the orchestration. The response includes a few useful URLs. For now, let's query the status of the orchestration.

  6. Copy the URL value for statusQueryGetUri, paste it in the browser's address bar, and execute the request. Alternatively, you can also continue to use Postman to issue the GET request.

    The request will query the orchestration instance for the status. You must get an eventual response, which shows the instance has completed and includes the outputs or results of the durable function. It looks like:

    {
        "name": "HelloOrchestrator",
        "instanceId": "9a528a9e926f4b46b7d3deaa134b7e8a",
        "runtimeStatus": "Completed",
        "input": null,
        "customStatus": null,
        "output": [
            "Hello Tokyo!",
            "Hello Seattle!",
            "Hello London!"
        ],
        "createdTime": "2020-03-18T21:54:49Z",
        "lastUpdatedTime": "2020-03-18T21:54:54Z"
    }
    
  7. To stop debugging, press Shift+F5 in Visual Studio Code.

After you've verified that the function runs correctly on your local computer, it's time to publish the project to Azure.

Sign in to Azure

Before you can publish your app, you must sign in to Azure.

  1. If you aren't already signed in, choose the Azure icon in the Activity bar. Then in the Resources area, choose Sign in to Azure....

    Screenshot of the sign-in to Azure window within VS Code.

    If you're already signed in and can see your existing subscriptions, go to the next section. If you don't yet have an Azure account, choose Create and Azure Account.... Students can choose Create and Azure for Students Account....

  2. When prompted in the browser, choose your Azure account and sign in using your Azure account credentials. If you create a new account, you can sign in after your account is created.

  3. After you've successfully signed in, you can close the new browser window. The subscriptions that belong to your Azure account are displayed in the sidebar.

Create the function app in Azure

In this section, you create a function app and related resources in your Azure subscription.

  1. Choose the Azure icon in the Activity bar. Then in the Resources area, select the + icon and choose the Create Function App in Azure option.

    Create a resource in your Azure subscription

  2. Provide the following information at the prompts:

    Prompt Selection
    Select subscription Choose the subscription to use. You won't see this prompt when you have only one subscription visible under Resources.
    Enter a globally unique name for the function app Type a name that is valid in a URL path. The name you type is validated to make sure that it's unique in Azure Functions.
    Select a runtime stack Choose the language version on which you've been running locally.
    Select a location for new resources For better performance, choose a region near you.

    The extension shows the status of individual resources as they're being created in Azure in the Azure: Activity Log panel.

    Log of Azure resource creation

  3. When the creation is complete, the following Azure resources are created in your subscription. The resources are named based on your function app name:

    • A resource group, which is a logical container for related resources.
    • A standard Azure Storage account, which maintains state and other information about your projects.
    • A function app, which provides the environment for executing your function code. A function app lets you group functions as a logical unit for easier management, deployment, and sharing of resources within the same hosting plan.
    • An App Service plan, which defines the underlying host for your function app.
    • An Application Insights instance connected to the function app, which tracks usage of your functions in the app.

    A notification is displayed after your function app is created and the deployment package is applied.

    Tip

    By default, the Azure resources required by your function app are created based on the function app name you provide. By default, they're also created in the same new resource group with the function app. If you want to either customize the names of these resources or reuse existing resources, you need to publish the project with advanced create options instead.

Deploy the project to Azure

Important

Deploying to an existing function app always overwrites the contents of that app in Azure.

  1. Choose the Azure icon in the Activity bar, then in the Workspace area, select your project folder and select the Deploy... button.

    Deploy project from the Visual Studio Code workspace

  2. Select Deploy to Function App..., choose the function app you just created, and select Deploy.

  3. After deployment completes, select View Output to view the creation and deployment results, including the Azure resources that you created. If you miss the notification, select the bell icon in the lower right corner to see it again.

    Screenshot of the View Output window.

Test your function in Azure

  1. Copy the URL of the HTTP trigger from the Output panel. The URL that calls your HTTP-triggered function must be in this format: http://<functionappname>.azurewebsites.net/api/orchestrators/HelloOrchestrator

  2. Paste this new URL for the HTTP request in your browser's address bar. You must get the same status response as before when using the published app.

Next steps

You have used Visual Studio Code to create and publish a Python durable function app.