ACID Properties

ACID is an acronym representing four key properties of database transactions to ensure data integrity, reliability, and consistency. These properties are:

1. Atomicity:
   • Ensures that all operations within a transaction are completed successfully. If any operation fails, the transaction is aborted, and all operations are rolled back. This guarantees that the database remains in a consistent state even in the event of a failure.
2. Consistency:
   • Ensures that a transaction takes the database from one valid state to another valid state. This means that any transaction will bring the database from one consistent state to another, maintaining database rules, such as integrity constraints.
3. Isolation:
   • Ensures that transactions are isolated from each other, preventing concurrent transactions from interfering with each other. This means the partial effects of incomplete transactions are invisible to other transactions, thus avoiding issues like dirty reads, non-repeatable reads, and phantom reads.
4. Durability:
   • Ensures that once a transaction is committed, it will remain so, even in the event of a system failure. This is achieved through various mechanisms like logging, whereby changes made by the transaction are recorded in a persistent storage.

Concurrency Control in DBMS

Concurrency control in a database ensures that multiple transactions can be executed concurrently without leading to inconsistencies in the database. It aims to coordinate concurrent accesses to the database in a multi-user environment. The main techniques for concurrency control are:

1. Lock-Based Protocols:
   • Two-Phase Locking (2PL): Ensures serializability by acquiring all the locks before any locks are released. It has two phases: the growing phase, where all locks are acquired, and the shrinking phase, where locks are released.
   • Strict Two-Phase Locking: A stricter version where transactions do not release any of their exclusive (write) locks until after they commit or abort.
2. Timestamp-Based Protocols:
   • Each transaction is given a unique timestamp. Transactions are ordered based on their timestamps, ensuring a serializable order. If a transaction requests a data item that has been accessed by a younger transaction, it is either rolled back (to avoid conflicts) or made to wait.
3. Optimistic Concurrency Control (OCC):
   • Transactions execute without acquiring locks, instead checking for conflicts only at the end of the transaction. If a conflict is detected, the transaction is rolled back and restarted. This method is useful in environments with low conflict rates.
4. Multiversion Concurrency Control (MVCC):
   • Maintains multiple versions of data items. When a transaction reads a data item, it gets a snapshot of the database at a specific point in time, ensuring that reads are never blocked by writes. Writers can continue to write, creating new versions of data items, thus improving concurrency.

Conclusion

The ACID properties and concurrency control mechanisms are crucial for ensuring that databases operate reliably and efficiently in a multi-user environment. While ACID properties focus on maintaining the integrity and consistency of data, concurrency control ensures that multiple transactions can proceed concurrently without causing data inconsistencies. Together, these concepts provide a robust framework for managing transactions in database systems.