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 Crash What happen ??? A:=A-500 2 goupee.com fb.com/groups/goupee 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 Atomicity Concurrency Consistency Control Isolation Recovery Durability 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; crash Write(B,t2); t1:=t1-500; Write(A,t1); } 4 goupee.com fb.com/groups/goupee 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); T’: A+B t1:=t1-500; (= 5000+3500) Write(A,t1); (A+B = 4500+3500) } 6 goupee.com fb.com/groups/goupee 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 goupee.com fb.com/groups/goupee 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 the data integrity guarantees that no effect of committed transactions is lost, and no effect of aborted (rolled back) transactions remains in the related database. Example T0: read(A); T1: read(A); A := A -50; temp := A *0.1; write(A); A := A -temp; read(B); write(A); B := B + 50; read(B); write(B); B := B + temp; write(B); 10 goupee.com fb.com/groups/goupee 5
Scheduling (2) (3) 11 (1) Serializability A schedule of a set of transactions is a linear ordering of their actions e.g. for the simultaneous deposits example: R1(X) R2(X) W1(X) W2(X) A serial schedule is one in which all the steps of each transaction occur consecutively A serializable schedule is one which is equivalent to some serial schedule goupee.com fb.com/groups/goupee 6
Lock Definition a synchronization mechanism for enforcing limits on access to DB in concurrent way. one way of enforcing concurrency control policies Lock types Shared lock (LS) readable but can not write Exclusive lock (LX): read and write UN(D): unlock Compatibility LS LX LS true false LX false false 13 Example T0: LX(A); T1: LX(A); read(A); read(A); A := A -50; temp := A *0.1; write(A); A := A -temp; LX(B); write(A) read(B); LX(B); B := B + 50; read(B); write(B); B:=B+temp; UN(A); write(B); UN(B); UN(A); UN(B); 14 goupee.com fb.com/groups/goupee 7
Well-Formed, two-phased transaction A transaction is well-formed if it acquires at least a shared lock on Q before reading Q or an exclusive lock on Q before writing Q and doesn’t release the lock until the action is performed Locks are also released by the end of the transaction A transaction is two-phased if it never acquires a lock after unlocking one i.e., there are two phases: a growing phase in which the transaction acquires locks, and a shrinking phase in which locks are released 2Phase Locking (2PL) Phase 1 locks are acquired and no locks are released Phase 2 locks are released and no locks are acquired BOT EOT t Phase lock Phase unlock goupee.com fb.com/groups/goupee 8
Example T T3 1 Lock(A) T Lock(B) 2 T4 Read(A) Lock(B) Read(B) Lock(A) Lock(B) Read(B) B=B-50 Read(A) Read(B) Lock(A) Write(B) Unlock(A) B:=B+A Read(A) Unlock(B) Lock(B) Write(B) Unlock(B) Lock(A) Read(B) Unlock(A) A:=A+B Read(A) Unlock(B) Unlock(B) Write(A) A=A+50 Pritn(A+B) Unlock(A) Write(A) Unlock(A) 2PL Not 2PL Deadlock T0: LX(B); (1) T1: LX(A); (4) read(B); (2) read(A); (5) B := B +50; (3) temp := A *0.1; (6) write(B); (8) A := A -temp; (7) LX(A); (10) write(A) (9) read(A); LX(B); A := A - 50; read(B); write(A); B:=B+temp; UN(A); write(B); UN(B); UN(A); UN(B); 18 goupee.com fb.com/groups/goupee 9
Resolving Deadlock Detecting Recovery when deadlock happen rollback Used waiting-graph Avoiding Resource ordering Timeout Wait-die Wound-wait Waiting Graph Graph Node handling lock or waiting for lock Edge TU U handle L(A) T wait to lock A T must wait until U unlock A If there exists a cycle in the waiting graph deadlok goupee.com fb.com/groups/goupee 10
Timeout Set a limit time for each transaction If time-out do rollback goupee.com fb.com/groups/goupee 11
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