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A database transaction is a unit of work, typically encapsulating a number of operations over a database (e.g., reading a database object, writing, acquiring lock, etc.), an abstraction supported in database and also other systems. Each transaction has well defined boundaries in terms of which program/code executions are included in that ...
Optimistic concurrency control (OCC), also known as optimistic locking, is a non-locking concurrency control method applied to transactional systems such as relational database management systems and software transactional memory. OCC assumes that multiple transactions can frequently complete without interfering with each other.
This occurs nowhere in the above example, as there is no read after write. For each case in S where T j executes a write_item(X) after T i executes a read_item(X), create an edge (T i → T j) in the precedence graph. This results in a directed edge from T 1 to T 2 (as T 1 has R(A) before T 2 having W(A)).
Examples of such operations are requesting a read operation, reading, writing, aborting, committing, requesting a lock, locking, etc. Often, only a subset of the transaction operation types are included in a schedule. Schedules are fundamental concepts in database concurrency control theory. In practice, most general purpose database systems ...
The requirement of mutual exclusion was first identified and solved by Edsger W. Dijkstra in his seminal 1965 paper "Solution of a problem in concurrent programming control", [1] [2] which is credited as the first topic in the study of concurrent algorithms. [3] A simple example of why mutual exclusion is important in practice can be visualized ...
In databases and transaction processing, two-phase locking (2PL) is a pessimistic concurrency control method that guarantees conflict-serializability. [1] [2] It is also the name of the resulting set of database transaction schedules (histories).
Isolation is typically enforced at the database level. However, various client-side systems can also be used. It can be controlled in application frameworks or runtime containers such as J2EE Entity Beans [2] On older systems, it may be implemented systemically (by the application developers), for example through the use of temporary tables.
In databases, and transaction processing (transaction management), snapshot isolation is a guarantee that all reads made in a transaction will see a consistent snapshot of the database (in practice it reads the last committed values that existed at the time it started), and the transaction itself will successfully commit only if no updates it has made conflict with any concurrent updates made ...