Azure Functions have officially reached ‘hammer’ status
I’ve been enjoying the ease with which we can now respond to events in SharePoint and perform automation tasks, thanks to the magic of Azure Functions. So many nails, so little time!
The seminal blog post that started so many of us on that road, was of course John Liu’s Build your PnP Site Provisioning with PowerShell in Azure Functions and run it from Flow and that pattern is fantastic for many event-driven scenarios.
One where it currently (at time of writing) falls down is when dealing with a list item delete event. MS Flow can’t respond to this and nor can a SharePoint Designer workflow.
Without wanting to get into Remote Event Receivers (errgh…), the other way to deal with this is after the fact via the SharePoint change log (if the delete isn’t time sensitive). In my use case it wasn’t – I just needed to be able to clean up some associated items in other lists.
SharePoint Change Logs
SharePoint has had an API for getting a log of changes of certain types, against certain objects, in a certain time window since the dawn of time. The best post for showing how to query it from the client side is (in my experience) Paul Schaeflin’s Reading the SharePoint change log from CSOM and was my primary reference for the below PowerShell-based Function.
In my case, I am only interested in items deleted from a single list, but this could easily be scoped to an entire site and capture more/different event types (see Paul’s post for the specifics).
The biggest challenge in getting this working was persisting the change token to Azure Storage, and this wasn’t that difficult in and of itself – it’s just that the PowerShell bindings for Azure are as of yet woefully under-documented (TIP: Get-Content and Set-Content are the key to the PowerShell bindings… easy when you know how). In my case I have an input and output binding to a single Blob Storage blob (to persist the change token for reference the next time the Function runs) and another output to Queue Storage to trigger another function that actually does the cleanup of the other list items linked to the one now sitting in the recycle bin. The whole thing is triggered by an hourly timer. If nothing has been deleted, then no action is taken (other than the persisted token blob update).
A Note on Scaling
Note that if multiple delete events occurred since the last check, then these are all deposited in one message. This won’t cause a problem in my use case (there will never be more than a handful of items deleted in one pass of the Function), but it obviously doesn’t scale well, as too many being handled by the receiving Function would threaten to bump up against the 5 min execution time limit. I wanted to use the OOTB message queue binding for simplicity, but if you needed to push multiple messages, you could simple use the Azure Storage PowerShell cmdlets instead of an out binding.
Code Now Please
Here’s the Function code (following the PnP PowerShell Azure Functions implementation as per John’s article above and liberally stealing from Paul’s guide above).
This is obviously a simple example with a single objective, but you could take this pattern and ramp it up as high as you like. By targeting the web instead of a single list, you could push a lot of automation through this single pipeline, perhaps ramping up the execution recurrence to every 5 mins or less if you needed that level of reduced latency. Although watch out for your Functions consumption if you turn up the executions too high!