0: BEGIN TRANSACTION 1: W = 1 2: X = W + 1 3: Y = X * 2 4: COMMIT TRANSACTION

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Dale Marshall
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1 Transactions

2 1. a) Show how atomicity is maintained using a write-ahead log if the system crashes when executing statement 3. Main memory is small, and can only hold 2 variables at a time. Initially, all variables are zero. 0: BEGIN TRANSACTION 1: W = 1 2: X = W + 1 3: Y = X * 2 4: COMMIT TRANSACTION

3 Transactions Atomic: Appear to be indivisible Consistent: Invariants hold before and after Isolated: Concurrent transactions do not interfere Durable: Once committed, the changes are permanent

4 Write-Ahead Logs Modify files in place. Before making a change, log it: What block is being changed What are the old and new values Once logged, the change is made

5 1. a) Show how atomicity is maintained using a write-ahead log if the system crashes when executing statement 3. 0: BEGIN TRANSACTION 1: W = 1 2: X = W + 1 3: Y = X * 2 4: COMMIT TRANSACTION

6 1. a) Before statement 3 Write-ahead log contains: Begin W = 0/1 (from statement 1) X = 0/2 (from statement 2) To execute 3, W must be swapped out Value of W on disk is 1 (not valid) System crashes

7 1. a) Bringing system back Examining the log: Begin W = 0/1 (from statement 1) X = 0/2 (from statement 2) Examining disk: W = 1 X = 0 Y = 0 The log can be rolled back by resetting W = 0

8 1. b) Show how durability is maintained using a write-ahead log if the system crashes after the commit but before pages 1 and 2 are written to disk.

9 1. b) Bringing system back Write-ahead log contains: Begin W = 0/1 (from statement 1) X = 0/2 (from statement 2) Y = 0/4 (from statement 3) End Disk: W = 1 (swapped out) X = 0 Y = 0

10 1. b) Bringing system back System examines each value on disk and in the log Sets the disk values to the final values as recorded in the log Hence, the transaction is durable

11 2. Define the operation R i (x) as transaction i reads variable x. Likewise, define W i (x) as transaction i writes variable x. Which of the following execution histories is serializable and why:

12 Serializability A schedule or history is serializable if the result is identical to running the transactions in some sequential order Systems want to execute transactions concurrently, where possible We need to be able to find serializable orderings of operations

13 Conflicting Operations Read - Write Write - Read Write - Write Read - Read: Order does not matter

14 2. (a) Transaction 1 Transaction 2 R1(A) R2(A) R1(B) R2(B) W1(A) W2(A)

15 2. (a) Transaction 1 Transaction 2 R1(A) R2(A) R1(B) R2(B) W1(A) W2(A) Not Serializable

16 2. (b) Transaction 1 Transaction 2 R1(B) R2(B) R1(A) W1(A) R2(A) W2(A)

17 2. (b) Transaction 1 Transaction 2 R1(B) R2(B) R1(A) W1(A) R2(A) W2(A) Serializable: 1, 2

18 2. (c) Transaction 1 Transaction 2 R1(B) R2(B) R1(A) R2(A) W2(A) W1(A)

19 2. (c) Transaction 1 Transaction 2 R1(B) R2(B) R1(A) R2(A) W2(A) W1(A) Not Serializable

20 2. (d) Transaction 1 Transaction 2 R2(A) R1(B) R2(B) W2(A) R1(A) W1(A)

21 2. (d) Transaction 1 Transaction 2 R2(A) R1(B) R2(B) W2(A) R1(A) W1(A) Serializable: 2, 1

22 2. (e) Transaction 1 Transaction 2 R1(A) R1(B) R2(B) W1(A) R2(A) W2(A)

23 2. (e) Transaction 1 Transaction 2 R1(A) R1(B) R2(B) W1(A) R2(A) W2(A) Serializable: 1, 2

24 3. Suppose you have three transactions, α, β and γ, that you wish to perform. The time for each of these transactions to execute is 10µs if there is no resource conflict. They each reserve resources according to the following diagram. All resources are released immediately upon terminating the transaction, whether the transaction is completed or aborted.

25 3. Resource Acquisition

26 3. a) If all three transactions start at time 0, when will each of the transactions terminate if we use wait-die deadlock prevention? It takes 1µs to restart a killed transaction. If events happen simultaneously, α is first, then β, then γ

27 Concurrency Control Optimistic Execute without controls Perform some verification Re-execute if there was a problem Pessimistic Control operations to guarantee correctness

28 Ensuring Serializability Pessimistic: Use locks 2 phase locking guarantees serializability Optimistic: Use validation Check that the order of operations is serializable before committing

29 Dealing with Deadlock Pessimistic: Deadlock prevention Wait-die Wound-wait Optimistic: Deadlock detection Check for cycles in the wait-for graph

30 Wait-Die Deadlock Prevention If a resource is currently being used by an older transaction, restart (die) If it is being used by a younger transaction, wait

31 3. a) Wait-Die

32 3. b) When will each of the transactions terminate if we use wound-wait deadlock prevention? It takes 1µs to restart a killed transaction. If events happen simultaneously, A is first, then B, then C

33 Wound-Wait Prevention If a resource is being used by an older transaction, wait If it is being used by a younger transaction, restart it (wound it)

34 3. b) Wound-Wait

35 3. c) When will each of the transactions terminate if we use deadlock detection? Each transaction informs the detector after requesting, receiving or releasing a resource. The detector will determine if a deadlock has occurred after 2 µs. It selects the youngest transaction in the deadlock cycle for termination.

36 Deadlock Detection Allow all transactions to wait for resources If a cycle is detected in the wait-for graph, kill one of the transactions

37 3. c) Deadlock Detection

38 4. In the two-phase commit protocol used to commit transactions, after a participant has informed the coordinator that it it prepared to commit, it is susceptible to a period of uncertainty while it is waiting for the final decision (either commit or abort) from the coordinator.

39 Two-Phase Commit 1. Coordinator: VOTE_REQUEST 2. Participant: VOTE_COMMIT/ABORT 3. Coordinator: GLOBAL_COMMIT (all VOTE_COMMIT) GLOBAL_ABORT (any VOTE_ABORT) 1. Participant commits or aborts

40 Two-Phase Commit

41 4. a) What can prolong this period of uncertainty?

42 2PC Uncertainty Period Coordinator dies Participant dies Network failure Network delay

43 4. b) Can the participant time out and unilaterally abort the transaction? Why or why not?

44 2PC: Abort Transaction No: Coordinator sends GLOBAL_COMMIT One participant does not receive it Others commit Abort would corrupt global state!

45 4. c) What would you recommend doing in order to shorten the delay? Justify your answer.

46 2PC: Reducing Uncertainty Decision Request Message Attempts to determine what happened Cannot always recover Use 3 Phase Commit: Non-blocking, less uncertainty Still has some: requires that a majority of processes can be contacted

) Intel)(TX)memory):) Transac'onal) Synchroniza'on) Extensions)(TSX))) Transac'ons)

) Intel)(TX)memory):) Transac'onal) Synchroniza'on) Extensions)(TSX))) Transac'ons) Transactions - Definition A transaction is a sequence of data operations with the following properties: * A Atomic All