Agenda. Transactions Concurrency & Locking Lock Wait Deadlocks IBM Corporation

Transcription

1 Agenda Transactions Concurrency & Locking Lock Wait Deadlocks IBM Corporation

2 Concurrency and Locking App C App D ID Name Age 3 Peter 33 5 John Mary Ann 55 Concurrency: Multiple users accessing the same resources at the same time Locking: Mechanism to ensure data integrity and consistency IBM Corporation

3 Locking Locks are acquired automatically as needed to support a transaction based on isolation levels COMMIT and ROLLBACK statements release all locks Two basic types of locks: Share locks (S locks) acquired when an application wants to read and prevent others from updating the same row Exclusive locks (X locks) acquired when an application updates, inserts, or deletes a row IBM Corporation

4 Problems if there is no concurrency control Lost update Uncommitted read Non-repeatable read Phantom read IBM Corporation

5 Lost update seat name 7C IBM Corporation

6 Lost update seat name 7C update set name = 'John' IBM Corporation

7 Lost update seat name 7C John update set name = 'John' IBM Corporation

8 Lost update seat name 7C John update set name = 'John' update set name = 'Mary' IBM Corporation

9 Lost update seat name 7C John Mary update set name = 'John' update set name = 'Mary' IBM Corporation

10 Lost update seat name 7C 7B? John Mary update set name = 'John' update set name = 'Mary' IBM Corporation

11 Uncommitted read (also known as dirty read ) seat name 7C IBM Corporation

12 Uncommitted read (also known as dirty read ) seat name 7C update set name = 'John' IBM Corporation

13 Uncommitted read (also known as dirty read ) seat name 7C John update set name = 'John' IBM Corporation

14 Uncommitted read (also known as dirty read ) seat name 7C John update set name = 'John' Select name from where seat is '7C' IBM Corporation

15 Uncommitted read (also known as dirty read ) seat name 7C John update set name = 'John' Select name from where seat is '7C' John IBM Corporation

16 Uncommitted read (also known as dirty read ) seat name 7C John update set name = 'John' Select name from where seat is '7C' John Roll back IBM Corporation

17 Uncommitted read (also known as dirty read ) seat name 7C update set name = 'John' Select name from where seat is '7C' John Roll back IBM Corporation

18 Uncommitted read (also known as dirty read ) seat name 7C update set name = 'John' Roll back Select name from where seat is '7C' Further Further processing processing in in App App B B uses uses incorrect incorrect / / uncommitted uncommitted value value of of John John John IBM Corporation

19 Non-repeatable read seat name 7C IBM Corporation

20 Non-repeatable read seat name 7C from IBM Corporation

21 Non-repeatable read seat name 7C 7C 7B from IBM Corporation

22 Non-repeatable read seat name 7C 7C 7B from update set name = 'John' IBM Corporation

23 Non-repeatable read seat name 7C John 7C 7B from update set name = 'John' IBM Corporation

24 Non-repeatable read seat name 7C John 7C 7B from from update set name = 'John' IBM Corporation

25 Non-repeatable read seat name 7C John 7C 7B 7B from from update set name = 'John' IBM Corporation

26 Non-repeatable read seat name 7C John 7C 7B 7B from from update set name = 'John' The The same same SELECT SELECT (read) (read) returns returns a a different different result: result: Less Less rows rows (in (in this this case case '7C' '7C' doesn't doesn't show show anymore). anymore). This This is is a a non-repeatable non-repeatable read read IBM Corporation

27 Phantom read seat name 7C Susan IBM Corporation

28 Phantom read seat name 7C Susan from IBM Corporation

29 Phantom read seat name 7C Susan 7B from IBM Corporation

30 Phantom read seat name 7C Susan 7B from update set name = NULL IBM Corporation

31 Phantom read seat name 7C 7B from update set name = NULL IBM Corporation

32 Phantom read seat name 7C 7B from from update set name = NULL IBM Corporation

33 Phantom read seat name 7C 7B 7C 7B from from update set name = NULL IBM Corporation

34 Phantom read seat name 7C 7B 7C 7B from update set name = NULL The The same same SELECT SELECT (read) (read) returns returns a a different different result: result: More More rows rows (phantom (phantom from rows, rows, in in this this case case '7C', '7C', is is shown) shown) This This is is a a phantom phantom read read IBM Corporation