Isolation

The property of isolation in database transactions ensures that concurrent transactions do not interfere with each other, creating the illusion that only one transaction is executed in the system, even though multiple transactions may be executed in parallel. It provides a way to maintain the integrity and consistency of the data in a multi-user environment.

Concurrency Control

Concurrency control is the mechanism used to manage concurrent access to the database by multiple transactions. It ensures that the execution of transactions is properly synchronized to avoid conflicts and maintain data consistency.

Isolation Levels

There are different isolation levels defined in database systems to control the level of isolation provided by transactions. The most commonly used isolation levels are:

1. Read Uncommitted: This is the lowest isolation level where transactions can read uncommitted changes made by other transactions. It does not provide any isolation and can lead to dirty reads, non-repeatable reads, and phantom reads.

2. Read Committed: In this isolation level, transactions can only read committed data. It avoids dirty reads but can still lead to non-repeatable reads and phantom reads.

3. Repeatable Read: This isolation level guarantees that a transaction will see a consistent snapshot of the database throughout its execution. It prevents dirty reads and non-repeatable reads, but phantom reads can still occur.

4. Serializable: This is the highest isolation level that provides full isolation between transactions. It ensures that transactions are executed as if they were executed one after another, in a serial manner. It prevents dirty reads, non-repeatable reads, and phantom reads, but it can lead to a high degree of contention and may impact performance.

Locking Mechanisms

To achieve isolation, database systems use locking mechanisms to control concurrent access to data. Locks can be obtained on different levels (e.g., row-level, table-level) and can be shared or exclusive. Transactions acquire locks before accessing data and release them after completing the operation.

Concurrency Control Techniques

To ensure isolation and manage concurrent access, various concurrency control techniques are used, such as:

1. Locking: Transactions acquire locks on data items to prevent other transactions from accessing or modifying them concurrently.

2. Timestamp Ordering: Transactions are assigned timestamps, and their execution order is determined based on these timestamps. Conflicts are resolved by aborting and restarting transactions.

3. Multiversion Concurrency Control (MVCC): Each transaction sees a snapshot of the database at the start of its execution. Changes made by other transactions are stored as separate versions, allowing concurrent access without conflicts.

Conclusion

The isolation property in database transactions ensures that concurrent transactions do not interfere with each other, providing the illusion that only one transaction is executed in the system. Concurrency control mechanisms, such as locking, timestamp ordering, and MVCC, are used to manage concurrent access and maintain data consistency. The choice of isolation level depends on the application requirements and trade-offs between isolation and performance.