Editing Isolation (database systems) (section)

==DBMS concurrency control==
[[Concurrency control]] comprises the underlying mechanisms in a [[DBMS]] which handle isolation and guarantee related correctness. It is heavily used by the database and storage engines both to guarantee the correct execution of concurrent transactions, and (via different mechanisms) the correctness of other DBMS processes. The transaction-related mechanisms typically constrain the database data access operations' timing ([[database transaction schedule|transaction schedules]]) to certain orders characterized as the [[serializability]] and [[recoverability]] schedule properties. Constraining database access operation execution typically means reduced performance (measured by rates of execution), and thus concurrency control mechanisms are typically designed to provide the best performance possible under the constraints. Often, when possible without harming correctness, the serializability property is compromised for better performance. However, recoverability cannot be compromised, since such typically results in a [[Data integrity|database integrity violation]].

[[Two-phase locking]] is the most common transaction concurrency control method in DBMSs, used to provide both serializability and recoverability for correctness. In order to access a database object a transaction first needs to acquire a [[Lock (database)|lock]] for this object. Depending on the access operation type (e.g., reading or writing an object) and on the lock type, acquiring the lock may be blocked and postponed, if another transaction is holding a lock for that object.