Presented by Dr. Greg Speegle

 Concurrency Control  Multiple Versions  Version number timestamp of writing transaction  Read last committed value  Ex. w0(x)c0R1(x)W1(x)R2(x)  First committer wins  Ex. w0(x)c0R1(x)W1(x)R2(x)W2(x)c2c1 => abort T1  rw conflicts  Ti reads x concurrent with Tj writes x  Popular in commercial databases

 Lower overhead  No read locks  No blocking between reads and writes  Ex: R1(x)R2(x)W1(x)  Non-serializable schedules allowed  R1(x)R1(y)R2(x)R2(y)W1(x)W2(y)c1c2  R1(y) T1->T2  R2(x) T2->T1  Write skew

 Performance better than 2PL  Correctness better than SI

 SI plus  Booleans InConflict and OutConflict for every transaction  SIRead locks  Does not block writes  Does not block on writes  Indicate rw-conflicts

 Request SIRead Lock  Write lock held by T2 => T1.InConflict = true & T2.OutConflict = true  For all versions (Ti) later than version read by T1 T1.InConflict = True Ti.OutConflict = True  Continue as in SI

 Request Write Lock (in SI)  SIREAD lock by T2 T1.OutConflict = true T2.InConflict = true  Create new version with timestamp T1  Continue as in SI

 T1.InConflict && T1.OutConflict => abort  Otherwise, same as SI

 R1(x)R1(y)R2(x)R2(y)W1(x)W2(y)  W1(x) sets T2.InConflict & T1.OutConflict  W2(y) sets T2.OutConflict & T1.InConflict  Commit request causes abort

 R1(x)R1(y)R2(x)R2(y)W1(x)c1W2(y)  SIREAD by T1 released at c1 problem  T2.OutConflict not set  Maintain locks past commit  Release when all concurrent transaction terminated

 Assume Not: Cyclic SG and follow protocol  All ww/wr conflicts => commit cycle  One rw conflict  WLOG Tn -> T0 is rw  T0 commits before Tn-1 commits, before Tn starts => rw not possible  Two consecutive rw conflict  InConflict && OutConflict

 Two non-consecutive rw conflicts (to appear paper)  WLOG, Tn->T0 & Ti->Tj  T0 & Tn concurrent, Ti & Tj concurrent  Path from T0 to Ti and Tj to Tn non-rw conflicts  T0 commits before Ti starts  Tj commits before Tn starts  Ti and Tj concurrent => T0 committed and Tn not started (not concurrent)

 Conservative scheduler  R1(x) W2(x)R3(y)W1(y)c1  Equivalent to T3 T1 T2  W2(x) sets T1.InConflict  W1(y) sets T1.OutConflict  Abort T1 at c1

 Positive Theoretical Results  Excellent Performance under High Contention  Some problems under Low Contention Due to False Positives  Future Work - study increased detail to reduce false positives