Accessing data Transactions

Agenda Questions from last class? Transactions concurrency Locking rollback

Transactions 1+ SQL statements that are performed completely, or not at all ACID properties –Atomic –Consistent –Isolation –Durability Why develop these?

Banking CustomerAccount has

Transactions T1 T2 T3 Checking Savings Loan T1 – CustomerA checks her balance and withdraws money from checking account T2 – An automatic withdrawal from CustomerA’s checking account goes towards a mortgage loan that she holds with the same bank T3 – An automatic transfer moves money from CustomerA’s checking account into her savings account

Airline Reservations CustomerReservation makes Ticket purchases converts to

Concurrency Problems: Lost Update T1 - read R1 T2 - read R1 T1 - write new R1 T2 - write new R1

Concurrency Problems: Temporary Update or Dirty Read T1 - read R T1 - update R T2 - read R T1 - rollback to original value of R

Concurrency Problems: Incorrect Summary T1 - read T2 - update R1 R2 R3 R1 R2 R3

Concurrency Problems: Phantom Record T1 - query DB, retrieve set of records T2 - insert new record that satisfies T1’s query

Dealing with concurrency (OR: how to handle the pressure!) Timestamp ordering Locking Multiversion protocols Optimistic protocols (validation protocol)

Locks Granularity – lock on table, record, page, ets. Escalation – raising granularity in the middle of a transaction Index locking – primarily for adding/deleting records, but also changes to index field

Locking: Binary Lock 2 states: locked or unlocked Rules: 1.Transaction must issue lock on item before doing any reads or writes of it. 2.Transaction must issue unlock of item after completing all reads and writes of it.

Locking: Multiple-mode Lock 2 types of locks: Shared, S, or Read lock. Any number of S locks are allowed on an item. Exclusive, X, or Write lock. If an item is X- locked, there can be no other locks on it. Transaction must have appropriate lock on item before performing the action.

Locking: Multiple-mode Lock (1) Rules: 1.Transaction must issue S or X lock before reading an item. 2.Transaction must issue an X lock before writing an item. 3.Transaction must release locks after all reads and writes are completed.

Locking: Multiple-mode Lock (2) 4.Transaction may upgrade S lock to X lock if no one else has S lock. 5.Transaction may downgrade X lock to S lock when all writes are completed.

Intention locking Acts on next higher level – to write to a record, acquire intention lock on table and then acquire lock on record 3 types of locks –Intention read –Intention write –Read intention write Must check for locks on at least two levels

Concurrency with Locking: Lost Update T1 - request S lock on R T1 - read R - S lock T2 - request S lock on R T2 - read R - S lock T1 - request X lock - wait T2 - request X lock - wait

Concurrency with Locking: Temporary Update T1 - request S lock on R T1 - read R - S lock T1 - request X lock on R T1 - update R - X lock T2 - request S lock on R - wait T1 - release X lock on R T2 - read R - S lock

Concurrency with Locking: Incorrect Summary (1) T1 - request S lock on R1 T1 - read R1 - S lock T1 - request S lock on R2 T1 - read R2 - S lock T2 - request S lock on R3 T2 - read R3 - S lock

Concurrency with Locking: Incorrect Summary (2) T2 - request X lock on R3 T2 - update R3 - X lock T2 - request S lock on R1 T2 - read R1 - S lock T2 - request X lock on R1 - wait T1 - request S lock on R3 - wait

Concurrency with Locking: Deadlock Simplest example: T1 - write R1 - X lock T2 - write R2 - X lock T1 - request X lock on R2 - wait T2 - request X lock on R1 - wait