Practice and Applications of Data Management CMPSCI 345. Lecture 13: Transactions

Transcription

1 Practice and Applications of Data Management CMPSCI 345 Lecture 3: Transactions

2 Transactions } Problem: An applica0on must perform several writes and reads to the database, as a unit. } Example: Two people a>empt to book the last seat on a flight. } Solu0on: Mul0ple ac0ons of the applica0on are bundled into one unit called Transac0on } Transac0ons guarantee certain proper0es to hold that prevent such problems.

3 Transactions in Applications } Default: each statement = one transac0on } Mul0- statement transac0ons: START TRANSACTION [SQL statements] COMMIT or ROLLBACK (=ABORT) 3

4 Practice } Start psql instances on our prac0ce database 4

5 practice=# start transaction; START TRANSACTION practice=# practice=# select * from product; 5

6 practice=# start transaction; START TRANSACTION practice=# update product practice-# set price = price+0 practice-# where pname='gizmo ; practice=# select * from product; 6

7 ... practice=# select * from product; 7

8 ... practice=# select * from product; practice=# commit; COMMIT practice=# practice=# select * from product; 8

9 practice=# start transaction; START TRANSACTION practice=# practice=# start transaction; START TRANSACTION practice=# 9

10 ... practice=# update employees practice-# set phone = '34' practice-# where name= 'John';... practice=# update employees practice-# set phone = NULL practice-# where name= 'Cecilia'; 0

11 ... practice=# update employees practice-# set phone = '34' practice-# where name= 'John';... practice=# update employees practice-# set phone = NULL practice-# where name= 'Cecilia'; UPDATE practice=# update employees practice-# set phone = '9876' practice-# where name= 'John';

12 ... practice=# commit;... practice=# select * from employees;... practice=# rollback;

13 practice=# start transaction; START TRANSACTION practice=# practice=# start transaction; START TRANSACTION practice=# 3

14 ... practice=# update employees practice-# set phone = '0000' practice-# where name= 'John';... practice=# update employees practice-# set phone = NULL practice-# where name= 'Cecilia'; 4

15 ... practice=# update employees practice-# set phone = '0000' practice-# where name= 'John';... practice=# update employees practice-# set phone = NULL practice-# where name= 'Cecilia'; UPDATE practice=# update employees practice-# set phone = NULL practice-# where name= 'John'; 5

16 ... practice=# update employees practice-# set phone = ' ' practice-# where name= 'Cecilia'; 6

17 ... practice=# update employees practice-# set phone = ' ' practice-# where name= 'Cecilia'; ERROR: deadlock detected 7

18 ... practice=# update employees practice-# set phone = ' ' practice-# where name= 'Cecilia'; ERROR: deadlock detected practice=# commit; ROLLBACK... practice=# commit; COMMIT 8

19 practice=# start transaction; START TRANSACTION practice=# practice=# start transaction; START TRANSACTION practice=# 9

20 ... practice=# update employees practice-# set phone = ' ' practice-# where name= 'John';... practice=# select * from employees; 0

21 ... practice=# update employees practice-# set phone = ' ' practice-# where name= 'John'; practice=# commit; COMMIT Unrepeatable read... practice=# select * from employees;... practice=# select * from employees;

22 Isolation Levels in SQL. Dirty reads SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED. Commi>ed reads SET TRANSACTION ISOLATION LEVEL READ COMMITTED 3. Repeatable reads SET TRANSACTION ISOLATION LEVEL REPEATABLE READ 4. Serializable transac0ons SET TRANSACTION ISOLATION LEVEL SERIALIZABLE ACID

23 Choosing Isolation Level } Trade- off: efficiency vs correctness } DBMSs give user choice of level Always read DBMS docs Beware Default level is o]en NOT serializable Default level differs between DBMSs Some engines support subset of levels Serializable may not be exactly ACID 3

24 Isolation levels (PostgreSQL doc.) In Postgres: SELECT current_se_ng('transac0on_isola0on'); 4

25 READ-ONLY Transactions Client : START TRANSACTION INSERT INTO SmallProduct(name, price) SELECT pname, price FROM Product WHERE price <= 0.99 DELETE FROM Product WHERE price <=0.99 COMMIT Client : START TRANSACTION SET TRANSACTION READ ONLY SELECT count(*) FROM Product Can help DBMS improve performance SELECT count(*) FROM SmallProduct COMMIT 5

26 . Isolation Level: Dirty Reads Implementa0on using locks: } Long dura0on WRITE locks } A.k.a Strict Two Phase Locking } Do not use READ locks } Read- only transac0ons are never delayed Possible problems: dirty and inconsistent reads 6

27 . Isolation Level: Read Committed Implementa0on using locks: } Long dura0on WRITE locks } Short dura0on READ locks } Only acquire lock while reading (not PL) } Possible problems: unrepeatable reads } When reading same element twice, } may get two different values 7

28 3. Isolation Level: Repeatable Read Implementa0on using locks: } Long dura0on READ and WRITE locks } Full Strict Two Phase Locking } This is not serializable yet 8

29 practice=# start transaction; START TRANSACTION practice=# set transaction practice-# isolation level practice-# repeatable read; SET practice=# start transaction; START TRANSACTION practice=# set transaction practice-# isolation level practice-# repeatable read; SET 9

30 ... practice=# select count(*) practice-# from projects practice-# where empid=;... practice=# select count(*) practice-# from projects practice-# where empid=; 30

31 ... practice=# insert into projects practice-# values(,'web design'); INSERT 0... practice=# insert into projects practice-# values(,'web design'); INSERT 0 3

32 ... practice=# commit; COMMIT... practice=# commit; COMMIT 3

33 practice=# start transaction; START TRANSACTION practice=# set transaction practice-# isolation level practice-# serializable; SET practice=# start transaction; START TRANSACTION practice=# set transaction practice-# isolation level practice-# serializable; SET 33

34 ... practice=# select count(*) practice-# from projects practice-# where empid=;... practice=# select count(*) practice-# from projects practice-# where empid=; 34

35 ... practice=# insert into projects practice-# values(, Android game'); INSERT 0... practice=# insert into projects practice-# values(, Android game'); INSERT 0 35

36 ... practice=# commit; COMMIT... practice=# commit; ERROR: could not serialize access due to read/write dependencies among transactions 36

37 ... practice=# commit; COMMIT... practice=# commit; ERROR: could not serialize access due to read/write dependencies among transactions... practice=# select * from projects; 37