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T2 could read a database object A, modified by T1 which hasn't committed. This is a dirty or inconsistent read. T1 may write some value into A which makes the database inconsistent. It is possible that interleaved execution can expose this inconsistency and lead to an inconsistent final database state, violating ACID rules.
Write skew is possible at this isolation level in some systems. Write skew is a phenomenon where two writes are allowed to the same column(s) in a table by two different writers (who have previously read the columns they are updating), resulting in the column having data that is a mix of the two transactions. [3] [4]
Write-ahead logging: Any change to an object is first recorded in the log, and the log must be written to stable storage before changes to the object are written to disk. Repeating history during Redo: On restart after a crash, ARIES retraces the actions of a database before the crash and brings the system back to the exact state that it was in ...
In computer science, in the field of databases, read–write conflict, also known as unrepeatable reads, is a computational anomaly associated with interleaved execution of transactions. Specifically, a read–write conflict occurs when a "transaction requests to read an entity for which an unclosed transaction has already made a write request."
For 2PL, the only used data-access locks are read-locks (shared locks) and write-locks (exclusive locks). Below are the rules for read-locks and write-locks : A transaction is allowed to read an object if and only if it is holding a read-lock or write-lock on that object.
At least one of the actions is a write operation. The actions access the same object (read or write). [4] [5] Equivalently, two actions are considered conflicting if and only if they are noncommutative. Equivalently, two actions are considered conflicting if and only if they are a read-write, write-read, or write-write conflict.
In computer science, in the field of databases, write–write conflict, also known as overwriting uncommitted data is a computational anomaly associated with interleaved execution of transactions. Specifically, a write–write conflict occurs when "transaction requests to write an entity for which an unclosed transaction has already made a ...
In computer science, particularly the field of databases, the Thomas write rule is a rule in timestamp-based concurrency control. It can be summarized as ignore outdated writes . It states that, if a more recent transaction has already written the value of an object, then a less recent transaction does not need to perform its write since the ...