This is not about chemistry, medication or transcendent music. But it is a fussy topic. I assume the ACID (Atomicity Consistency Isolation Durability) paradigm is well known and understood from working with databases or in the scope of transactional logic. But this is about to change in large distributed applications running in the clouds (of whatever flavor) with distributed data repositories (e.g.: partitioned databases on distributed nodes). Running in clouds implies the utilization of web services predominantly. This environment leads to the CAP theorem which is about Consistency, Availability and Partition Tolerance with the restriction that only two goals (of CAP) can be achieved in a real world implementation. So it is up to application development to decide between availability and consistency and to conceal this conflict by different means, for instance by using a smart user interface logic and design as well as sophisticated update and messaging strategies in order to achieve consistency in the second step. - I apologize for oversimplifying this whole topic for the sake of a concise post. – Anyhow, this is already in place in large online-stores. To bring the third acronym into the game, an approach to make this happen is called BASE. It stands for Basically Available, Soft state, Eventually consistent). An architecture based on decoupled subsystems is an important prerequisite to achieve this.
There is nothing new about these acronyms and the paradigm change. Great articles can be found on the web (for instance Werner Vogel’s “Eventually consistent”) or other stuff especially available at ACM QUEUE. I just wanted to emphasize the need to check on this from the architectural perspective in order to answer the question: Is this application the appropriate choice to be deployed in the clouds or not? Or say it the other way around: Is BASE good enough for you? Or, is ACID essential for your system and the underlying requirements that are basically quality attributes? For many systems in the industrial domain (where data integrity and consistency is king) the answer might be not (or probably not yet). To answer these question correctly in order to adhere to such paradigms (or not), is essential because the clouds are already on the horizon.