Log-based recovery is a technique used in database management systems (DBMS) to ensure data consistency and recover from system failures or crashes. It involves the use of transaction logs, which are sequential records of all modifications made to a database during the execution of transactions.
Here’s an overview of how log-based recovery works:
- Transaction Logs: A transaction log is a file that records the sequence of operations performed on the database. It includes information such as the identification of the transaction, the type of operation (e.g., insert, update, delete), and the old and new values affected by the operation.
- Write-Ahead Logging: The write-ahead logging (WAL) protocol is a key principle in log-based recovery. It requires that all changes to the database must be written to the transaction log before they are written to the database itself. This ensures that the log always reflects the most recent state of the database.
- Transaction Execution: When a transaction is executed, its operations are first recorded in the transaction log, along with a unique identifier. The changes are then applied to the actual database.
- Commit and Abort: When a transaction commits, a log record called a commit record is written to the log to indicate the successful completion of the transaction. In case of an abort or failure, an abort record is written instead.
- Recovery Process: When a system failure occurs, such as a power outage or a crash, the database needs to be recovered to a consistent state. The recovery process involves two phases: redo and undo.
a. Redo Phase: In this phase, the DBMS examines the log starting from the last checkpoint (a known consistent state) up to the end of the log. It reapplies all the operations recorded in the log to the database, including the changes that were not yet written to the database before the failure. This ensures that all committed transactions are properly restored.
b. Undo Phase: After the redo phase, the DBMS examines the log in reverse order, starting from the end, to find transactions that were active but not committed at the time of the failure. It uses the undo information in the log to roll back these transactions, restoring the database to a consistent state.
- Recovery Completion: Once the redo and undo phases are complete, the recovery process finishes, and the DBMS can resume normal operation. The system is now in a consistent state, and transactions can continue executing.
Log-based recovery provides a reliable mechanism to ensure data integrity and recover from failures. It guarantees that committed transactions are not lost and that the database can be restored to a consistent state after a system crash.
There are two approaches to modify the database:
The two approaches to modify a database are:
- Immediate Update:
- In this approach, any modifications made by a transaction are immediately applied directly to the database.
- The changes take effect immediately and are visible to other transactions.
- If a transaction encounters an error or failure during its execution, the modifications made by that transaction may have already been permanently applied to the database, resulting in an inconsistent state.
- Immediate update is simpler to implement but does not provide built-in support for easy recovery from failures.
- Deferred Update:
- In deferred update, the modifications made by a transaction are not immediately applied to the database.
- Instead, the changes are recorded in a transaction log or buffer and are only applied to the database when the transaction successfully completes (commits).
- Until the transaction commits, the changes are kept separate from the actual database.
- Other transactions cannot see the modifications made by the transaction until it commits.
- If a transaction encounters an error or failure, the changes made by that transaction can be discarded, and the database remains unchanged.
- Deferred update provides better control over the transaction’s effect on the database and allows for easy recovery by simply discarding the uncommitted changes.
- However, implementing deferred update requires additional bookkeeping to manage the transaction log and handle concurrency control to ensure the isolation of transactions.
Both approaches have their advantages and trade-offs. Immediate update simplifies transaction management but can lead to inconsistent states in case of failures. Deferred update provides better control and recovery capabilities but requires additional overhead to manage the transaction log and ensure transaction isolation. The choice between the two approaches depends on the specific requirements and characteristics of the database system being used.
Recovery using Log records:
Recovery using log records, also known as log-based recovery, is a technique employed in database management systems (DBMS) to restore a consistent state of the database after a system failure or crash. It relies on the information stored in transaction logs to recover the database to a state that existed before the failure occurred. Here’s an overview of the recovery process using log records:
- Types of Log Records: Transaction logs contain various types of log records that capture different stages of transaction execution and database modifications. Common log record types include:
a. Start Record: Marks the beginning of a transaction.
b. Update Record: Records the modification made to a database item (e.g., insert, update, delete).
c. Commit Record: Indicates the successful completion of a transaction.
d. Abort Record: Represents the termination of a transaction due to failure or cancellation.
- Redo Phase: The recovery process begins with the redo phase, where the DBMS examines the transaction logs from the last checkpoint up to the end of the log. The objective of this phase is to reapply the changes recorded in the log to the database, ensuring that committed transactions are properly restored. The redo phase follows these steps:
a. Analyze: The DBMS scans the log records, identifying the transactions that were active or committed at the time of the failure.
b. Redo: For each logged modification that was not yet applied to the database, the DBMS reapplies the change to the corresponding data item.
- Undo Phase: After the redo phase, the undo phase is performed to handle the transactions that were active but not committed at the time of the failure. The purpose of this phase is to roll back these transactions and remove their effects on the database. The undo phase involves the following steps:
a. Analyze: The DBMS scans the log records backward, starting from the end, to identify the active transactions that need to be rolled back.
b. Undo: For each active transaction, the DBMS uses the undo information in the log to reverse the modifications made by the transaction. This involves applying the opposite operation (e.g., undo an update by restoring the original value).
- Recovery Completion: Once the redo and undo phases are completed, the database has been restored to a consistent state. At this point, the recovery process is finished, and the DBMS can resume normal operations. Transactions can continue to execute, and the system is prepared to handle any future failures.
Log-based recovery is an essential component of database systems, ensuring data consistency and durability. By leveraging the information stored in transaction logs, it provides a reliable mechanism to recover from system failures and maintain the integrity of the database.