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Database/DBMS Interview Questions And Answers - Part XVIII.

73) What Do You Mean By Serializability In Transaction Processing?

Answer: The objective of Serializability is to find non-serial schedules that allow transactions to execute concurrently without interfering with one another, and thereby produce a database state that could be produce by a serial execution.

  • Two transaction that read data only.
  • Two transactions that read or update separate data item.

74) What Are The Two Methods That Guarantee Serializability?

Answer: The two methods that guarantee Serializability are:
  • Two Phase Locking.
  • Timestamping.

75) What Is Locking? What Are The Types Of Locking?

Answer: A procedure used to control concurrent access to data. When one transaction is accessing the database, a lock may deny access to other transactions to prevent incorrect results.

The two types of locking are:
  • Shared lock: If a transaction has a shared lock on a data item, it can read the item but not update it.
  • Exclusive lock: If a transaction has an exclusive lock on a data item, it can both read and update the item.

76) Discuss Two Phase Locking (2PL) Protocol?

Answer: One protocol that is said to guarantee serializability is the Two Phase Protocol (2PL). The 2PL Protocol oversees locks by determining when transactions can acquire and release locks. The 2PL protocol forces each transaction to make a lock or unlock request in two steps:
  • Growing Phase: A transaction may obtain locks but may not release any locks.
  • Shrinking Phase: A transaction may release locks but not obtain any new lock.
The transaction first enters into the Growing Phase, makes requests for required locks, then gets into the Shrinking phase where it releases all locks and cannot make any more requests. Transactions in 2PL Protocol should get all needed locks before getting into the unlock phase.

While the 2PL protocol guarantees serializability, it does not ensure that deadlocks do not happen. To avoid deadlocks in 2PL Protocols, distributed applications like database management systems, employ deadlock detection mechanisms by construction of transaction wait for graphs and detecting for cycles. Any cycle detected in the wait for graph is terminated by breaking the cycle in the wait for graphs.


77) What Is Meant By Deadlock In DBMS?
78) What Are The Three Techniques For Handling Deadlocks?
79) How You Can Identify, Prevent & Resolve A Deadlock?
80) What Is Timestamping? Between Timestamping & Locking Which Is A Better Concurrency Control Technique?

81) What Is A Wait For Graph?

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