What is SQLite's WAL mode? What are its advantages?

SQLite's WAL (Write-Ahead Logging) mode is an important performance optimization mechanism:

1. WAL Mode Principle

   • Traditional rollback journal mode: Write operations directly modify the database file, using rollback journal to record changes
   • WAL mode: Write operations write changes to the WAL file instead of directly modifying the database file
   • During checkpoint, the contents of the WAL file are merged into the main database file

2. Advantages of WAL Mode

   • Improved Concurrency: Read operations do not block write operations, write operations do not block read operations
   • Reduced Disk I/O: Write operations are usually sequential, faster than random writes
   • Better Crash Recovery: The WAL file contains all uncommitted changes, recovery is more reliable
   • Improved Write Performance: Multiple writes can be batch committed, reducing synchronization operations

3. Enabling WAL Mode

   sql
   PRAGMA journal_mode = WAL;

4. WAL File Management

   • WAL files are in the same directory as the database file, with suffixes -wal and -shm
   • -wal file stores the actual WAL data
   • -shm file is a shared memory file used to coordinate concurrent access

5. Checkpoint Mechanism

   • Automatic checkpoint: Automatically triggered when the WAL file reaches a certain size
   • Manual checkpoint: Use PRAGMA wal_checkpoint(TRUNCATE); to manually trigger
   • Checkpoint merges valid changes from the WAL file into the main database file

6. Limitations of WAL Mode

   • Not supported by some older SQLite versions
   • May be unreliable on network file systems
   • Requires additional disk space to store WAL files
   • May cause database file growth in some cases

7. Applicable Scenarios

   • High concurrent read-write scenarios
   • Need better crash recovery capabilities
   • Write-intensive applications
   • Mobile applications and desktop applications

WAL mode is the recommended default configuration for SQLite, which can significantly improve performance in most application scenarios.