Definition from Gregor (New ACID)
“In the world of distributed systems, idempotency translates to the fact that an operation can be invoked repeatedly without changing the result.”
http://www.eaipatterns.com/ramblings/68_acid.html
http://codebetter.com/gregyoung/2010/08/12/idempotency-vs-distibuted-transactions/
Idempotency Patterns
http://blog.jonathanoliver.com/idempotency-patterns/
