The PAC Principle

The CAP Theorem is well understood in the world of data management: a system can only be two of Consistent, Available, and Partition Tolerant. Similarly, in project management you only get to choose two of the three Good, Fast, or Cheap.

As it turns out, there is a similar trade-off in the enterprise architecture world. An enterprise architecture specification can only be two of Prescriptive, Autonomous, and Coherent. That is to say: If you want to create a highly prescriptive standard, you can either make it Coherent (across all lines of business and systems) or Autonomous (giving freedom to the local architects) - but not both. Similarly, you can make an architectural standard that is highly Coherent and Autonomous, but only if you make it sufficiently vague.

The PAC principle is an important one for enterprise architects to understand. Trying to create a standard that if completely prescriptive, gives autonomy to the individual teams, and still maintains the coherency of the portfolio is an impossible task: the amount of organizational concurrence that needs to occur is practically impossible in large organizations. Even if such a standard could be established, it would be brittle in the face of changes to the product needs, staffing of the organization, or technology changes.

Ignoring the PAC principle leads to the failure of enterprise architecture programs. In attempting to satisfy all the criteria, they either stiffle creativity (and productivity), or create something so vague as to be useless.

There is a way to use the PAC principle to our advantage. By thinking of the architectural models as having both a Logical and Physical component, we can create a logical specification (which, by nature, will be less prescriptive and therefore AC compliant) while leaving the physical specification to the local teams (AP compliant). This approach can even be expanded by including a Conceptual model, which is useful when creating standards for interoperating across business units or other large domains.

Going back to the CAP Theorem, it is the consequence of data management living with the constraints imposed by current network technologies. Enterprise architecture does not exist in a vacuum; it is part of and subject to organizational forces that shape what is practical. The PAC principle is, like the CAP theorem, a result of these organization realities.