Learn about common issues like Lost Update, Dirty Read, and Non-Repeatable Read.
When a DBMS allows multiple transactions to execute concurrently, it creates the potential for several problems if the isolation between them is not properly managed. These problems can compromise data integrity. The first is the Lost Update problem. This occurs when two transactions access and modify the same data item, and one of the updates is overwritten or 'lost'. For example, Transaction 1 reads a value, Transaction 2 reads the same value, Transaction 1 updates the value, and then Transaction 2 updates the value. The update made by Transaction 1 is lost. The second is the Dirty Read problem. This occurs when one transaction reads data that has been written by another transaction that has not yet committed. If the writing transaction subsequently aborts and rolls back its changes, the reading transaction is left with 'dirty' or invalid data that never officially existed in the database. The third is the Non-Repeatable Read problem. This happens when a transaction reads the same data item twice, but gets a different value each time because another committed transaction modified the data in between the two reads. A related issue is the Phantom Read problem, where a transaction re-runs a query and finds new rows that have been inserted by another committed transaction. These problems are why the 'Isolation' property of ACID is so important, and database systems use concurrency control mechanisms (like locks) to prevent them.