2.15.4 Timestamping

Operates on tentative versions of data

• Each Transaction receives global unique timestamp on initiation
• Every object, x, in the system or database carries the maximum (ie youngest) timestamp of last transaction to read RTM(x)^2.3 and maximum of last transaction to write WTM(x)^2.4
• If transaction requests operation that conflicts with younger transaction, older transaction restarted with new timestamp.
• Transactions committed in order of timestamps, so a transaction may have to wait for earlier transaction to commit or abort before committing.
• Since tentative version is only written when transaction is committed, read operations may have to wait until the last transaction to write has committed.

An operation in transaction T_i with start time TS_i is valid if:

• The operation is a read operation and the object was last written by an older transaction ie TS_i > WTM(x). If read permissible, RTM(x) = MAX(TS_i, RTM(x))
• The operation is a write operation and the object was last read and written by older transactions ie TS_i > RTM(x) and TS_i > WTM(x). If permissible, WTM(x) = TS_i