Optimistic Concurrency Control

1

• Optimistic Concurrency Control

2
Optimistic

• Updates are applied to local copies of the data
  (transaction workspace)
• If serializability is not violated, transactions
  are committed
• The DB is updated from local copies
• Otherwise T is aborted and restarted

3
Validation Concurrency Control Techniques (or
certification)

• Optimistic (validation) Concurrency Control
  Techniques (or certification)
• In other concurrency control techniques, checking
  is done to the DB before operations are executed
• e.g. TO, 2PL
• In optimistic, checking is done after

4
Optimistic

• 3 phases
• R phase - Read items from DB, updates to local
  copies
• validation phase - check for serializability
• W phase
• if validation successful, update DB     else,
  restart transaction

5
Optimistic

• If little interference, T's validated
  successfully and optimistic protocol works
  well     else lots of interference, so
  optimistic
• doesnt work well
• This protocol (there are others) uses TS's and
  W-sets and R-sets
• R, W sets different from R, W phase

6
Optimistic

• Validation phase
• For each Ti ready to commit and in validation
  phase,
• check for each Tj, committed or in validation
  phase 1)  Tj completes W-phase before Ti starts
  to read 2)  Ti starts W-phase after Tj
  completes W-phase and R-set of Ti has no items in
  common with W-set of Tj 3)  Both R-set and
  W-set of Ti, have no items in common with W-set
  of Tj and Tj completes R-phase before Ti
  completes R-phase

7
Optimistic

• If any 1 of the previous holds, no interference
•      Ti validated successfully,      Else Ti
  aborted and restarted later. 
• Interference may have occurred.

8
Examples

• Apply optimistic to these schedules
• R1(X)R2(X)R1(Y)W1(X)R2(Z)C1W2(Y)C2
• R1(X)R2(X)W1(Y)C1R3(Y)R4(W)W2(Z)C2W3(Z)R4(Y)W4(X)C
  3C4

9
Is Concurrency Control relevant?

• Check out the Data Concurrency and Consistency
  link for Oracle
• http//download-west.oracle.com/docs/cd/B10501_01/
  server.920/a96524/c21cnsis.htm2570