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How does a DBMS handle concurrent access to data? What issues can arise and how are they addressed?

Question

How does a DBMS handle concurrent access to data? What issues can arise and how are they addressed?

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Solution

A Database Management System (DBMS) handles concurrent access to data through a process known as concurrency control. This process ensures that transactions are executed concurrently without violating data consistency.

Here are the steps involved:

  1. Locking Mechanism: The DBMS uses a locking mechanism where any data item being used by a transaction is locked, preventing other transactions from accessing it. There are two types of locks: shared locks and exclusive locks. Shared locks allow multiple transactions to read (but not write) the same data item simultaneously, while exclusive locks allow only one transaction to read and write a data item.

  2. Timestamp-Based Protocols: The DBMS assigns a timestamp to each transaction. If a transaction wants to read or write a data item, the DBMS compares the transaction's timestamp with the data item's read-timestamp or write-timestamp. This helps to maintain the order of transactions and prevent conflicts.

  3. Optimistic Concurrency Control: This method assumes that conflicts between transactions are rare and allows multiple transactions to access the same data item without locks. However, before committing, it checks whether conflicts have occurred. If a conflict is detected, the transaction is rolled back and restarted.

  4. Multi-version Concurrency Control (MVCC): In this method, the DBMS creates a new version of a data item each time it is written, allowing multiple transactions to read different versions of the same data item. This reduces the need for locks and increases concurrency.

Issues that can arise during concurrent access include:

  1. Deadlocks: This occurs when two or more transactions indefinitely wait for each other to release resources. DBMSs handle deadlocks by using a deadlock detection and prevention system that rolls back one of the transactions to break the deadlock.

  2. Dirty Reads: This occurs when a transaction reads data that has been written by another uncommitted transaction. To prevent this, DBMSs use isolation levels that define how and when the changes made by one transaction are visible to others.

  3. Non-repeatable Reads and Phantom Reads: These occur when a transaction reads the same data twice and gets different results each time. This can be prevented by using higher isolation levels or locking mechanisms.

  4. Update Lost: This occurs when two transactions that access the same database item have their operations interleaved in a way that makes the value of some database item incorrect. The DBMS can use a lock-based protocol to prevent this problem.

In summary, a DBMS handles concurrent access to data by using various concurrency control techniques and isolation levels to ensure data consistency and prevent conflicts.

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