MVCC in Databases Quiz

Multiversion Concurrency Control (MVCC) in Databases

• MVCC in a database ensures that changes made by a writer are not visible to other users until the changes are committed, creating multiple versions of the data item.
• Snapshot isolation is the most common isolation level implemented with MVCC, providing point-in-time consistent views for transactions.
• MVCC allows read and write transactions to be isolated from each other without the need for locking, although some MVCC databases, like Oracle, still use locks.
• MVCC introduces the challenge of removing obsolete versions of data, which may involve periodic sweeping and deleting of obsolete versions, as seen in PostgreSQL's VACUUM FREEZE process.
• Some databases use a storage block split into two parts: the data part keeps the last committed version, and the undo log enables recreation of older versions of data.
• MVCC uses timestamps and incrementing transaction IDs to achieve transactional consistency, ensuring that a transaction never has to wait to read a database object.
• When a transaction wants to write to an object, it must have a timestamp that is not earlier than the object's current read timestamp, or the transaction is aborted and restarted.
• The drawback of MVCC is the cost of storing multiple versions of objects in the database, but it ensures that reads are never blocked, important for workloads involving mostly reading values from the database.
• MVCC is adept at implementing true snapshot isolation, which other methods of concurrency control may do incompletely or with high performance costs.
• Multiversion concurrency control was described in detail in a 1981 paper by Phil Bernstein and Nathan Goodman, and was first featured in the 1984 VAX Rdb/ELN database software by Digital Equipment Corporation.
• MVCC was also featured in the second commercially successful database, InterBase, created by Jim Starkey, who also worked on VAX Rdb/ELN.
• MVCC allows snapshot isolation reads without any locks, and it was originally described in a 1978 dissertation by David P. Reed.