๐ข๏ธ Database Transactions and Recovery Techniques | Engineer Exam Prep
Engineer ExamDatabaseTransactionACIDRecovery TechniquesCertification
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Published: 2025-07-10
Last modified: 2025-07-10
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Summary
A summary of the ACID properties (Atomicity, Consistency, Isolation, Durability) of database transactions and various recovery techniques (Log-based, Checkpoint, Shadow Paging), including a practice problem for the certification exam.
Transaction Properties: ACID
To ensure the integrity and consistency of a database, a transaction must satisfy the following four main properties (ACID).
| Property | Description |
|---|---|
| Atomicity | All operations of a transaction must be executed successfully, or none of them should be (All or Nothing). |
| Consistency | The database must always remain in a consistent state after a transaction is executed. |
| Isolation | No other transaction can interfere while one transaction is being executed. |
| Durability | The results of a successfully completed transaction must be permanently stored in the database. |
Database Recovery Techniques
These are techniques for restoring data to a previous consistent state when a database failure occurs.
Core Components of Recovery Techniques
| Component | Description |
|---|---|
| REDO | If a transaction has successfully committed but its changes have not been written to disk, REDO applies those changes again. |
| UNDO | If a transaction fails, UNDO reverts the database to its original state before the changes were made. |
Types of Recovery Techniques
| Technique | Description |
|---|---|
| Log-based Recovery | Changes to the database are sequentially recorded in a log file. In case of failure, this log is used for recovery. |
| ย ย - Deferred Update | Delays applying changes to the database until the transaction successfully completes. It only requires REDO, not UNDO. |
| ย ย - Immediate Update | Changes made during a transaction are immediately reflected in the database. Both REDO and UNDO may be necessary. |
| Checkpoint Recovery | Saves a consistent state of the database at a specific point in time (a checkpoint). If a failure occurs, recovery starts from the most recent checkpoint, reducing recovery time. |
| Shadow Paging | Creates a copy (shadow page) of the page that needs to be modified. When the transaction completes successfully, the original page is replaced with the shadow page. |
โ๏ธ Practice Problem for Engineer Exam
| Problem | Describe the core component of a database recovery technique that re-executes the changes by referring to the log when a transaction has completed successfully, but its changes have not been reflected on the disk. |
| Your Answer | |
| Correct Answer | Reveal Answer |