Yan Huang - CSCI5330 Database Implementation – Concurrency Control Concurrency Control I 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control T1 T2 … Tn DB (consistency constraints) 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Enforce Conflict Serializable Schedules Prevent cycles in precedence graph from occurring T1 T2 ….. Tn Scheduler DB 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control A locking protocol For transaction i Use li to lock an item Use ui to unlock the lock enforced by transaction i T1 T2 lock table scheduler 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Well behaved transactions Ti: … li(A) … pi(A) … ui(A) ... 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Example of a transaction performing locking T1: l1(A); read (A); u1(A); l1(B); read (B); u1(B); display(A+B) Sufficient to guarantee serializability ? 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Example: T2: Read(A) T3: Read(A) A  A+100 A  A2 Write(A) Write(A) Read(B) Read(B) B  B+100 B B2 Write(B) Write(B) Constraint: A=B 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Schedule A A B T2 T3 25 25 l1(A);Read(A) A A+100;Write(A);u1(A) 125 l2(A);Read(A) A Ax2;Write(A);u2(A) 250 l2(B);Read(B) B Bx2;Write(B);u2(B) 50 l1(B);Read(B) B B+100;Write(B);u1(B) 150 250 150 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Two-Phase Locking Protocol Phase 1: Growing Phase transaction may obtain locks transaction may not release locks Phase 2: Shrinking Phase transaction may release locks transaction may not obtain locks 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Ti = ……. li(A) ………... ui(A) ……... no unlocks no locks 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control # locks held by Ti Time Growing Shrinking Phase Phase 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control What happens to a transaction which tries to lock an item but failed? 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Schedule B T2 T3 l1(A);Read(A) A A+100;Write(A) l1(B); u1(A) l2(A);Read(A) A Ax2;Write(A); l2(B) Read(B);B B+100 Write(B); u1(B) l2(B); u2(A);Read(B) B Bx2;Write(B);u2(B); delayed 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control 2PL  conflict-serializable schedules? To help in proof: Definition Shrink(Ti) = SH(Ti) = first unlock action of Ti 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control First: Ti  Tj in S  SH(Ti) <S SH(Tj) Proof: Ti  Tj means that S = … pi(A) … ui(A) … lj(A) ... qj(A) … 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Then: (1) Assume P(S) has cycle T1  T2 …. Tn  T1 (2) By lemma: SH(T1) < SH(T2) < ... < SH(T1) (3) Impossible, so P(S) acyclic (4)  S is conflict serializable 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Deadlock To handle a deadlock one of T4 or T5 must be rolled back and its locks released. T4 T5 l3(B) read(B) write(B) l4(A) read(A) l4(B) l3(A) 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Starvation A transaction does not get its turn for a long time Example: A transaction may be waiting for a lock on an item, while a sequence of other transactions request and are granted an lock on the same item. The same transaction is repeatedly rolled back due to deadlocks. Concurrency control manager can be designed to prevent starvation. 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control 2PL and Deadlock Are schedules from 2PL transactions deadlock free? 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

2PL and Possible Schedules Does 2PL allow all possible conflict serializable schedules? 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control

Yan Huang - CSCI5330 Database Implementation – Concurrency Control Beyond this simple 2PL protocol, it is all a matter of improving performance and allowing more concurrency…. Shared locks Multiple granularity Inserts, deletes and phantoms Other types of C.C. mechanisms 3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control