Commutative data types and their operations is a powerful idea.
Somewhere this fortnight I read that I should read "CRDTs: Consistency without concurrency control" <http://hal.inria.fr/inria-00397981/en/>. The basic idea is to have a data structure where for the same set of operations applied in any order will lead to the same state. If I have a number of replicants of this data structure then as long as all the operations are applied to each replicant -- in any order -- then the replicants will have the same end state. With concurrent replicants then all replicants will eventually have the same state. Commutative data types and their operations is a powerful idea.