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# CSE 414 Homework 5 Transactions solved

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## Assignment Details Part 1: Schedules and Anomalies (10 points)

Consider a database with objects X, Y, and Z and assume that there are two transactions T1
and T2 that attempt the following operations.
T1: R(X), R(Y), W(X)
T2: R(X), R(Y), W(Y), R(X), R(Y), W(X), R(Z), W(Z)

A) Write an example schedule that interleaves operations between T1 and T2, that is NOT
conflict serializable.
B) If T1 is instead just “R(X)”, this corresponds to T1 just being a single query like
SELECT * FROM Flights WHERE id=1024;
Should the database treat a single SQL statement like this as a transaction? Why or why not?

## Part 2: Conflict Serializability (20 points)

Consider the following three transactions and schedule (time goes from top to bottom). Is this
schedule conflict-serializable? Show why or why not.
T1 T2 T3
R(A)
W(A)
R(A)
W(A)
R(A)
R(B)
R(B)
W(B)
W(B)
R(B)
commit
commit
commit

## Part 3: Two-Phase Locking (20 points)

A) Now modify the above schedule by adding locks, which may block some transactions from
doing their operations until the lock is released. You’ll need to rewrite the above schedule in a
table form. (The lecture slides show how to represent blocking in your schedules.)

Use two-phase locking (doesn’t need to be “strict”) in your modified schedule to ensure a
conflict-serializable schedule for the transactions above.
Use the notation L(A) to indicate that the transaction acquires the lock on element A and U(A) to
indicate that the transaction releases its lock on.
B) If 2PL ensures conflict-serializability, why do we need strict 2PL? Explain briefly.

## Submission Instructions

The files you will need to submit to Gradescope
● Part1.pdf
● Part2.pdf
● Part3.pdf
Points may be deducted for incorrect file names.