Assignment: Log-based recovery, distributed concurrency control
1. Recovery: Assuming a redo-only recovery, what should the database do to recover if the following is the contents of the log upon recovery:
• T1 write X: was 3, now 6
• T2 write Y: was 2, now 0
• T1 commit
• T3 write X: was 6, now 4
• T2 commit
2. Recovery: Assuming a undo-redo recovery, what should the database do to recover if the following is the contents of the log upon recovery:
• T1 write X: was 3, now 6
• T2 write Y: was 2, now 0
• T1 commit
• Checkpoint
• T3 write X: was 6, now 4
• T2 write Z: was 1, now 5
• T3 commit
3. Suppose we have a MongoDB database with 4 replicas:
• Replica 1 is the primary, the others are secondaries
• Replica 1 writes document A and replicas 2 and 3 get the updated document
• Replica 1 writes document B, but crashes before updating any of the secondaries
• Replica 2 detects that replica 1 has crashed
• How does the system recover?
• What happens to documents A and B?
4. Suppose we have a MongoDB database with 4 replicas:
• Replica 1 is the primary, the others are secondaries
• Replica 1 writes document A and replica 2 gets the updated document
• A partition occurs: replicas 1 and 2 are in one partition, while replicas 3 and 4 are in the other partition (the 2 partitions now cannot communicate)
• Replicas 1 and 3 detect the partition
• What does the system do?
• Discuss what you think should be done with document A, and why.
5. Suppose we had a distributed database which uses locking where a transaction must lock all copies to do a write, 1 copy to do a read. If a transaction T1 cannot get a lock because of a conflicting lock by another transaction T2, then T1 is put on a queue for the lock. What happens in the following schedule (this is like the 2-phase locking questions). Each operation is preceded by the transaction issuing the request. Each row should be treated as happening concurrently on the 3 servers during one time period, so (unrealistically), the execution is synchronized.
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Server 1
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Server 2
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Server 3
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T1:Read(A)
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T2:Read(A)
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T3:Read(B)
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T1:Write(A)
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T2:Write(B)
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T2:Write(B)
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T2:Write(B)
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T1:Write(A)
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T3:Read(A)
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T1:Write(A)
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T3:Commit
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T2:Commit
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T1:Commit
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Format your assignment according to the following formatting requirements:
1. The answer should be typed, double spaced, using Times New Roman font (size 12), with one-inch margins on all sides.
2. The response also includes a cover page containing the title of the assignment, the student's name, the course title, and the date. The cover page is not included in the required page length.
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