CAP Therom and Digital Identity

If you read this blog chances are you are familier with the CAP theorem, it basically states that any distributed system operating at scale can choose at most two of the followings three:
- Consistency
- Availability
- Partition Tolerance

There other examples of pick any "2 out of 3" in life are:

- The management rule of thumb: Good, Cheap, Fast
- Graduate student dilemma: Fun, Grades, Sleep (replace fun with your own idea of it)
- Investment advice: Low Risk, High Return, Legality - if you pick low risk and high return chances are you are compromising legality :-)

They way I look at all these "rules" is that the space of each of these domains offer only two degrees are freedom and once you choose two points (that effectively determine or fix your degree of freedom, the third point will be chosen for you)

For example in "Good, Cheap, Fast", your degrees of freedom are basically time and money, once you choose how much time and money you want to spend all three qualities are determined. So now, if you choose time and money not directly, but indirectly via the choice of say good and fast, you automatically also chosen "not cheap".

Interestingly digital identity offers the same 2 out of 3 dynamics among the three main attributes of

- Quality of Identity
- Usability
- Cost

"Quality of Identity" is a measure of how uniquely a set of data represents a real world person and how strongly an IDP stands behind such assertion (for example whether IDP guarantees up to a certain amount damages resulting from inaccurate data), usability is how easy it is for IDP to provision such identities.

It is clear that if an IDP chooses to provide high quality identity and also wants to makes its provisioning easy to use (or scalable for that matter), it has to spend a lot of money.

In practice though, IDPs segment the user base and only provide high quality identity for users to whom maximum credit are extended (e.g. users who can sell the most on eBay).