We will continue our discussion of parallel and distributed query processing. You should read:

• C. Mohan, Bruce Lindsay, and R. Obermarck. Transaction Management in the R* Distributed Database Management Systems. ACM Transactions On Database Systems 11(4), 1986. In Red Book. [PDF]

This paper discusses distributed transactions, addressing the problem of providing ACID-style semantics in a shared nothing environment.

As you read the paper, consider the following questions:

1. How does the two phase commit (2PC) protocol work? What problem does it solve? What are the costs of using it?
2. What is the significance of the Presumed Abort/Presumed Commit variants of 2PC? How do they reduce the overhead of 2PC? When should you choose one over the other?