Chapter 7: Isolation

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1 Handout #11 Chapter 7: Isolation Overview Isolation Concepts Isolation Theorem Degrees of Serializability Read Past and Notify Locks Lock Conversions Phantoms - Granular Locking - Static & Dynamic Key Range Locking Field Calls Escrow Locking CS346 - Transaction Processing Markus Breunig - 7 / 1 -

2 Isolation Concepts Schedule, History, Execution Serializable Schedules Locking Protocols - well formed (wf) lr i (w) r i (x) ur i (x) lw i (x) w i (x) uw i (x) - legal schedule conflicting locks not granted concurrently - two phase (2p) lpi(x) uqi(x) no unlocks no locks CS346 - Transaction Processing Markus Breunig - 7 / 2 -

3 Theorem well formed legal serializable schedule (degree 3 or 2.99) two phase CS346 - Transaction Processing Markus Breunig - 7 / 3 -

4 Degrees of Serializability Degree 3: Degree 2.99: Degree 2.1: Degree 2: Degree 1: Degree 0: serializable - 2p for exclusive and shared - wf for reads and writes - phantom protection repeatable reads - no phantom protection - same as 3 if no inserts/deletes cursor stability - 2p for exclusive, not 2p for shared - wf for read and write - crabbing enabled - save as 2.1 not no crabbing - 2p for exclusive, not 2p for read - wf for write, not wf for read - not 2p - wf for write CS346 - Transaction Processing Markus Breunig - 7 / 4 -

5 Degree 2 T i : lr i (x) r i (x) ur i (x) lp i (y) no dirty reads no lost updates but not serializable Degree 2 could cause problems with cursors Solution: crabbing do not release lock until we have secured next read lock no one can come in and modify the current one cursor stability, Deg. 2.1 CS346 - Transaction Processing Markus Breunig - 7 / 5 -

6 Read Past and Notify Locks Example declare cursor C on select * from R where cond for update read past. skips records that are locked Example notify from R where cond cond is on key field of R if lock on range specified by cond is released, we get triggered useful for waiting on empty table or set (e.g. server waiting for work) CS346 - Transaction Processing Markus Breunig - 7 / 6 -

7 Example for Deg. 2 causing non-serializable execution Constraint: A > B Initially: A=100, B=0 T1 Read A,B A=50 T2 Read A,B B = 70 CS346 - Transaction Processing Markus Breunig - 7 / 7 -

8 A Question From Last Year T1 slock A read A unlock A xlock A A=5 unlock A commit T2 xlock A A=10 unlock A commit is this Deg 2? lost update NO 2p exclusive, not 2p shared YES CS346 - Transaction Processing Markus Breunig - 7 / 8 -

9 Modified Example T1 slock A read A -> TEMP unlock A xlock A A=TEMP+5 unlock A commit T2 xlock A A=10 unlock A commit not a lost write as meant by book example does show Deg. 2 control use lock protocol definitions for degrees of concurrency! CS346 - Transaction Processing Markus Breunig - 7 / 9 -

10 Lock Conversions e.g. have S lock on object, want to upgrade to X request X lock e.g. have IS lock on object, want U lock what combined lock do I want? Lock Conversion Lattice X U SIX S IX IS CS346 - Transaction Processing Markus Breunig - 7 / 10 -

11 Phantoms locking of non-existent records Example 1: relation R with key K T1 : insert tuple K=5 T2 : insert tuple K=5 Execution: (a) T1, T2 read all records, see no K=5 (b) both insert tuples (c) key constraint violated we would like to lock the non-existing record K=5! CS346 - Transaction Processing Markus Breunig - 7 / 11 -

12 Example 2: relation R, records with field S object T = S R T1 reads all R records, stores T T2 inserts new record S=10, increments T by 10 Execution: (a) T1 reads all R records, sums S's (b) T2 runs and commits (c) T1 updates T CS346 - Transaction Processing Markus Breunig - 7 / 12 -

13 One Solution to Phantoms: Granular Locks use multi-level granularity e.g. DB R 1 R 2 R n T 11 T 1n T 21 T 2n T n1 T nn CS346 - Transaction Processing Markus Breunig - 7 / 13 -

14 Compatibility Matrix Compatibility Matrix for Granular Locks Granted Mode Requested Mode None IS IX S SIX U X IS IX S SIX U X CS346 - Transaction Processing Markus Breunig - 7 / 14 -

15 Another Solution for Phantoms: Key Range Locking both special cases of predicate locking: granular - lock sets key range - lock key range Example relation R, key K define ranges: (-, 0], (0,1], (1,2],... fixed ranges not related to index organization RULE: lock range first, then records very similar to granular locks how does it prevent phantoms? e.g. insert K=1.5 CS346 - Transaction Processing Markus Breunig - 7 / 15 -

16 Dynamic Key Range Locking range defined by records in file e.g. R 1 K 1 = 5 (-, 5] R 2 K 2 = 7 (5,7] R 3 K 3 = 13 (7,13] record & range: single lock "next key" "previous key" would be [5,7), [7,13), (13, ) Example: record X with K=5 (-,5] record Y with K=8 (5,8] T1: insert record Z with K=7 T2: insert record W with K=6 CS346 - Transaction Processing Markus Breunig - 7 / 16 -

17 Execution: (a) T1 locks (5,8], inserts record Z new ranges! (b) what does T2 lock? X K=5 (-,5] Z K=7 (5,7] T2 locked? Y K=8 (7,8] T1 locked and T1 locks (5,7] too! (b')t2 tries to get (5,7] and waits (c) T1 aborts! record Z disappears! (d) T2 is very confused! (d') when T2 wakes up, is needs to check if it has lock it was waiting for! CS346 - Transaction Processing Markus Breunig - 7 / 17 -

18 Field Calls T1:... if c>10 then c = c - 10 else abort... commit T1:... if c<= 10 then abort release c lock... commit violates 2p locking protocol, but here ok! commit processing: if c<=10 then abort c=c-10 CS346 - Transaction Processing Markus Breunig - 7 / 18 -

19 Example of one detail to worry about Initially: c=15 T1:... if c>10 then c=c-10 else abort... if c>8 then c=c-8 else abort... commit T1:... if c<=10 then abort... if c<=8 then abort... commit processing: if c<=10 then abort if c<=8 then abort c = c-10 c = c-8 Simple Solution: Disallow! CS346 - Transaction Processing Markus Breunig - 7 / 19 -

20 Escrow Locking Initially: c=15 reserved = 0 T1 exec: check c and reserv ed [c-reserved > need] if enough, reserve += need (update reserve without 2p locking!) T1 commit: do update CS346 - Transaction Processing Markus Breunig - 7 / 20 -

21 Summary Locking Theory and Application Advanced Locking Schemes Read Textbook about Time Domain Addressing & Update Mode Next: How to implement Locking! CS346 - Transaction Processing Markus Breunig - 7 / 21 -

Concurrency Control. R &G - Chapter 19

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