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Level: Power Up

Accessing Polling Trigger Parameters

Hey all! Currently building out a Custom Connector on MS Flow, and need a bit of help here. Actually becoming quite frustrated over this. Here is an abstraction of the issue I am facing.

I need to build a trigger that polls an endpoint to detect if an instance of `ParentObject` has had a new instance of `ChildObject` appended to its `contents` property.


The request/response for the poll will look something like this.


    GET "/trigger/newChildObject"
    Query parameters:
        parentObjectId: <string>
        # potential solution - "after" UTC string
after: <UTC DateTime> Response: Body<application/json>: "contents": [ ChildObject{ id: <string>, name: <string>, created: <UTC DateTime>, updated: <UTC DateTime> }]


The `parentObjectId` query parameter is determined by the user via a dynamic dropdown when setting up the flow. That much I have working.


What I'm getting hung up on, and actually quite frustrated over, is the lack of documentation on MS Flow Polling Trigger. Polling inherently involves caching/storing the value of some property, right? By the very nature of polling, there must be a mechanism via which the previous poll is compared to the current one, so as to determine if some action/operation should be executed. 


So in my case, I do have the ability to include an `after` query parameter, a UTC Date-Time. When included, the API will return only the `childObject` items which were created after that time code. Well, even when I include that in the HTTP Request definition for the Polling Trigger definition, I do not get the option to choose this parameter from the "Query Parameter" dropdown in the "Trigger Configuration" section. Furthermore, there is no mention of how to dynamically pass the Date-Time of the previous poll within the MS Flow Documentation.


Setting a query parameter via Policy Template seemed like the next logical route of investigation, but has turned up nothing, as again, the documentation on available `ConnectionParameters` dynamic variables for templating is completely lacking, particularly for polling.


The last option would be to return all childObject instances, regardless of create times, and compare the length of the array. But again, no mention as to how that might be done, and given the nature of Flow, I'd hedge my bets that it can't be.


The closest thing I can find is this document but it is of no help in creating an MS Flow Polling Trigger via UI or OpenAPI Extension.


Let's figure this out, yeh?


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