Timestamp-based concurrency control

Timestamp method (English timestamp ordering) are used in database systems to meet the demand of the isolation of the ACID principle in transactions.

Timestamp methods fall into the category of optimistic synchronization methods, ie it is assumed that at the start of a transaction from the fact that the probability of a conflict with a concurrent transaction ( TA) is low.

Transactions will only be reset when it came to a conflict, or may have occurred.

In order to recognize the conflicts that get transactions and objects timestamp. Now can be checked before each read or write access to an object if the object has already been changed from a parallel TA, ie, a conflict has arisen, otherwise the timestamps can be updated.

In practical applications, not apply before each access to an object, a validation instead, but the changes of a transaction are initially in a buffer, so that the actual insertion into the database (write to disk) only after successful validation phase ( before running the commit, the write operation ) is performed.

Basic validation techniques

For each transaction, the database system is a read- set ( set of objects, the TA has read them) and a write- set ( set of objects that have been written by the TA) out. There is a conflict between two transactions prior if and only if the Write and Read set of transactions involve the same objects.

We distinguish: