ADVANCED RECOVRY TECHNIQUES
Advanced recovery techniques in Database Management Systems (DBMS) are essential for ensuring data integrity, consistency, and availability in case of failures.
1. Write-Ahead Logging (WAL)
Write-Ahead Logging ensures that all modifications to the database are logged before they are actually applied. This technique provides the foundation for many recovery methods.
- Undo Logging: Logs the old value of a data item before it is updated, allowing the system to reverse (undo) changes.
- Redo Logging: Logs the new value of a data item after it is updated, allowing the system to reapply (redo) changes if a transaction is incomplete.
2. Checkpointing
Checkpointing periodically saves the current state of the database to a stable storage. This helps to reduce the amount of work needed during recovery.
- Consistent Checkpointing: Ensures that the database state at the checkpoint is consistent.
- Fuzzy Checkpointing: Allows the database to continue processing transactions while taking a checkpoint.
3. ARIES (Algorithm for Recovery and Isolation Exploiting Semantics)
ARIES is a comprehensive recovery method that combines several techniques to provide efficient and reliable recovery.
- Write-Ahead Logging: Ensures all changes are logged before they are applied.
- Repeating History: During recovery, ARIES repeats the history of actions to reconstruct the state at the time of failure.
- Logical Undo Logging: Uses compensation log records (CLRs) to handle undo operations efficiently.
- Dirty Page Table (DPT): Tracks pages that have been modified but not yet written to disk.
- Transaction Table (TT): Tracks the status of all active transactions.
4. Shadow Paging
Shadow paging involves maintaining two versions of the database pages: a current version and a shadow version. Modifications are made to the current version, and the shadow version is used for recovery.
- Advantages: Immediate recovery, no need for log analysis.
- Disadvantages: High overhead due to page copying, potential fragmentation.
5. Replication
Replication involves maintaining copies of the database on multiple servers to ensure high availability and fault tolerance.
- Synchronous Replication: Ensures that all copies are updated simultaneously.
- Asynchronous Replication: Updates copies with a delay, which can improve performance but may lead to temporary inconsistencies.
6. Log-Based Recovery
Log-based recovery methods use the transaction log to recover from failures.
- Undo/Redo Logging: Combines undo and redo logging to handle both types of recovery.
- Physiological Logging: Logs changes at a lower level (e.g., record level) to reduce log size and improve performance.
7. Transaction Rollback and Rollforward
These techniques involve rolling back incomplete transactions and rolling forward completed transactions during recovery.
- Rollback: Reverts the changes made by a transaction that did not commit.
- Rollforward: Reapplies the changes made by committed transactions to ensure the database is up-to-date.
8. Nested Transactions
Nested transactions allow sub-transactions within a main transaction, providing finer granularity for recovery and isolation.
- Sub-Transaction Rollback: If a sub-transaction fails, it can be rolled back independently of the main transaction.
- Parent-Child Relationship: Ensures that changes in sub-transactions are consistent with the main transaction.
9. Time-Travel Queries
Some databases support time-travel queries, which allow users to query the state of the database at a specific point in time.
- Temporal Tables: Store historical versions of data to support time-travel queries.
- Snapshot Isolation: Provides a consistent view of the database at a particular timestamp.
10. Hybrid Techniques
Combining multiple recovery techniques can provide a more robust and efficient recovery mechanism.
- Hybrid Logging: Combines physiological and logical logging.
- Adaptive Checkpointing: Adjusts the frequency and type of checkpoints based on the workload and system state.