Concurrency control
- 1. MCA, MSc(CS), PGDCA Pratibha Rashmi
- 2. When many people try to make modifications to a database at any point of time, there should be a system of controls in place to ensure that changes made by one do not adversely affect the other, this is calledadversely affect the other, this is called concurrency control. Concurrency control deals with interleaved execution of more than one transaction.
- 3. When multiple transactions execute concurrently in an uncontrolled or unrestricted manner, then it might lead to several problems. Such problems are called as concurrency problems. concurrency problems In transaction Dirty Read Problem Unrepeatable Read Problem Phantom Read Problem Lost Update Problem
- 4. The concurrency problems are:- Dirty Read Problem Unrepeatable Read Problem Phantom Read Problem Lost Update Problem
- 5. Dirty Read: Reading the data written by an uncommitted transaction is called as dirty read. This read is called as dirty read because- There is always a chance that the There is always a chance that the uncommitted transaction might roll back later. Thus, uncommitted transaction might make other transactions read a value that does not even exist. This leads to inconsistency of the database.
- 6. Dirty read does not lead to inconsistency always. It becomes problematic only when the uncommitted transaction fails and roll backs later due to some reason.later due to some reason.
- 8. Here, T1 reads the value of A. T1 updates the value of A in the buffer. T2 reads the value of A from the buffer. T2 writes the updated the value of A. T2 commits.T2 commits. T1 fails in later stages and rolls back. In this example, 1. T2 reads the dirty value of A written by the uncommitted transaction T1. 2. T1 fails in later stages and roll backs. 3. Thus, the value that T2 read now stands to be incorrect. 4. Therefore, database becomes inconsistent.
- 9. This problem occurs when a transaction gets to read unrepeated i.e. different values of the same variable in its different read operations even when it has not updated its value.value.
- 10. Here, T1 reads the value of X (= 10 say). T2 reads the value of X (= 10). T1 updates the value of X (from 10 to 15 say) in the buffer. T2 again reads the value of X (but = 15). In this example, 1. T2 gets to read a different value of X in its second reading. 2. T2 wonders how the value of X got changed because according to it, it is running in isolation.
- 11. This problem occurs when a transaction reads some variable from the buffer and when it reads the same variable later, it finds that the variable does not exist.
- 12. Here, T1 reads X. T2 reads X. T1 deletes X. T2 tries reading X but does not find it.T2 tries reading X but does not find it. In this example, 1. T2 finds that there does not exist any variable X when it tries reading X again. 2. T2 wonders who deleted the variable X because according to it, it is running in isolation.
- 13. This problem occurs when multiple transactions execute concurrently and updates from one or more transactions get lost.
- 14. Here, T1 reads the value of A (= 10 say). T2 updates the value to A (= 15 say) in the buffer. T2 does blind write A = 25 (write without read) in the buffer. T2 commits. When T1 commits, it writes A = 25 in the database. In this example, In this example, 1. T1 writes the over written value of X in the database. 2. Thus, update from T1 gets lost. NOTE- This problem occurs whenever there is a write-write conflict. In write-write conflict, there are two writes one by each transaction on the same data item without any read in the middle.
- 15. To ensure consistency of the database, it is very important to prevent the occurrence of above problems. Concurrency Control Protocols help to prevent the occurrence of above problemsprevent the occurrence of above problems and maintain the consistency of the database.
- 16. If the transaction is rolled back, all the database changes made inside the transaction are ___________ A. made permanent B. made temporary C. copied to the logC. copied to the log D. undone In ________ one transaction overwrites the changes of another transaction. A. uncommitted read problem B. lost update problem C. update lost problem D. dirty read problem
- 17. In a dirty read problem _________ A. one transaction reads an uncommitted value of another transaction B. one transaction reads the committed value for another transaction C. one transaction reads another transaction D. one transaction commits another transaction. C. one transaction reads another transaction D. one transaction commits another transaction. Isolation of the transactions is ensured by A. Transaction management B. Application programmer C. Concurrency control D. Recovery management
- 18. Define the term Concurrency Control. What is dirty read and phantom read? What is inconsistent read problem? What is the mean of write- write problem? Reference: https://www.gatevidyalay.com/, http://beginnersbook.com