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

Customer Accounthas

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

Concurrency Problems: Lost Update

T1 - read R1T2 - read R1T1 - write new R1T2 - write new R1

Concurrency Problems: Temporary Update or Dirty Read

T1 - read RT1 - update RT2 - read RT1 - rollback to original value of R

Concurrency Problems: Incorrect Summary

T1 - read

T2 - update

R1

R2

R3

R1R2 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 RT1 - read R - S lockT2 - request S lock on RT2 - read R - S lockT1 - request X lock - waitT2 - request X lock - wait

Concurrency with Locking:Temporary Update

T1 - request S lock on RT1 - read R - S lockT1 - request X lock on RT1 - update R - X lockT2 - request S lock on R - waitT1 - release X lock on RT2 - read R - S lock

Concurrency with Locking:Incorrect Summary (1)

T1 - request S lock on R1T1 - read R1 - S lockT1 - request S lock on R2T1 - read R2 - S lockT2 - request S lock on R3T2 - read R3 - S lock

Concurrency with Locking:Incorrect Summary (2)

T2 - request X lock on R3T2 - update R3 - X lockT2 - request S lock on R1T2 - read R1 - S lockT2 - request X lock on R1 - waitT1 - request S lock on R3 - wait

Concurrency with Locking:Deadlock

• Simplest example:T1 - write R1 - X lockT2 - write R2 - X lockT1 - request X lock on R2 - waitT2 - request X lock on R1 - wait