In order to make sure I answer all your questions, I will restate them and then answer them
1. What value does a pull request from the server receive if I have disabled caching for a property?
A pull request from the server retrieves the current remote VirtualThing state. This state is independent of any pushed values that are being delivered to the server via a call to updateSubscribedProperties(), either folded or not. Think of the property values you are pushing as a queue of updates, each with a timestamp. These property changes are delivered to the server and applied to the current property value in the cache in the order their timestamps say they should be. The result of all these property changes are stored in the server cache. A property of a Thing on the server will either consult the cache for this value or it will ignore it and make a call over your edge connection to get the latest value, completely bypassing the cached value, if you configure it to disable caching.
2. If a "pull request" method does not invoke processScanRequest() then how does it get the current values from an actual physical device?
There are two ways you can get data from a physical device. You can poll it periodically (processScanRequest()) or it can deliver data when it arrives via an interrupt or an asynchronous message when it has received an update. Either way, when new data arrives, you must store it in your VirtualThing on the device the hardware is attached to as a property value. If this change is significant, you can choose to record the value change immediately by calling updateSubscribedProperties() at the time the value changes. If folded, your change is recorded but not necessarily delivered to the server right away.
Your VirtualThing on your edge device contains in it a map of property names and current values which reflects its current state. It is also recording and forwarding changes to that state when you call updateSubscribedProperties() to the server so that the current value in the cache can be in sync with the remote state. It can also log these changes to a ValueStream on the server for use in historical mashups.
When you perform a "pull request", you bypass the server cached value and make a remote request to the getProperty() function of the remote VirtualThing for each property that is configured to not use cached values. As you can imagine, doing this too often can create a performance issue for your mashup because obtaining these values is a function of how long it takes to make multiple remote service calls, each representing a round trip over the network to your remote device. If you want to trigger a request to your hardware to collect data in response to when the server requests a property, you can also override the processPropertyRead() function in your VirtualThing. This function will be called right before getProperty() and would give you an opportunity to update the property value before it gets read. Note that the default implementation of processPropertyRead() does nothing.
I hope this answers your questions. Understanding the mechanisms at work in edge connectivity, server state, remote state, property change events and how the cache works to improve mashup performance, are very important to the design decisions a developer has to make when implementing a solution with our Edge SDKs.
