Lecture 8 Transactions & Concurrency UFCE8K-15-M: Data Management

Transactions o ‘Transaction’ in a DBMS means an elemental unit of work which is either completed in full or fails totally o DB Transactions are not the same as a user transaction such as ‘place an order’ o Each transaction has o begin transaction o some work - reads and writes to the db o either COMMIT (make changes permanent) o or ROLLBACK (destroy all evidence of work) UFIE8K-15-M Data Management 2014/152

ACID o (A) Atomicity o Transaction is indivisible unit of work - can’t partly succeed o (C) Consistency o Transaction (failed or completed) must leave DB in a consistant state o (I) Isolation o Each transaction must appear to run in isolation to any other transactions o (D) Durability o Work done must be permanent UFIE8K-15-M Data Management 2014/153

Implementing ACID o (A) Atomicity o system must be able to undo work if transaction fails - e.g. due to failed integrity constraint o system could record the before state of an record and restore these when rollback required o (C) Consistency o fail if an integrity or transaction constraint violated o (I) Isolation o concurrency problem >> o (D) Durability o DB backup and recovery, transaction logging UFIE8K-15-M Data Management 2014/154

Concurrency Control o If transactions could be serialised - executed one at a time - each would execute in Isolation o but this would slow the system down - most of the time is spent waiting for disk access o If not prevented, interaction between transactions can cause anomalies UFIE8K-15-M Data Management 2014/155

Lost Update o – bot – read(x) – x=x+1 – write(x) – commit o if x=2 at start, it should be 4 at end o T2 – bot – read(x) – x=x+1 – write(x) – commit o but its only 3! UFIE8K-15-M Data Management 2014/156

Dirty Read o T1 – bot – read(x) – x=x+1 – write(x) – abort o x should be 3 o T2 – bot – read(x) – x=x+1 – write(x) – commit o but it is 4! UFIE8K-15-M Data Management 2014/157

Inconsistent Read o T1 – bot – read(x) – commit o T1 sees different values of x during its execution o T2 – bot – read(x) – x=x+1 – write(x) – commit UFIE8K-15-M Data Management 2014/158

Ghost update o x+y must = 100 o T1 – bot – read(y) – read(x) – ? x+y=90 – commit o T2 – bot – read(y) – read(x) – x=x -10 – write(x) – y=y+10 – write(y) – commit o now x+y = 100 again UFIE8K-15-M Data Management 2014/159

Scheduling o A schedule is a sequence of operations (reads or writes) from multiple transactions o Reads and writes are assumed to be atomic themselves o The Scheduler tries to create a schedule which preserves Isolation o Serial schedule puts one transaction after another, but this will be inefficient o Need to find equivalent, shorter schedules UFIE8K-15-M Data Management 2014/1510

Pessimistic/Optimistic o Pessimistic o assume transactions will interact and prevent it o Locking o Timestamping o Optimistic o assume transactions will NOT interact but take action if they do UFIE8K-15-M Data Management 2014/1511

Locking o Read lock (shared lock) o any number can read but no one can write o readers need counting o Write lock (exclusive lock) o no one else can read or write o Unlock - drop the lock o Transaction waits if resource already locked o Locks held in DB - lock table o Lock granularity o best if only a record is locked but lock table large UFIE8K-15-M Data Management 2014/1512

Two-phase locking o Discipline on transactions to ensure schedule is serialiable o Growing phase: o Transaction must acquire all its locks in one phase o Lock level can be escalated ( read > write) o Shrinking phase o Transaction must release all its locks in the second phase o Lock level can reduce (write > read) o Can’t acquire a lock after releasing a lock UFIE8K-15-M Data Management

Lost Update with Locking o T1 – bot – wlock(x) – read(x) – x=x+1 – write(x) – unlock(x) – commit o T2 – bot – wlock(x) – wait – read(x) – x=x+1 – write(x) – commit UFIE8K-15-M Data Management 2014/1514

Dirty Read with locking o T1 – bot – wlock(x) – read(x) – x=x+1 – write(x) – unlock(x) – abort o x should be 3 but it is still 4 ! o T2 – bot – wlock(x) – wait – read(x) – x=x+1 – write(x) – unlock(x) – commit UFIE8K-15-M Data Management 2014/1515

Strict Two-phase locking o Locks can only be released after commit o Deadlock o T1 wlocks(x) and waits to wlock(y) o T2 wlocks(y) and waits to wlock(x) UFIE8K-15-M Data Management 2014/1516

Deadlock o Deadlock – T1 wlocks(x) and waits to wlock(y) – T2 wlocks(y) and waits to wlock(x) o Approaches – timeout - lock expires so transaction aborts – deadlock detection - identifying deadlocks and killing one transaction UFIE8K-15-M Data Management 2014/1517