The ACID properties are a set of principles that ensure reliable processing of database transactions. They stand for Atomicity, Consistency, Isolation, and Durability. These properties guarantee that transactions are processed in a way that maintains the integrity of the database.
ACID Properties:
1. Atomicity: Atomicity ensures that a transaction is treated as a single, indivisible unit of work. It means that all the operations within a transaction are either fully completed or not executed at all. Example: Suppose you are transferring money from Account A to Account B. The transaction involves debiting Account A and crediting Account B. Atomicity ensures that both operations are completed successfully. If one operation fails, the entire transaction is rolled back, and neither account is updated.
2. Consistency: Consistency ensures that a transaction transforms the database from one valid state to another valid state, preserving the integrity constraints defined in the database schema.
Example: If a database has a constraint that the total balance of two accounts (A and B) must be equal to a certain amount, a transaction transferring money between these accounts should maintain this invariant. The database should remain consistent before and after the transaction.
3. Isolation: Isolation ensures that the operations of a transaction are isolated from the operations of other concurrent transactions. It means that the intermediate state of a transaction is invisible to other transactions until it is completed.
Example: If two transactions are running concurrently—one transferring money from Account A to Account B and another transferring money from Account C to Account D—they should not interfere with each other. Each transaction should execute as if it is the only transaction in the system, providing an isolated view of the data.
4. Durability: Durability ensures that once a transaction is committed, its effects are permanently stored in the database, even in the case of a system failure.
Example: After successfully transferring money from Account A to Account B, the changes should be stored in non-volatile storage. Even if the system crashes immediately after the transaction is committed, the updated balances of Account A and Account B should be preserved when the system is restarted.
Key points about ensuring atomicity and durability in distributed systems:
i. Two-Phase Commit (2PC): This widely used protocol is a standard method for achieving atomicity in distributed transactions, where all participating nodes must agree on the outcome of a transaction before committing any changes.
ii. Preparation Phase: In 2PC, the coordinator node sends a “prepare” message to all involved nodes, asking them to prepare to commit the transaction, but not to finalize the changes yet.
iii. Commit Phase: If all nodes respond positively in the prepare phase, the coordinator sends a “commit” message to all nodes, allowing them to finalize the transaction. If any node fails or responds negatively, the coordinator sends a “rollback” message to all nodes, causing them to undo any changes made.
iv. Write-Ahead Logging (WAL): This technique is often used to ensure durability by writing transaction logs to persistent storage before updating the actual data, guaranteeing that even if a system crashes, the changes can be recovered from the logs.
v. Replication: Replicating data across multiple nodes can also enhance durability by allowing for data recovery from a backup node if a primary node fails.
Thus, ACID properties ensure reliable database transactions. Techniques like 2PC, WAL, and replication help maintain consistency and durability.