Schedules

Question 1

Serial Schedule

Answer 1

• Execute correctly
• Maintain consistency of database
• Inefficient in multi-user environments (transactions will wait for other transactions)

Question 2

Concurrent Schedules

Answer 2

• may not execute correctly
• may not guarantee consistency
• efficient in multi-user environments

Question 3

Basic Operations of Transactions

Answer 3

begin
read(X)
write(X)
commit

Question 4

read(X)

Answer 4

Reads a database item X into program variable (also named X, for simplicity):

• Find address of disk block/page containing item X
• Copy disk block into a buffer in main memory
• Copy item X from buffer to program variable X

Question 5

write(X)

Answer 5

Writes value of program variable X into database item named X:

• find address of page that contains item X
• Copy disk block into buffer in main memory
• Copy item X from program variable X into its correct location
• Store the updated block from buffer back to disk either immediately or some time later

Question 6

Serializable Schedules

Answer 6

A schedule S is serializable if there is a serial schedule S’ that has the same effect as S on every initial database state.
Essentially, it is serializable if it is a concurrent schedule that we can change to a serial one without changing the outcome.

Question 7

Serialisable Schedule ACID Properties

Answer 7

Serializable schedules guarantee correctness and consistency, but does not satisfy isolation (can be fixed).

Question 8

Conflict-equivalent Schedule

Answer 8

Conflict-equivalent schedules are found when you can get one transaction from another by swapping pairs of non-conflicting operations.

Question 9

Conflict-serialisable Schedule

Answer 9

A schedule in which is performed in a serial manner would cause conflicts between read and writes of different transactions. It can also be defined as a schedule which is conflict-equivalent to a serial schedule.

Question 10

Precedence Graph

Answer 10

A directed graph in which the nodes represent the transactions, and is drawn to show the conflicts in the transactions, and used to represent if a group of transactions are conflict-serializable.