How does SQLite handle concurrent access?

SQLite's concurrency control mechanism is one of its core features:

1. Lock Levels SQLite uses different levels of locks to manage concurrent access:

   • UNLOCKED: Unlocked state
   • SHARED: Shared lock, allows multiple read operations simultaneously
   • RESERVED: Reserved lock, indicates preparation for writing, still allows read operations
   • PENDING: Pending lock, prevents new read operations, waits for existing read operations to complete
   • EXCLUSIVE: Exclusive lock, exclusive access, blocks all other operations

2. Read-Write Concurrency Model

   • Multiple Readers, Single Writer: SQLite supports multiple read operations simultaneously, but only allows one write operation at a time
   • Read operations do not block other read operations
   • Write operations require exclusive locks and block all other operations

3. Transaction Isolation Modes

   sql
   -- DEFERRED (default): Acquires shared lock on first read, upgrades to exclusive lock on first write
   BEGIN DEFERRED TRANSACTION;
   
   -- IMMEDIATE: Immediately acquires reserved lock, prevents other write operations
   BEGIN IMMEDIATE TRANSACTION;
   
   -- EXCLUSIVE: Immediately acquires exclusive lock, blocks all other operations
   BEGIN EXCLUSIVE TRANSACTION;

4. WAL Mode (Write-Ahead Logging)

   • WAL mode significantly improves concurrency performance
   • Read operations do not block write operations, write operations do not block read operations
   • 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

5. Deadlock Handling

   • SQLite automatically detects deadlocks and rolls back one of the transactions
   • Applications should catch SQLITE_BUSY errors and retry
   • Use sqlite3_busy_timeout() to set busy wait timeout

6. Concurrency Optimization Recommendations

   • Use WAL mode to improve concurrency performance
   • Keep transactions short to reduce lock holding time
   • Separate read operations and write operations into different transactions
   • Use appropriate isolation modes to avoid unnecessary blocking

SQLite's concurrency control mechanism provides reasonable concurrency performance while ensuring data consistency, suitable for the concurrency needs of small to medium-sized applications.