Quiz questions and answers in distributed database MCQ 08

Distributed database quiz questions with answers, MCQ in distributed database, solved exam questions in distributed database, distributed transaction processing, two phase commit protocol

Multiple Choice Questions with Answers in Distributed Database

Let us suppose that there are a transaction coordinator T_c and two participants P₁ and P₂. They are working on a transaction T₁.

[Refer here to understand more and to answer – 2PC protocol]

 

1. Which of the following will happen after the following sequence of events is completed as per Two Phase Commit (2PC) protocol?

T_C sends <Prepare T₁> to P₁ and P₂.

P₁ sends <Ready T₁> to T_C.

P₂ sends <Ready T₁> to T_C.

a) T_C sends <Commit T₁> to P₁.

b) T_C sends <Commit T₁> to P₂.

c) T_C sends <Abort T₁> to P₁.

d) T_C sends <Abort T₁> to P₂.

Answer: (a) TC sends <Commit T1> to P1 and (b) TC sends <Commit T1> to P2 It is given that the transaction coordinator sent a prepare message to all the participating sites and in-turn the participants responded with ready message. If all the participating sites are responded with ready message (these responses only known to coordinator at this moment), then the coordinator sends the commit message to all the participants.

 

2. Which of the following will happen after the following sequence of events is completed as per 2PC?

T_C sends <Prepare T₁> to P₁ and P₂.

P₁ sends <Ready T₁> to T_C.

P₂ sends <Ready T₁> to T_C.

P₂ crashed after sending <Ready T₁> to T_C.

P₂ recovered immediately after crash.

a) T_C sends <Commit T₁> to P₁ and P₂

b) T_C sends <Abort T₁> to P₁ and P₂.

c) T_C sends <Commit T₁> only to participant P₁.

d) T_C sends <Abort T₁> only to participant P₂.

Answer: (a) TC sends <Commit T1> to P1 and P2 It is given that the transaction coordinator sent a prepare message to all the participating sites and in-turn all the participants responded with ready message. After that P2 crashed and recovered. We can understand that the crash and recovery happened before the coordinator responds. Whether the participating sites crashed or not, the coordinator will respond based on the messages it received from the participating sites. Hence, in this case, Tc will send the commit message to all the participants.

 

3. After the event “T_C sends <Prepare T₁> to P₁ and P₂”, P₁ decides to commit transaction T₁. Which of the following is true about P₁ with respect to 2PC protocol?

a) P₁ sends <Ready T₁> to T_C and then writes <Ready T₁> into its local log

b) P₁ writes <Ready T₁> into its local log and then sends <Ready T₁> to T_C

c) P₁ sends <Ready T₁> to T_C and then writes <Commit T₁> into its local log

d) P₁ writes <Commit T₁> into its local log and then sends <Ready T₁> to T_C

Answer: (b) P1 writes <Ready T1> into its local log and then sends <Ready T1> to TC In 2PC, if a participating site decided to commit or abort, it has to first register the decision into local log file, flush the log file into disk and then reply with the appropriate message to the coordinator site.

 

4. Which of the following will happen if P₂ is not ready to commit T₁ after the following events?

T_C sends <Prepare T₁> to P₁ and P₂.

P₁ sends <Ready T₁> to T_C.

a) P₂ sends <Commit T₁> to T_c.

b) P₂ writes <Abort T₁> to its local log and sends <Abort T₁> to T_c.

c) P₂ writes <Abort T₁> to its local log and sends <Abort T₁> to T_c and P₁.

d) P₂ writes <No T₁> to its local log and sends <Abort T₁> to T_C.

Answer: (d) P2 writes <No T1> to its local log and sends <Abort T1> to TC According to 2PC, if a participating site has decided not to commit, it has to write <No T> message into its log and send <Abort T> to the transaction coordinators.

 

5. Which of the following will happen after the following sequence of events is completed as per Two Phase Commit (2PC) protocol?

T_C sends <Prepare T₁> to P₁ and P₂.

P₁ sends <Ready T₁> to T_C.

P₂ sends <Abort T₁> to T_C.

a) T_C sends <Commit T₁> to P₁ and P₂

b) T_C sends <Abort T₁> to P₁ and P₂.

c) T_C sends <Commit T₁>, only to participant P₁.

d) T_C sends <Abort T₁>, only to participant P₂.

Answer: (b) TC sends <Abort T1> to P1 and P2 It is given that the transaction coordinator sent a prepare message to all the participating sites. One of the sites is not ready to commit the transaction. In this case, the coordinator has to abort the transaction and inform all the participating sites to abort T1.

 

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