[concurrent processing] Optimistic concurrency control

Optimistic concurrency control


More specifically, OCC transactions involve these phases:

• Begin: Record a timestamp marking the transaction's beginning.
• Modify: Read and write database values. 
• Validate: Check whether other transactions have modified data that this transaction has used (read or wrote). Always check transactions that completed after this transaction's start time. Optionally, check transactions that are still active at validation time. 
• Commit/Rollback: If there is no conflict, make all changes part of the official state of the database. If there is a conflict, resolve it, typically by aborting the transaction, although other resolution schemes are possible. 

Inventor:
http://en.wikipedia.org/wiki/HT_Kung