However, it is often difficult to test just the REST API without inadvertently involving the authentication scheme, the web application configuration, or even network connectivity. Those are a lot of hoops to jump through just to test the code within your dispatch class.

Another way you can do this is by splitting web (rest) api from the business logic. I think ideally your REST handler method does only the following two things:

• Parses incoming request
• Verifies incoming request (debatable, might also be offloaded to the underlying API)
• Calls underlying API

This way you can focus on unit testing your underlying api layer and keep the REST parts of your implementation as simple as possible.

You can use BPs to do that automatically.

BS (pool size 1) sends file to BP (1 request, pool size 1).

BP parses the file and sends X async requests to BO (pool size >1) for processing, total request count is stored in BP.

BP OnError/OnResponse methods are called once rows are being processed, maintain count here.

Once OnError/OnResponse count matches original count send a final batch request.  

a-z, A-Z only. Everything else is considered non-alphabetic, although I guess non-latin characters would be a more correct term.