Custom logic app connector

Question

We are creating a multi tenant application. To allow the users to create bussiness logic, we want to use Logic apps.

Therefore I want to create a web app which will expose the DocumentDB change feed. When creating a logic app, you can choose between different out of the box connectors. How can we get ours included in the list? Is there any documentation on that?

The idea is to get the logic app running with every document insert. To achieve this, I have two options: Polling triggers and Webhook triggers. I prefer the polling trigger because this will be less work than implementing logic to handle all the subscribed URL's per tenant. Anyone who has concerns/suggestions on this approach?

The location header should become my continuation token from the DocumentDB change feed, is that correct?

1. Logic app will call my api the first time without location header

2. My api will call DocDb without continuation tokens, which will return all docs one by one, because the max doc count is set to 1

3. My api will return the first document that is retrieved, and will set the retry-after to 0 and the location to new continuation token that I have received. If no documents are found, the api will return the result like in step 5.

4. Logic app will start a new instance to handle the document and will call the API again with the continuation token in the header.

Step 3 to 4 will be repeated until all documents are processed. Because I am only processing one document per logic app instance, Azure should be able to scale for me automatically?

5. When all documents are processed, the api will return a 202 statuscode with a location header set to the latest continuation token and a retry-after to 15.

6. After 15 seconds, logic app will call our api with the latest continuation token. This will trigger the process again.

Is my solution feasible? What if I need to stop, or clone the logic app configuration for some reason, how can I know what the latest continuation was or do I need to save my continuation tokens in some data store?

Answer 1

Yes what you've described here should be supported. You can use your own connector in a logic app by clicking the dropdown above the search and selecting to use an API from API Management or App Services as detailed here and here.

The continuation token can be preserved in the "trigger state" of the location header assuming you are using the 202 polling pattern above. So for example the header may be https://mydocdbconnector.azurewebsites.net/api/trigger?triggerstate={thisCouldBeTheContinuationToken} -- that way on subsequent polls the last continuation token is sent back to the trigger and can be used in the operation. Trigger state is preserved as long as the trigger remains unchanged in the definition (enabled/disabling/etc all preserve trigger state).

The only part I'm not clear on is the multi-tenant requirements you have. I assume you mean you want each of the users to be able to trigger on their own documentDb instance -- the best supported pattern for this today is to have a logic app per customer - each with it's own triggerState and trigger. This could be leveraging a custom connector as well. This is the pattern that services like Microsoft Flow use which are built on Logic Apps.

Let me know if that helps.