Transactions and Concurrency Control
Tags: beginners
Published: 2024-02-13 07:02:13

**Transactions:**

A transaction is a sequence of one or more operations on a database that is executed as a single unit of work. The primary goal of transactions is to maintain the consistency and integrity of the database even in the presence of failures. Transactions adhere to the ACID properties:

1. **Atomicity:**
   - Atomicity ensures that a transaction is treated as a single, indivisible unit of work. Either all the changes made by the transaction are committed to the database, or none of them are. If any part of the transaction fails, the entire transaction is rolled back.

2. **Consistency:**
   - Consistency ensures that a transaction brings the database from one valid state to another. The database should always be in a consistent state, meaning it adheres to the defined integrity constraints and business rules.

3. **Isolation:**
   - Isolation ensures that the execution of one transaction is isolated from the execution of other transactions. Even if multiple transactions are executed concurrently, each should see the database as if it is the only transaction in progress. Isolation levels (e.g., Read Uncommitted, Read Committed, Repeatable Read, Serializable) define the degree of isolation.

4. **Durability:**
   - Durability guarantees that once a transaction is committed, its effects will persist even in the face of system failures. The changes made by committed transactions are permanently stored in the database.

**Concurrency Control:**

Concurrency control is a mechanism used to manage the simultaneous execution of multiple transactions in a way that maintains the consistency of the database. It addresses issues that can arise when multiple transactions are executed concurrently, such as lost updates, uncommitted data, and inconsistent reads. Key concepts and techniques in concurrency control include:

1. **Locking:**
   - Locking is a common technique used to control access to data. Transactions acquire locks on data items, preventing other transactions from accessing or modifying the same data simultaneously. There are different types of locks, such as read locks and write locks.

2. **Isolation Levels:**
   - Isolation levels define the degree to which one transaction is isolated from the effects of other concurrently executing transactions. Common isolation levels include Read Uncommitted, Read Committed, Repeatable Read, and Serializable. Higher isolation levels provide stronger guarantees but may lead to more contention.

3. **Serializability:**
   - Serializability ensures that the execution of a set of concurrent transactions is equivalent to some serial execution of those transactions. In other words, the final result is as if transactions were executed one after another, even though they may execute concurrently.

4. **Timestamp Ordering:**
   - Timestamp ordering uses timestamps to order transactions and their operations. Transactions are assigned timestamps, and the system ensures that transactions are executed in timestamp order. This helps maintain a consistent and predictable order of transactions.

5. **Two-Phase Locking (2PL):**
   - Two-Phase Locking is a concurrency control protocol that ensures transactions follow a set of rules regarding acquiring and releasing locks. The protocol has two phases: the growing phase, where locks can be acquired, and the shrinking phase, where locks can only be released.

6. **Optimistic Concurrency Control:**
   - Optimistic Concurrency Control assumes that conflicts between transactions are rare. Instead of locking data, it allows transactions to proceed without interference and checks for conflicts only at the end of the transaction. If conflicts are detected, appropriate actions are taken.

7. **Multiversion Concurrency Control (MVCC):**
   - MVCC maintains multiple versions of a data item to allow concurrent transactions to read and modify the data without blocking each other. Each transaction sees a snapshot of the database at a specific point in time.

Concurrency control is crucial in a multi-user database environment where multiple transactions may try to access and modify data simultaneously. By ensuring that transactions are executed in a controlled manner, concurrency control mechanisms help prevent data inconsistencies and maintain the integrity of the database.