We will be analyzing schedules of transactions in a system that uses some form of timestamp-based concurrency control.
In each exercise, there are three transactions (T1, T2 and T3) and three data items (X, Y and Z).
You should assume that:
The read and write timestamps of all of the data items are initially 0.
The timestamps assigned to the transactions are:
These timestamps are consistent with the fact that, in the schedules we’ll be analyzing below, T1 starts first, followed by T2, followed by T3.
In giving your answers, you should fill in a table that begins as follows:
| request | response | state changes and/or explanation |
|---|---|---|
| r1(Y) | allowed | RTS(Y) = 100 |
The third column should include any changes to the state maintained for the data item. If a request is denied or ignored, that column should also include a brief explanation of why.
You do not need to restart a transaction that is rolled back, which means that you can skip any requests by a transaction that come after the point at which it is rolled back.
In this exercise, we will assume that the DBMS is using timestamp-based concurrency control with commit bits and with only one version of each data item.
What is the purpose of maintaining commit bits?
Consider the following schedule:
s1; r1(Y); s2; w2(Y); w2(X); s3; r3(Z); r2(Z); w3(Y); r3(X); c2; c3; w1(Y); r1(X); c1
How would the system respond to each of the requested actions?
In addition to the assumptions mentioned in the preliminaries, you should assume that all of the commit bits are initially true.
Make sure to include the responses to the commit requests (c1, c2, c3), since they are relevant when a system uses commit bits.
If a transaction is made to wait, you should include the relevant request a second time when the transaction is allowed to try again.
In this exercise, we will assume that the DBMS is using timestamp-based concurrency control without commit bits and with multiple versions of each item.
What is the purpose of maintaining multiple versions?
Consider the following schedule (which is slightly different than the one from Exercise 1):
s1; r1(Y); s2; w2(Y); w2(X); s3; r3(Z); r2(Z); w3(Y); r3(X); c2; c3; r1(X); w1(Z); c1
How would the system respond to each of the requested actions?
You should make the same assumptions as before about the timestamps of the transactions and the initial timestamps of the data items.
Make sure to create a new version of a data item whenever necessary.
Because commits don’t have an effect when we’re not using
commit bits, you do not need to include the commit actions
(c1, c2, etc.) in your table.
Last updated on April 2, 2025.