Write-Ahead Logging 편집하기

'''Write-Ahead Logging (WAL)''' is a technique used in [[Database Management System|database management systems (DBMS)]] to ensure the '''durability''' and '''atomicity''' of transactions. WAL ensures that all changes to the database are recorded in a log before the changes are written to the database itself. This enables reliable crash recovery and consistency in case of a failure.
==Key Concepts==
*'''Sequential Logging:''' All modifications are recorded sequentially in the log, making logging operations efficient.
*'''Durability Guarantee:''' The log is stored in stable storage (e.g., disk) to ensure it survives crashes.
*'''Redo and Undo Records:''' WAL maintains sufficient information to redo committed changes and undo uncommitted ones during recovery.
==How Write-Ahead Logging Works==
WAL operates with the following principles:
#'''Log Before Data:''' Before any changes are written to the database, the corresponding log entries are recorded.
#'''Atomic Writes:''' The log entry is written atomically to prevent partial updates.
#'''Recovery Mechanism:''' During recovery, the system replays the log to restore committed transactions and roll back uncommitted ones.
'''Example of WAL Workflow'''

# A transaction modifies a row in the database. 
# The system writes a log entry describing the change (e.g., "update row X: old value = A, new value = B"). 
# The log entry is flushed to stable storage. 
# The actual modification is written to the database.

If a crash occurs after step 3 but before step 4, the log ensures that the change can still be applied during recovery.
==Benefits of Write-Ahead Logging==
*'''Crash Recovery:''' WAL enables recovery to a consistent state after a system crash.
*'''Performance Optimization:''' Logs are written sequentially, which is faster than random database writes.
*'''Concurrency Support:''' Allows multiple transactions to run concurrently while maintaining isolation and consistency.
==Applications of WAL==
WAL is widely used in various systems to ensure reliability and durability:
*'''Relational Databases:''' Systems like PostgreSQL, MySQL (InnoDB), and Oracle use WAL for transaction management.
*'''Distributed Systems:''' Ensures consistency in distributed databases and storage systems.
*'''File Systems:''' Some journaling file systems use WAL to guarantee file system integrity.
==Limitations of Write-Ahead Logging==
*'''Disk I/O Overhead:''' Writing log entries increases the number of disk writes, which can impact performance.
*'''Log Size Management:''' Logs must be periodically truncated or archived to prevent unbounded growth.
*'''Recovery Complexity:''' Recovery procedures using WAL can be intricate and resource-intensive.
==WAL and ARIES==
WAL is a foundational component of the [[ARIES (Algorithm for Recovery and Isolation Exploiting Semantics)]] recovery algorithm. In ARIES:
*WAL ensures that redo and undo operations have all necessary information.
*Checkpoints are used to reduce recovery time by marking the last consistent state.
==Example Implementation in Python==
A simple example of logging changes using WAL:<syntaxhighlight lang="python">
class WriteAheadLog:
    def __init__(self):
        self.log = []

    def write_log(self, transaction_id, operation, old_value, new_value):
        entry = {
            "transaction_id": transaction_id,
            "operation": operation,
            "old_value": old_value,
            "new_value": new_value
        }
        self.log.append(entry)
        print("Log entry written:", entry)

# Example usage
wal = WriteAheadLog()
wal.write_log(1, "update", "A", "B")
</syntaxhighlight>
==Related Concepts and See Also==
*[[Transaction Management]]
*[[Durability (ACID)]]
*[[ARIES (Database)]]
*[[Checkpointing]]
*[[Database Recovery]]
*[[Log-Structured File System]]
*[[Journaling]]
[[Category:Database]]