lørdag den 10. august 2019

Azure Function App Managed Service Identity in Visual Studio Code (PowerShell)

I started using MSI in Function Apps a few years ago. Back then it was still a nice quality of life addition, but you had to jump through a few hoops to get it working, ex. this was necessary to use the MSI to login to Azure.

$apiVersion = "2017-09-01"
$resourceURI = ""
$tokenAuthURI = $env:MSI_ENDPOINT + "?resource=$resourceURI&api-version=$apiVersion"
$tokenResponse = Invoke-RestMethod -Method Get -Headers @{"Secret"="$env:MSI_SECRET"} -Uri $tokenAuthURI
$accessToken = $tokenResponse.access_token
Login-AzAccount -Tenant $TenantId -AccountId $env:WEBSITE_SITE_NAME -AccessToken $accessToken -Scope Process

After not having done too much with Function Apps for a while I jumped back in and just could not wrap my head around how to get MSI working in a local development environment (I use VS Code). No help to find on the Internet, at least not explicitly stating how to do it. And for a good reason; the trick is to do nothing at all!

When running locally everything is done in the context of the logged in user, often the developer him/herself.
Above code would also fail (there is no MSI_ENDPOINT and much less an MSI_SECRET). But all the magic happens in profile.ps1 that is auto-created for each function app project. It is already outfitted with below code.

# Authenticate with Azure PowerShell using MSI.
# Remove this if you are not planning on using MSI or Azure PowerShell.
if ($env:MSI_SECRET -and (Get-Module -ListAvailable Az.Accounts)) {
    Connect-AzAccount -Identity

If there is an MSI_SECRET in the environment variables (which there should not be in a local development environment), then connect using an MSI, which I imagine doing something similar to the first piece of code I posted.
Another advantage is that you only do the login once (when the function app does a "cold start"). Previously it would be rather dodgy, and to be on the safe side, I would login in each function run, adding significant overhead.

And when running locally the entire thing is piggybacking off the logged in user. If you want to simulate the access the MSI has when running in Azure, just create a service principal, assign it the same roles as the MSI and login before running your code locally.

Running PowerShell in Azure Function App has come a long way. With the new Push-OutputBinding it is even easier than before to put the result of a function execution into a queue, table storage, etc.

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