Homework 5

2PL

Answer Questions 1&2 [True/False+ Explanation]

In a distributed DB, there are three sites, A, B, and C, with communication lines between A & B and B & C (but no direct line between A &C).

The three sites co-operate in a distributed transaction using the 2PC protocol; A is the coordinator.

1. If communication is lost between A and B, A could wait indefinitely in a blocked state till communication is restored?

2. Say that B receives the commit instruction from A but cannot forward such instruction to C since the line between B & C is down. Then B must wait in a blocked state till the line is restored and the instruction to commit can be sent to C.

… Questions 3, 4 and 5: next slide.

Questions 3,4 and 5 [True/False+ Explanation]

Now say that B receives the commit instruction from A, but immediately after that both the line between A & B and that between B & C fail. Also say that the line between B & C is restored well before the line between A & B. Then:

3. B remains in a blocked state until the line with A is restored, and at that point, it asks A for instructions?

4. C remains in a blocked state till its line with B is restored, and then it ask B for its decision and replicates it?

5. C remains in a blocked state until both lines are restored, and and at that point, it asks A for instructions?