Concurrency Vu Tuyet Trinh trinhvt@it-hut.edu.vn Department of Information Systems, Faculty of Information Technology Hanoi University of Technology Example 500USD Account A Account B read(A) If A > 500 then B:=B+500 A:=A-500 Crash What happen ??? 2 fb.com/groups/ 1 Transaction  A sequence of read and write operations on data items that logically functions as one unit of work   Assuring data integrity and correction ACID Properties Concurrency Control Atomicity  Consistency  Isolation  Durability  Recovery 3 Automicity  guarantee that either all of the tasks of a transaction are performed or none of them are  Example T: Read(A,t1); If t1 > 500 { Read(B,t2); t2:=t2+500; Write(B,t2); t1:=t1-500; Write(A,t1); } crash 4 fb.com/groups/ 2 Consistency  ensures that the DB remains in a consistent state before the start of the transaction and after the transaction is over  Example A+B = C T: Read(A,t1); If t1 > 500 { Read(B,t2); t2:=t2+500; Write(B,t2); t1:=t1-500; Write(A,t1); } A+B = C 5 Isolation   ability of the application to make operations in a transaction appear isolated from all other operations. Example A= 5000, B= 3000 T: Read(A,t1); If t1 > 500 { Read(B,t2); t2:=t2+500; Write(B,t2); t1:=t1-500; Write(A,t1); } T’: A+B (= 5000+3500) (A+B = 4500+3500) 6 fb.com/groups/ 3 Durability guarantee that once the user has been notified of success, the transaction will persist, and not be undone  Ví dụ: A= 5000, B= 3000  T: Read(A,t1); If t1 > 500 { Read(B,t2); t2:=t2+500; Write(B,t2); t1:=t1-500; Write(A,t1); } crash 7 A= 4500, B=3500 Transaction States 8 fb.com/groups/ 4 Transaction Management Interfaces  Begin Trans  Commit ()  Abort()  Savepoint Save()  Rollback (savepoint) (savepoint = 0 ==> Abort) 9 Concurrency Control  Objective: ensures that database transactions are performed concurrently without the concurrency violating