Publisher's Synopsis
Excerpt from The Five Color Concurrency Control Protocol: Non-Two-Phase Locking in General Databases, April, 1986
A transaction that intends to read (or write) a data item x, issues a read (or write) request to the transaction manager. The transaction manager is responsible for determining whether or not granting of the request may cause a violation of the correctness criterion (generally serializability). The transaction manager then takes appropriate action by granting, rejecting or delaying the request.
A trace of a transaction is a sequence of read and write requests it makes to the transaction manager. A history is written as a sequence of actions of the form Ri(x) 01' Wi(x)' where Ri(x) (or Wi(x)) means a transaction Ti issues a read (write) on data item x. Note that we are not interested in the values read or written, but in the syntactic properties of the string that lists the sequence of reads and writes on the data items.
We will assume at most one read and at most one write per transaction per data item in any trace. If a transaction reads as well as writes a particular data item, we assume the read will precede the write. Multiple reads and writes are handled in an obvious way: The first read is used to read the value of the data item and Store it in local storage, and the other reads on the same data item are processed locally. Similarly, all writes except the last one are written to local storage, and the last one appears on the log. Thus there is no loss of generality.
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