How does SQLite's transaction mechanism work?

SQLite's transaction mechanism follows ACID principles:

1. Atomicity

   • All operations in a transaction either succeed completely or fail completely
   • Use BEGIN TRANSACTION to start a transaction, COMMIT to commit, ROLLBACK to rollback
   • SQLite uses Write-Ahead Logging (WAL) or rollback journals to ensure atomicity

2. Consistency

   • The database must remain in a consistent state before and after transaction execution
   • All constraints (primary key, foreign key, unique constraint, check constraint) are validated when the transaction commits
   • Operations that violate constraints cause transaction rollback

3. Isolation

   • SQLite defaults to serializable isolation level
   • Read operations do not block other read operations
   • Write operations acquire an exclusive lock on the database, preventing other write operations
   • Supports multiple isolation modes: DEFERRED, IMMEDIATE, EXCLUSIVE

4. Durability

   • Once a transaction is committed, modifications to the database are permanent
   • Data is written to disk before returning commit success
   • Uses synchronous mode to control data persistence levels

5. Transaction Control Commands

   sql
   BEGIN TRANSACTION;  -- Start transaction
   -- Execute SQL operations
   COMMIT;  -- Commit transaction
   -- Or
   ROLLBACK;  -- Rollback transaction

6. Auto Transactions

   • If no explicit transaction is started, SQLite automatically creates a transaction for each statement
   • Recommended to use explicit transactions in complex operations to improve performance and ensure data consistency

7. Savepoints

   • Supports nested transactions and partial rollbacks
   • Use SAVEPOINT to create savepoints, RELEASE SAVEPOINT to release, ROLLBACK TO SAVEPOINT to rollback to a specific point

SQLite's transaction mechanism ensures data security and consistency, making it particularly suitable for application scenarios requiring strong consistency guarantees.