Quorum-based voting can be used as a
replica control method, as well as a commit method to ensure
transaction atomicity in the presence of
network partitioning. • Before a transaction aborts, it must obtain an abort quorum VaThe total of zero or more sites that are prepared to abort or any sites waiting \ge Va. The first rule ensures that a transaction cannot be committed and aborted at the same time. The next two rules indicate the votes that a transaction has to obtain before it can terminate one way or the other.
Quorum-based voting for replica control In replicated databases, a data object has copies present at several sites. To ensure
serializability, no two transactions should be allowed to read or write a data item concurrently. In case of replicated databases, a quorum-based replica control protocol can be used to ensure that no two copies of a data item are read or written by two transactions concurrently. The quorum-based voting for replica control is due to [Gifford, 1979]. Each copy of a replicated data item is assigned a vote. Each operation then has to obtain a
read quorum (Vr) or a
write quorum (Vw) to read or write a data item, respectively. If a given data item has a total of V votes, the quorums have to obey the following rules: • Vr + Vw > V • Vw > V/2 The first rule ensures that a data item is not read and written by two transactions concurrently. Additionally, it ensures that a read quorum contains at least one site with the newest version of the data item. The second rule ensures that two write operations from two transactions cannot occur concurrently on the same data item. The two rules ensure that one-copy serializability is maintained. ==See also==