19 Questions
What does a lower isolation level in database systems increase?
Ability of many users to access the same data at the same time
In two-tier systems, what is required to maintain isolation?
Transaction processing (TP) manager
Isolation is one of the four ACID properties, along with which of the following?
Atomicity, consistency and durability
What do n-tier systems require to commit a booking and send confirmation to the customer?
Combination of stored procedures and transaction management
What is the highest isolation level according to the ANSI/ISO SQL standard?
Serializable
Which isolation level can result in phantom reads?
Repeatable reads
In which isolation level are write locks released at the end of the transaction, but read locks are released as soon as the SELECT operation is performed?
Read committed
Which isolation level allows a transaction to be performed at a higher level than requested?
Serializable
Which method is the most common transaction concurrency control method in DBMSs?
Two-phase locking
What does a dirty read refer to in the context of database transactions?
Retrieving uncommitted data
What is the purpose of lock-based concurrency control and multiversion concurrency control?
Prevent non-repeatable reads and phantom reads
What do most DBMSs offer to control the degree of locking that occurs when selecting data?
Transaction isolation levels
What is the primary trade-off involved in maintaining the highest level of isolation in a DBMS?
Loss of concurrency
According to ANSI/ISO standard SQL 92, what is a non-repeatable read in a database transaction?
Retrieving a row twice with an update in between retrievals by another committed transaction
What is the purpose of two-phase locking in database management systems?
Ensuring both serializability and recoverability for correctness
In database systems, what are read phenomena as per ANSI/ISO standard SQL 92?
Dirty read, non-repeatable read, phantom read
What strategy is used to prevent non-repeatable reads and phantom reads in database management systems?
Lock-based concurrency control and multiversion concurrency control
What does reducing the locking overhead for a system involve in database applications?
Constructing transactions to avoid requiring high isolation levels
What is the primary purpose of offering multiple transaction isolation levels in most DBMSs?
To provide flexibility in balancing between consistency and performance requirements
Study Notes
Database Concurrency Control and Isolation Levels
- Database and storage engines heavily rely on concurrency control mechanisms to ensure correct execution of concurrent transactions and other DBMS processes.
- Transaction-related mechanisms constrain database data access operations’ timing to certain orders characterized as serializability and recoverability schedule properties.
- Concurrency control mechanisms are designed to provide the best performance possible under constraints, often compromising serializability for better performance without compromising recoverability.
- Two-phase locking is the most common transaction concurrency control method in DBMSs, ensuring both serializability and recoverability for correctness.
- ANSI/ISO standard SQL 92 refers to three different read phenomena when a transaction retrieves data that another transaction might have updated.
- A dirty read occurs when a transaction retrieves a row that has been updated by another uncommitted transaction, potentially leading to incorrect views of the data.
- A non-repeatable read occurs when a transaction retrieves a row twice, and that row is updated by another committed transaction in between the retrievals.
- A phantom read occurs when a transaction retrieves a set of rows twice, and new rows are inserted into or removed from that set by another committed transaction in between the retrievals.
- Lock-based concurrency control and multiversion concurrency control are the two basic strategies used to prevent non-repeatable reads and phantom reads.
- Most DBMSs offer a number of transaction isolation levels, which control the degree of locking that occurs when selecting data.
- To maintain the highest level of isolation, a DBMS usually acquires locks on data, which may result in a loss of concurrency, or implements multiversion concurrency control.
- For many database applications, the majority of database transactions can be constructed to avoid requiring high isolation levels, reducing the locking overhead for the system.
Test your knowledge of database isolation levels with this quiz. Determine how transaction integrity is visible to other users and systems, and understand the trade-offs between lower and higher isolation levels.
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