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Transactions and Concurrency Control

Introduction​

Transactions and concurrency control are fundamental concepts in database management systems (DBMS). They play crucial roles in ensuring data consistency, reliability, and performance. This guide aims to provide a comprehensive overview of these topics, making them accessible to students pursuing degrees in computer science and related fields.

What are Transactions?​

A transaction is a sequence of operations performed on a database that must execute atomically—either all or none of its operations succeed. Transactions are used to maintain data integrity and consistency across multiple operations.

Key Characteristics of Transactions​

  1. Atomicity: All operations within a transaction must complete successfully or fail entirely. If one operation fails, the entire transaction is rolled back.

  2. Consistency: The database must remain in a consistent state before and after the transaction. Any transaction must adhere to predefined rules, constraints, and triggers.

  3. Isolation: Concurrent transactions must not interfere with each other. Each transaction should execute as if it is the only transaction in the system.

  4. Durability: Once committed, the effects of a transaction are permanent, even in the event of a system failure.

Types of Transactions​

  1. Simple Transaction: A single operation that modifies one or more tables.

    Example:

    BEGIN TRANSACTION;
    UPDATE Accounts
    SET Balance = Balance - 100
    WHERE Account_ID = 1;
    COMMIT;

  2. Complex Transaction: A sequence of multiple operations that involve multiple tables or steps.

    Example:

    BEGIN TRANSACTION;
    UPDATE Accounts
    SET Balance = Balance - 100
    WHERE Account_ID = 1;

    UPDATE Accounts
    SET Balance = Balance + 100
    WHERE Account_ID = 2;

    COMMIT;

What is Concurrency Control?​

Concurrency control refers to the techniques used to manage simultaneous operations on a database without causing data inconsistencies. It ensures that database transactions are processed reliably and independently.

Concurrency Control Techniques​

  1. Locking: Prevents other transactions from accessing the same data simultaneously.

    • Shared Lock: Allows multiple transactions to read a data item but prevents writing.
    • Exclusive Lock: Allows only one transaction to write to a data item and prevents both reading and writing by others.

    Example:

    LOCK TABLE Accounts IN EXCLUSIVE MODE;

  2. Timestamp Ordering: Uses timestamps to order transactions. Older transactions get higher priority.

    Example: Assign a timestamp to each transaction and process them in timestamp order.

  3. Optimistic Concurrency Control: Assumes that conflicts are rare and allows transactions to execute without locking. Conflicts are detected at commit time, and transactions are rolled back if necessary.

    Example: Use version numbers or timestamps to detect changes during a transaction.

  4. Two-Phase Locking (2PL): Ensures that transactions acquire all necessary locks before releasing any. It has two phases: growing phase (acquiring locks) and shrinking phase (releasing locks).

    Example:

    BEGIN TRANSACTION;
    -- Growing phase
    SELECT * FROM Accounts WHERE Account_ID = 1 FOR UPDATE;
    -- Perform updates
    UPDATE Accounts SET Balance = Balance - 100 WHERE Account_ID = 1;
    COMMIT;

  5. Serializable Transactions: Ensures that transactions are executed in a manner that produces the same result as if they were executed sequentially, one after the other.

    Example: Use isolation levels to achieve serializability, such as SERIALIZABLE in SQL.

Isolation Levels​

Isolation levels define the degree to which the operations in one transaction are isolated from the operations in other concurrent transactions.

  1. Read Uncommitted: Allows transactions to read uncommitted changes from other transactions. This level has the lowest isolation and can lead to dirty reads.

    Example:

    SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

  2. Read Committed: Ensures that transactions can only read committed changes from other transactions. This level prevents dirty reads but allows non-repeatable reads.

    Example:

    SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

  3. Repeatable Read: Ensures that if a transaction reads a data item, it will read the same value if it reads that data item again. It prevents dirty and non-repeatable reads but may allow phantom reads.

    Example:

    SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

  4. Serializable: The highest isolation level, which ensures complete isolation from other transactions, effectively serializing the execution of transactions.

    Example:

    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Conclusion​

Understanding transactions and concurrency control is essential for maintaining the integrity and performance of database systems. Transactions ensure atomicity, consistency, isolation, and durability, while concurrency control techniques manage simultaneous access to data. By mastering these concepts, you can design and implement robust and reliable database systems that handle complex and concurrent operations effectively.