Database Recovery and Transaction Logs: MySQL vs. PostgreSQL

Database failure recovery and transaction log: MySQL vs. PostgreSQL

Introduction:
Database failure recovery is a vital part of the database management system. When a database failure occurs, how to ensure data consistency and integrity and restore database operation as quickly as possible has become an important issue that database administrators need to face. This article will discuss the different implementation methods of database failure recovery and transaction logs in two popular relational database management systems, MySQL and PostgreSQL, and give corresponding code examples.

1. MySQL’s fault recovery mechanism:
1.1 Log
MySQL uses two different log types to support fault recovery: binary log (Binary log) and transaction log (InnoDB redo log) .

1.2 Binary log
The binary log is a log used to record all write operations on the MySQL server. Its main function is for failure recovery and database replication. When the database crashes or other failures occur, the state of the database is restored by replaying the operations recorded in the binary log. The following is a sample code for recovery using binary logs:

# 开启二进制日志
mysql> SET sql_log_bin=1;

# 执行写操作
mysql> INSERT INTO table_name (column1, column2, ....)
    -> VALUES (value1, value2, ...);

# 关闭二进制日志
mysql> SET sql_log_bin=0;

# 恢复数据库
shell> mysqlbinlog binlog.000001 | mysql -u root -p

1.3 Transaction log (InnoDB redo log)
The transaction log is used to record the data modification operations of the InnoDB storage engine, mainly used for crash recovery and transaction recovery roll. It consists of two files: redo log and undo log.

The redo log is a cyclically written log file used to record transaction modification operations. When the database crashes, the state of the database is restored by replaying the logs in the redo log. The following is a sample code for recovery using transaction logs:

# 开启事务
mysql> START TRANSACTION;

# 执行写操作
mysql> INSERT INTO table_name (column1, column2, ....)
    -> VALUES (value1, value2, ...);

# 提交事务
mysql> COMMIT;

# 恢复数据库
shell> innobackupex --apply-log /path/to/backup
shell> cp -R /path/to/backup /var/lib/mysql
shell> chown -R mysql:mysql /var/lib/mysql

2. PostgreSQL fault recovery mechanism:
2.1 Log
PostgreSQL uses two different log types to support fault recovery: transaction log (WAL) and checkpoint log (Checkpoint Log).

2.2 Transaction log (WAL)
The transaction log is a log used to record all modification operations in the database. Its main function is to ensure the consistency and durability of the database. When a database crashes or otherwise fails, the state of the database is restored by replaying the operations in the transaction log. The following is a sample code for recovery using transaction logs:

# 修改配置文件
wal_level = replica     # 选择合适的事务日志级别
archive_mode = on       # 打开归档模式
archive_command = 'cp %p /path/to/archive/%f'    # 设置归档命令

# 创建及配置归档目录
shell> mkdir /path/to/archive
shell> chown postgres:postgres /path/to/archive

# 执行写操作
postgres$ INSERT INTO table_name (column1, column2, ....)
        VALUES (value1, value2, ...);

# 手动触发归档（可选）
postgres$ SELECT pg_switch_xlog();

# 恢复数据库
shell> cp /path/to/archive/xlog.00000001 /var/lib/postgresql/data/pg_xlog/
shell> chown postgres:postgres /var/lib/postgresql/data/pg_xlog/xlog.00000001
shell> pg_ctl -D /var/lib/postgresql/data start

2.3 Checkpoint Log (Checkpoint Log)
The checkpoint log is a log used to record the latest checkpoint (checkpoint) operation in the database. It is mainly used for database recovery speed optimization. When a database crashes, the state of the database can be quickly restored by reading the checkpoint log.

Conclusion:
MySQL and PostgreSQL both use transaction logs to support failure recovery, but there are certain differences in specific implementations. MySQL uses binary logs and InnoDB redo logs, while PostgreSQL uses transaction logs (WAL) and checkpoint logs. Through the above code examples, we can see their specific operation processes when restoring the database. In order to ensure the security and reliability of the database, database administrators need to select an appropriate database management system based on specific needs and usage scenarios, and reasonably configure the corresponding fault recovery mechanism.

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