Databases implement ACID transactions using a combination of techniques designed to ensure atomicity, consistency, isolation, and durability. Here's how they achieve each component:
Ensures that a transaction is "all or nothing." Either all operations succeed, or none are applied.
- Transaction Logs (Write-Ahead Logging):Changes are first written to a log before being applied to the database. If the transaction fails, the log is used to roll back incomplete operations.Example: PostgreSQL uses a Write-Ahead Log (WAL).
- Rollback Mechanism:Keeps a record of the previous state of data to undo changes if the transaction fails.Example: MySQL’s InnoDB engine uses undo logs.
Ensures that a transaction moves the database from one valid state to another, maintaining all rules and constraints.
- Database Constraints:Enforcing rules like primary keys, foreign keys, and check constraints ensures that transactions don't violate data integrity.Example: Ensuring a foreign key references an existing record.
- Trigger Mechanisms:Custom business rules are enforced using triggers to validate data during a transaction.Example: Ensuring inventory is sufficient before deducting stock during an order.
Ensures that transactions execute independently without interfering with each other.
- Locks:Row-level Locks: Only the rows involved in the transaction are locked, reducing contention.Table-level Locks: Entire tables are locked, typically in older systems or for large operations.Example: MySQL and PostgreSQL support row-level locking.
- Multiversion Concurrency Control (MVCC):Instead of locking, transactions access snapshots of the database, ensuring consistency while allowing concurrency.Example: PostgreSQL and Oracle implement MVCC.
- Isolation Levels:Configurable levels such as Read Uncommitted, Read Committed, Repeatable Read, and Serializable control how much isolation is enforced.
Ensures that once a transaction is committed, its changes are permanent, even in case of power loss or system crashes.
- Transaction Logs:Changes are logged before committing, ensuring recovery from system crashes.
- Checkpointing:Periodically writes in-memory data and logs to disk to reduce recovery time in case of failure.
- Write-Ahead Logging (WAL):Ensures all changes are safely written to persistent storage before marking a transaction as committed.Example: WAL is standard in most modern databases, like PostgreSQL.
- Replication and Backup Systems:Replicating data across multiple nodes or maintaining backups ensures durability in distributed systems.
- MySQL (InnoDB Engine):
- PostgreSQL:
- Oracle Database:
- SQL Server:
