Distributed
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transaction processing, two phase commit protocol
Multiple Choice Questions with Answers in Distributed Database
Let us suppose that there are a
transaction coordinator Tc 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?
TC sends <Prepare T1> to P1
and P2.
P1 sends <Ready T1> to TC.
P2 sends <Ready T1> to TC.
a) TC sends <Commit T1>
to P1.
b) TC sends <Commit T1>
to P2.
c) TC sends <Abort T1>
to P1.
d) TC 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.
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2. Which of the following will happen
after the following sequence of events is completed as per 2PC?
TC sends <Prepare T1> to P1
and P2.
P1 sends <Ready T1> to TC.
P2 sends <Ready T1> to TC.
P2 crashed after sending <Ready T1>
to TC.
P2 recovered immediately after crash.
a) TC sends <Commit T1>
to P1 and P2
b) TC sends <Abort T1>
to P1 and P2.
c) TC sends <Commit T1>
only to participant P1.
d) TC 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.
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3. After the event “TC
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 TC and then writes <Ready T1> into its local log
b) P1 writes <Ready T1>
into its local log and then sends <Ready T1> to TC
c) P1 sends <Ready T1>
to TC and then writes <Commit T1> into its local log
d) P1 writes <Commit T1>
into its local log and then sends <Ready T1> to TC
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.
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4. Which of the following will happen if
P2 is not ready to commit T1 after the following events?
TC sends <Prepare T1> to P1
and P2.
P1 sends <Ready T1> to TC.
a) P2 sends <Commit T1>
to Tc.
b) P2 writes <Abort T1>
to its local log and sends <Abort T1> to Tc.
c) P2 writes <Abort T1>
to its local log and sends <Abort T1> to Tc and P1.
d) P2 writes <No T1>
to its local log and sends <Abort T1> to TC.
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.
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5. Which of the following will happen
after the following sequence of events is completed as per Two Phase Commit
(2PC) protocol?
TC sends <Prepare T1> to P1
and P2.
P1 sends <Ready T1> to TC.
P2 sends <Abort T1> to TC.
a) TC sends <Commit T1>
to P1 and P2
b) TC sends <Abort T1>
to P1 and P2.
c) TC sends <Commit T1>,
only to participant P1.
d) TC 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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