CAP Revisited

PACELC

I just found out about an excellent blog post on the CAP theorem by Daniel Abadi. I agree with his assertions that the CAP theorem is somewhat confusion and like the concept of PACELC. In my opinion, the main problem is an unclear understanding of what C, A, and P actually mean.

My 3 cents on consistency, availability, and partition-tolerance:

Consistency

Imho, it does not make much sense to talk about consistency without a specific application or system architecture in mind. For example, in the case of a relational database, consistency usually equals integrity constraints & atomic transactions; in the case of Dynamo/Cassandra it is specified by the N-R-W configuration of the quorum protocol (or hard-wired consistency level); et cetera.

Availability

In a similar sense, when defining availability, there should be a time constraint (e.g. if the ping does not return after 2 secs, the app is down). Everything in between [0, 2] is the “availability spectrum”, aka latency. Apparently, it is again application-specific what the upper bound should be and when we talk about latency as opposed to availability.

Partition-tolerance

I agree that from a client perspective there is no difference between unavailability due to server failure and unavailability due to network partition. However, different repair mechanisms will be used in either case so it might make sense to differentiate when looking from a system perspective (?)

What do you think?

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