DatabasesLesson 1 of 3 in the course Transactions and concurrency

Database Concurrency Control: Schedules, Mechanisms, and Recovery

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

Concurrency control helps concurrent components interact without producing inconsistent or incorrect results. This lesson introduces its purpose and the database transaction properties that support reliable work.

Computer systems contain components that may run at the same time and communicate by messages or shared data. Each component may be correct when operating alone, yet one component's concurrent actions can violate another's assumptions. Concurrency control provides rules and methods that preserve consistency across interacting components.

All flashcards

Dirty read
Reading a value written by a transaction that has not yet committed.
Incorrect summary
An aggregate combines values from different points in time and may match no valid serial ordering.
Serial schedule
A schedule in which each transaction finishes before the next one starts.
Timestamp ordering
A concurrency-control method that checks or controls data access against transaction timestamps.
Strictness
A schedule property that can make recovery after failure more efficient.

Quiz questions

  1. Transaction T1 reads an item, transaction T2 reads the same item, and then T1's update is overwritten by T2's update based on its earlier read. What anomaly occurred?Answer: Lost update
  2. What does it mean for a schedule to be serializable?Answer: Its result is equivalent to a serial schedule containing the same transactions.
  3. A committed transaction read data written by another transaction that later aborted. What does this indicate about the schedule?Answer: It is not recoverable.
  4. How does optimistic concurrency control typically handle a transaction that fails a check at commit?Answer: It aborts the transaction, which can then be restarted.
  5. In conflict-serializability analysis, what does a cycle in the precedence graph indicate?Answer: The schedule is not conflict-serializable.

Based on the Wikipedia article Concurrency control, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.

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