Local Testing an Azure Function with a Timer Trigger

Following on from the last post on  Azure Function Deployed But Not Visible, when investigating this I wanted a way to test the function without having to deploy it to Azure. Here's how I did it, using Visual Studio Code and Postman API.

Extensions 

 First up was to install a couple of extra extensions in VS Code; Azurite and Azure Functions Core Tools.

Azurite is an extension that allows "mocking" Azure Blob Storage functionally and can be installed from the VS Code Extensions tab. More info on installing can be found here, https://learn.microsoft.com/en-us/azure/storage/common/storage-install-azurite?tabs=visual-studio-code%2Cblob-storage, and the page has links to other pages on what Azurite is and how to use it.

Azure Functions Core Tools is the tool set that lets you run functions locally. You can try installing by hitting the F1 key then choosing Azure Functions: Install or Update Azure Functions Core Tools but may find you need to download and install manually. The download and further instructions can be found here, https://learn.microsoft.com/en-us/azure/azure-functions/functions-run-local

Running the Function

Before we can run the function we need to start the function runtime environment. To do this, hit the F1 key then find and select Azure Functions: Start. When this runs you'll get prompted to log into Azure and select the subscription and resource group you want to use. Alternatively enter func start in the terminal window. I've found the Azure Functions: Start command a bit hit and miss, so using func start seems to work more consistently.

 

Note that for Python, you'll need to start the Python virtual environment if using that config, otherwise you'll get errors about modules not being found. This will be something like running & <project_path>/.venv/Scripts/Activate.ps1 in the terminal.

 

Postman

Once the function is running in VS Code you'll be able to send a Postman request to trigger the function. By default the function runs on port 7071 (I don't know how to change this, as this worked fine for me) so the call for Postman will be a POST request to http://localhost:7071/admin/functions/function_name, e.g. http://localhost:7071/admin/functions/MyTestFunction. 

 

Authentication isn't needed, but there does need to be a body, so if the function doesn't take parameters then just add empty brackets to the Postman request body. 

Here's how the request should look

 

 

All going well, Postman will get an HTTP 202 response, and function output will appear in the VS Code terminal window.

Settings etc.

The functions local runtime uses settings in the local.settings.json file, so these might need to be changed temporarily for testing. Also, if using service principals or managed identities for any resource authentication these will probably fail, so updating these to a different auth (e.g. SAS key or SQL login) might be needed for testing.