On purpose and by necessity: Compliance under the GDPR

The European General Data Protection Regulation (GDPR) gives primacy to purpose: Data may be collected and stored only when (i) end-users have consented, often explicitly, to the purposes for which that data is collected, and (ii) the collected data is actually necessary for achieving these purposes. This development in data protection regulations begets the question: how do we audit a computer system’s adherence to a purpose?