1. Allocating Isolation Levels to Transactions by Alan Fekete
Presentation by Dr. Greg Speegle
November 10, 2005

2. Introduction Strict 2PL ensures serializability Snapshot isolation
Increases concurrency
Allows inconsistencies
Both supported by DBMS
What happens when both used?

3. Review of Strict 2PL Transactions must request locks before access
All locks released at commit time
Shared (read) locks compatible
All other combinations incompatible

4. Intro to Snapshot Isolation
T.start timestamp when T starts
Read last committed value before start timestamp
T.commit timestamp assigned at commit
T commits unless
Ti commit time between T.start and T.commit
writeset(t) overlaps writeset(Ti)
“First committer wins”

5. Example Snapshot Isolation
Ti.start = 0; Tj.start = 1;
Ri[x] Wi[x] Rj[x] Cj Ri[y] Ci
CSR
Not allowed S2PL
Not recoverable

6. Example Snapshot Isolation
Ti.start = 0; Tj.start = 1
Ri[x] Wi[x] Rj[x] Wj[x] Cj Ri[y] Ci
Ci fails (Tj.commit = 2; Ti.commit = 3)
CSR
Not allowed S2PL
Not strict (or recoverable)

7. Example Snapshot Isolation
Ti.start = 0; Tj.start = 1
Ri[x] Rj[x] Wi[x] Ri[y] Rj[y] Wj[y] Ci Cj
Not allowed by S2PL
Not CSR
Recoverable, Cascadeless & Strict

8. Snapshot Isolation No waiting for reads Requires multiversion support
Possible non-serializable executions
Allows non-strict executions
Conclusion: Useful for very high read-to-write ratio transactions (read only?)

9. Problem Statement
Can Snapshot Isolation and Strict 2PL be used at the same time and allow only serializable schedules?
Trivial answer – Yes, if all transactions use S2PL!

10. Allocations T is the set of transactions
Allocation – the set of transactions executing under SI (denoted S)
T – S is the set of transactions executing under S2PL.
T is acceptable if every allowed execution is CSR

11. Interference Graph (IG)
Nodes are transactions
Edges are labeled
Ti -> Tj is exposed if
rset(Ti) overlaps wset(Tj) AND wset(Ti) does not overlap wset(Tj)
Ti -> Tj is protected if
wset(Ti) overlaps wset(Tj) OR
rset(Ti) does not overlap wset(Tj) AND wset(Ti) overlaps rset(Tj)

12. Interference Graph (IG)
Edges are independent of schedule
Edges always in pairs
T1 = R[x]R[y]W[y]
T2 = R[x]W[x]
T3 = W[y]
T1 -> T2 (exposed)
T2 -> T1, T1->T3, T3->T1 (protected)
No edge between T2 and T3

13. Exposed Edges Key Point:
Exposed edges represent exactly the possibility of conflicts which are permitted under snapshot isolation which may lead to non-CSR schedules

14. Example Snapshot Isolation
Ti.start = 0; Tj.start = 1
Ri[x] Rj[x] Wi[x] Ri[y] Rj[y] Wj[y] Ci Cj
Not allowed by S2PL
Not CSR
Recoverable, Cascadeless & Strict
Ti -> Tj & Tj -> Ti are both exposed

15. Pivot Tb is a pivot if there exists Ta and Tc Ta -> Tb is exposed
Tb -> Tc is exposed
Ta, Tb, and Tc are consecutive nodes in a cycle in IG

16. Example Snapshot Isolation
Ti.start = 0; Tj.start = 1
Ri[x] Rj[x] Wi[x] Ri[y] Rj[y] Wj[y] Ci Cj
Not allowed by S2PL
Not CSR
Ti -> Tj & Tj -> Ti are both exposed
Ti and Tj are both pivots

17. Acceptable Allocations
An allocation is acceptable iff all pivots are under S2PL.
Example: Ti.start = 0; Tj.start = 1
Ri[x] Rj[x] Wi[x] Ri[y] Rj[y] Wj[y] Ci Cj
Ti and Tj both pivots
Ti and Tj both under S2PL to be acceptable

18. Conclusion Given set of transactions T Given allocation S Build IG(T)
Find pivots, P
If P does not overlap S, then ok

19. Future Work Performance issues Build tools for DBMS use
Maximal S best?
Build tools for DBMS use
Consider additional SQL isolation levels