It states that, if a more recent transaction has already written the value of an object, then a less recent transaction does not need to perform its write since the more recent one will eventually overwrite it.
The Thomas write rule is applied in situations where a predefined logical order is assigned to transactions when they start. For example, a transaction might be assigned a monotonically increasing timestamp when it is created. The rule prevents changes in the order in which the transactions are executed from creating different outputs: The outputs will always be consistent with the predefined logical order.
For example, consider a database with 3 variables (A, B, C), and two atomic operations C := A (T1), and C := B (T2). Each transaction involves a read (A or B), and a write (C). The only conflict between these transactions is the write on C. The following is one possible schedule for the operations of these transactions:
If (when the transactions are created) T1 is assigned a timestamp that precedes T2 (i.e., according to the logical order, T1 comes first), then only T2's write should be visible. If, however, T1's write is executed after T2's write, then we need a way to detect this and discard the write.
One practical approach to this is to label each value with a write timestamp (WTS) that indicates the timestamp of the last transaction to modify the value. Enforcing the Thomas write rule only requires checking to see if the write timestamp of the object is greater than the time stamp of the transaction performing a write. If so, the write is discarded
In the example above, if we call TS(T) the timestamp of transaction T, and WTS(O) the write timestamp of object O, then T2's write sets WTS(C) to TS(T2). When T1 tries to write C, it sees that TS(T1) < WTS(C), and discards the write. If a third transaction T3 (with TS(T3) > TS(T2)) were to then write to C, it would get TS(T3) > WTS(C), and the write would be allowed.
Referencesedit
Robert H. Thomas (1979). "A majority consensus approach to concurrency control for multiple copy databases". ACM Transactions on Database Systems. 4 (2): 180–209. doi:10.1145/320071.320076.
thomas, write, rule, computer, science, particularly, field, databases, rule, timestamp, based, concurrency, control, summarized, ignore, outdated, writes, states, that, more, recent, transaction, already, written, value, object, then, less, recent, transactio. In computer science particularly the field of databases the Thomas write rule is a rule in timestamp based concurrency control It can be summarized as ignore outdated writes It states that if a more recent transaction has already written the value of an object then a less recent transaction does not need to perform its write since the more recent one will eventually overwrite it The Thomas write rule is applied in situations where a predefined logical order is assigned to transactions when they start For example a transaction might be assigned a monotonically increasing timestamp when it is created The rule prevents changes in the order in which the transactions are executed from creating different outputs The outputs will always be consistent with the predefined logical order For example consider a database with 3 variables A B C and two atomic operations C A T1 and C B T2 Each transaction involves a read A or B and a write C The only conflict between these transactions is the write on C The following is one possible schedule for the operations of these transactions T1T2Read A Read B Write C Write C CommitCommit T1T2Read A Read B Write C CommitCommit displaystyle begin bmatrix T 1 amp T 2 amp Read A Read B amp amp Write C Write C amp Commit amp amp Commit end bmatrix Longleftrightarrow begin bmatrix T 1 amp T 2 amp Read A Read B amp amp Write C amp Commit amp amp Commit end bmatrix If when the transactions are created T1 is assigned a timestamp that precedes T2 i e according to the logical order T1 comes first then only T2 s write should be visible If however T1 s write is executed after T2 s write then we need a way to detect this and discard the write One practical approach to this is to label each value with a write timestamp WTS that indicates the timestamp of the last transaction to modify the value Enforcing the Thomas write rule only requires checking to see if the write timestamp of the object is greater than the time stamp of the transaction performing a write If so the write is discardedIn the example above if we call TS T the timestamp of transaction T and WTS O the write timestamp of object O then T2 s write sets WTS C to TS T2 When T1 tries to write C it sees that TS T1 lt WTS C and discards the write If a third transaction T3 with TS T3 gt TS T2 were to then write to C it would get TS T3 gt WTS C and the write would be allowed References editRobert H Thomas 1979 A majority consensus approach to concurrency control for multiple copy databases ACM Transactions on Database Systems 4 2 180 209 doi 10 1145 320071 320076 c nbsp This computer science article is a stub You can help Wikipedia by expanding it vte Retrieved from https en wikipedia org w index php title Thomas write rule amp oldid 1145777367, wikipedia, wiki, book, books, library,
