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Distributed Database 2PC Protocol: Communication Line Failures and Blocked States Analysis

This document explores the implications of communication failures in a distributed database scenario with three sites: A, B, and C. Using the Two-Phase Commit (2PC) protocol, it examines the states of each site when communication is interrupted. It details potential blocked states when lines between sites fail, particularly focusing on scenarios where instructions for committing transactions are not relayed due to broken connections. The evaluation includes questions regarding the conditions under which each site remains blocked and the possible outcomes once communication is restored.

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Distributed Database 2PC Protocol: Communication Line Failures and Blocked States Analysis

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  1. Homework 5 2PL

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

  3. 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?

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