Oncology app - the AG witness
One of the more interesting technical decisions with the whole SQL Server Availability Group set up for this application was the witness mechanism.
Before you start to even think about the witness, make sure you have a good grasp of all the concepts of an AG.
I recommend that following article:
Brent Ozar (Brent Ozar Unlimited https://www.brentozar.com/) had a free video by Kendra Little on where quorum is set up and explains witnesses, dynamic voting, and why your Windows version is so important.
For the oncology application we chose to use a File Share Witness (FSW) and combine this with Distributed File System Replication (DFSR https://msdn.microsoft.com/en-us/library/bb540025(v=vs.85).aspx) for high availability of the witness.