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The lost update problem: A second transaction writes a second value of a data-item (datum) on top of a first value written by a first concurrent transaction, and the first value is lost to other transactions running concurrently which need, by their precedence, to read the first value. The transactions that have read the wrong value end with ...
Concurrent use of shared resources can be a source of indeterminacy leading to issues such as deadlocks, and resource starvation. [7] Design of concurrent systems often entails finding reliable techniques for coordinating their execution, data exchange, memory allocation, and execution scheduling to minimize response time and maximise ...
This is a property of a system—whether a program, computer, or a network—where there is a separate execution point or "thread of control" for each process. A concurrent system is one where a computation can advance without waiting for all other computations to complete. [1] Concurrent computing is a form of modular programming.
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).
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 shared resource is a data object, which two or more concurrent threads are trying to modify (where two concurrent read operations are permitted but, no two concurrent write operations or one read and one write are permitted, since it leads to data inconsistency). Mutual exclusion algorithms ensure that if a process is already performing ...
For others, the application can implement an OCC layer outside of the database, and avoid waiting or silently overwriting records. In such cases, the form may include a hidden field with the record's original content, a timestamp, a sequence number, or an opaque token. On submit, this is compared against the database.
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)).