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Database Management Systems/Managing Database

Notes Motivation for Concurrent Execution

The schedule, involving two transactions shown in the following figure represents an interleaved
execution of the two transactions.

T1 T2
R(A)
W(A)
R(B)
W(B)
R(C)
W(C)

First, while one transaction is waiting for a page to be read in from disk, the CPU can process
another transaction This is because I/O activity can be done in parallel with CPU activity in a
computer. Overlapping I/O and CPU activity reduces the amount of time and increases system
throughput which is the average number of transactions completed in a given time.
Second, interleaved execution of a short transaction with a long transaction usually allows the
short transaction to complete quickly. In serial execution, a short transaction could get stuck
behind a long transaction, leading to unpredictable delays in response, time, or average time
taken to complete a transaction.

9.5 Serializability

A serializable schedule over a set S of committed transactions is a schedule whose effect on any
consistent database is guaranteed to be identical to that of some complete serial schedule over S.
That is, even-though the actions of transactions are interleaved, the result of executing transactions
serially in different orders may produce different results.

9.6 Recoverability

Unfortunately, the timestamp protocol presented above permits schedules that are not
recoverable, as illustrated by the schedule in Table below. If TS(T ) = 1 and TS(T ) = 2, this
1 2
schedule is permitted by the timestamp protocol (with or without the Thomas Write Rule). The
timestamp protocol can be modified to disallow such schedules by buffering all write actions
until the transaction commits. In the example, when T wants to write A, WTS(A) is updated to
1
reflect this action, but the change to A is not carried out immediately; instead, it is recorded in a
private workspace, or buffer. When T wants to read A subsequently, its timestamp is compared
2
with WTS(A), and the read is seen to be permissible. However, T is blocked until T completes.
2 1
If T commits, its change to A is copied from the buffer; otherwise, the changes in the buffer are
1
discarded. T is then allowed to read A.
2
T1 T2
W(A)
R(A)
W(B)
Commit

This blocking of T is similar to the effect of T obtaining an exclusive lock on A! Nonetheless,
2 1
even with this modification the timestamp protocol permits some schedules that are not permitted
by 2PL; the two protocols are not quite the same.

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