Temple University – CIS Dept. CIS661 – Principles of Data Management V. Megalooikonomou Concurrency control- Deadlocks (based on slides by C. Faloutsos at CMU and on notes by Silberchatz,Korth, and Sudarshan)

General Overview Relational model - SQL Functional Dependencies & Normalization Physical Design &Indexing Query optimization Transaction processing concurrency control recovery

Transactions - dfn = unit of work, eg. Atomicity (all or none) move $10 from savings to checking Atomicity (all or none) Consistency Isolation (as if alone) Durability recovery concurrency control

Isolation - concurrency control serializability <-> correctness precedence graph automatically correct interleavings: locks + protocol (2PL, 2PLC) but: deadlocks!

Deadlocks detection handling (prevention)

Deadlock detection T1 T2 lock manager L(A) Yes L(B) Yes ... L(B) No <waits> L(A) No time DEADLOCK

Algo for deadlock detection? wait-for graph: nodes -> transactions arcs -> Tsource waits for Tsink if cycle, then deadlock! Must invoke a deadlock-detection algorithm periodically to look for cycles.

Eg: time for ‘B’ T1 T2 ‘A’ ‘B’ for ‘A’ T1 T2 lock manager L(A) Yes L(B) Yes ... L(B) No <waits> L(A) No time for ‘B’ T1 T2 ‘A’ ‘B’ for ‘A’

Another example is there a deadlock? if yes, which xacts are involved?

Another example now, is there a deadlock? if yes, which xacts are involved? T1 T2 T3 T4

Deadlock detection how often should we run the algo? how many transactions are typically involved?

Deadlock handling Q: what to do? T1 T2 T3 T4

Deadlock handling Q0: what to do? A: select a ‘victim’ & ‘rollback’ Q1: which/how to choose? T1 T2 T3 T4

Deadlock handling Q1: which/how to choose? A1.1: by age A1.2: by progress A1.3: by # items locked already... A1.4: by # xacts to rollback Q2: How far to rollback? T1 T2 T3 T4

Deadlock handling Q2: How far to rollback? A2.1: completely A2.2: minimally Q3: Starvation?? T1 T2 T3 T4

Deadlock handling Q3: Starvation?? A3.1: include #rollbacks in victim selection criterion. T1 T2 T3 T4

SQL statement usually, conc. control is transparent to the user, but LOCK <table-name> [EXCLUSIVE|SHARED]

Concurrency control - conclusions serializability <-> correctness automatically correct interleavings: locks + protocol (2PL, 2PLC, ...) deadlock detection + handling

serializable schedules serializable schedules Conclusions 2PL schedules serializable schedules serializable schedules 2PLC serial sch’s