A-level Computing/AQA/Paper 2/Fundamentals of databases/Client server databases

Client server database system provides simultaneous access to the database for multiple clients through Database Management System (DBMS) with server software running on network server and client software

Know how concurrent access can be controlled to preserve the integrity of the database.
Concurrent access can result in the problem of updates being lost if two clients edit a record at the same time. This problem can be managed by the use of record locks, serialisation, timestamp ordering and commitment ordering

Record Lock
It is a technique that prevents simultaneous access to objects in database in order to prevent update being lost or arising data inconsistencies

Serialisation
Ensures that transactions do not overlap in time and therefore cannot interfere with each other or lead to updates being lost and implemented using the Timestamp ordering

Timestamp ordering
Timestamp is given to each transaction so that if 2 transaction affect the same object (record) the transaction with the earlier timestamp would be applied first and to ensure the data are not lost every object in database has a read and write timestamp which are updated whenever an object in database is read or written

Commitment Ordering
Serialisation technique ensures safe transaction without any lost of data when >2 users are simultaneously accessing the same object in database and ordered in terms of their dependencies on each other and the time they were initiated