G4G0-1/Semester 2/Database Systems/Week 5/Week 5 Database Systems.md

Conflict Serialisability

If a concurrent schedule organises conflicting operations in the same way as a serial schedule, the results are the same. This is called conflict serialisability, which can be tested using precedence graphs.

Directed Graph

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Precedence Graph

• Used to determine if a schedule is serialisable.
• It is serialisable if the graph has no cycles.

Example Using Serial Schedule

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• This schedule is serialisable because the graph is acyclic. ex. No directed cycle.

Example Using Non-Serial Schedule

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• This schedule is serialisable due to being acyclic.
• It is equivalent due to
  • T2 reading the written value of X from T1
  • T1 reading the value of X from the database
  • T1 is the last transaction to write Y
  • T2 is the last transaction to write X

Example Using Non-Serialisable Schedule

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• This schedule is non-serialisable since the graph is cyclic.

Example 4

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• Cyclic graph, non serialisable.

Tutorial Questions: Chapter 28

1. What is a serial schedule? A serial schedule is an order of operations where each is complete before a new operation starts. Ex. No interleaves operations.

2. Why is a serial schedule considered correct? Each operation is either successful or aborted before a new operation starts, meaning every operation is correct and the database is consistent.

3. What is a serialisable schedule? A serialisable schedule is a concurrent schedule with an acyclic precedence graph. The operation is the same as a serial schedule.

4. Why is a serialisable schedule considered correct? Since the operation is functionally the same as a serial schedule, we can consider it correct.

5. The precedence graph is serialisable since it is acyclic. No directed cycles.

6. T1.1 -> T2.2, T1.2 -> T2.1, T1.2 -> T2.2, T1.2 -> T3.1, T2.2 -> T3.1

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• The graph is serialisable since there are no directed cycles.

1. T2.1 -> T1.1, T1.1 -> T2.2, T2.2 -> T1.2

• The graph is not serialisable since there is a directed cycle

1. T1.1 -> T2.1, T1.2 -> T2.1

• The graph is serialisable since there are no directed cycles.

1. T1.1 -> T2.2, T2.1 -> T1.2, T1.2 -> T2.2

• The graph is not serialisable since there are directed cycles