I needed to create new SharePoint Online Document Library and amongst other things set a Retention Label on that newly created Document Library using Microsoft Flow. Creating new doclib is straightforward using Flow, but I just couldn’t set the the Retention Label via REST from Flow. There is an API for that, but due to reasons I couldn’t get it to work from Flow.
We had an Azure Automation Runbook that was called at the end of the Flow anyway, so I decided to use that to set the Label on the SPO Library using SharePoint Online PnP’s Set-PnPLabel. No problem. However, it takes a while for the Label to be applied to the Library and as email was sent to users at the end of the Flow, they found themselves in the library too early, i.e., the Label was not yet set.
I could’ve used a “Do Until” loop in Flow and poll the list but that’s not something we like to do, right? We like events and triggers, so why not use the HTTP Webhook action, sounds exciting!
In the screenshot above, I’m calling the Azure Automation Runbook, but you can really call anything that is capable of listening to your request and at the and making a HTTP request back to the callback URL you define. You define your endpoint address in the Subscribe - URI field.
In the Subscribe - Body you must at least pass in the listCallbackUrl() so that you know the endpoint of this specific Flow instance you need to call in your backend code. Note that listCallbackUrl() is generated automatically, is specific to this running instance of the Flow, and can only be called once. Flow processing will halt at this action and it will continue when your backend code calls the listCallbackUrl(). You can define timeout of how long the action waits for the callback in the action settings.
Here I’m also passing in the title of the SharePoint Library as I’m also doing some tricks on the library but you would pass in anything you need in your scenario.
In the backend code you will do whatever you need, but when you’re done, make HTTP POST call to the URL you received as a parameter in your backend code, in my case that would look like this in PowerShell.
If you look at the Flow when it is running, you see it pause at the HTTP Webhook action, and continue as soon as the backend calls the callback URL. You can also manually call the callback URL using e.g., Postman, just paste in the callback URL and make sure method is POST. You will get HTTP 200 when the callback call succeeds.