How Build the Proper B2B Relationships?

[Robert Campbell]

I'm torn between building relationships correctly and building them in an easy to understand way. The data flow is a simple flow. App1 -> Data Warehouse -> Event Hub -> App2. However, due to processes or lack of, App2 is not informed when App1 goes down. Data Warehouse is but that's all. Likewise, change approvals also only go one level. This means, if App1 is doing a change that could impact App2, App1 needs to know that and call it out so that App2 is informed. 

App1 doesn't know that App2 is dependent on it. App2 realized App1 had data that it needed and worked with the Data Warehouse and Event Hub to get the data. If App1 changes data type or something like that, the Data Warehouse can make the necessary changes so that it send the proper data to Event Hub and no changes are needed outside of the Data Warehouse but if App1 stops producing data and the Data Warehouse is setup that it just passes a NULL value to Event Hub, everything seems to be fine but App2 is now missing data.

To shine a light on this, we could make the Data Warehouse, Event Hub and App2 L1 B2B to App1. This way, when App1 makes a change, people [can] question if the Data Warehouse, Event Hub and App2 will be impacted. Otherwise, you rely on the information flowing up the chain. However, because it's a Data Warehouse and Event Hub, EVERY application could be involved because although every application doesn't receive data from App1, every application receives data from the Data Warehouse and/or Event Hub. It's simpler to show a B2B to the source and every "middleware" app in between. Or is there a better way to do this?