BACS 485—Database ManagementConcurrency Control

Concurrency Control

Overview of Database Concurrency Control

BACS 485—Database ManagementConcurrency Control

Lecture Objectives Learn the basic concepts associated

with concurrency control in a relational database environment.

Learn about serial and serializable schedules.

Learn the basic concurrency control protocols

Learn the basics of locks and timestamps as there are used in concurrency control.

BACS 485—Database ManagementConcurrency Control

Concurrency Background Modern databases gain their

usefulness by sharing data among many users.

The cost of sharing data is concurrency problems.

Modern DBMSs deal with concurrency problems by controlling the interaction among users to maintain database consistency (integrity).

BACS 485—Database ManagementConcurrency Control

Concurrency Background There are 5 basic background

concepts related to concurrency control: Multiprogramming Data Sharing Concurrent access vs. integrity

conflict Transactions Levels of concurrency control

BACS 485—Database ManagementConcurrency Control

Multiprogramming Multiprogramming is the process of

interleaving (shuffling) the operations of several programs in order to better utilize the system resources.

By quickly switching “contexts”, the illusion of simultaneous execution is possible on a single CPU computer.

BACS 485—Database ManagementConcurrency Control

Data Sharing Database systems share data to reduce

redundancy and gain better control through centralization.

Shared data that is dynamic (i.e., update, insert, delete) causes concurrency problems in database environments.

Strictly “read only” shared data does not cause concurrency problems.

BACS 485—Database ManagementConcurrency Control

Concurrency vs. Integrity There is a conflict between total

concurrent data access and database integrity.

The more concurrency that takes place, the potentially worse the integrity.

Conversely, the better the integrity, the less concurrency is possible.

The goal is to achieve a reasonable balance.

BACS 485—Database ManagementConcurrency Control

Transactions Transactions are a logical unit of

work in the database. By executing transactions, the

database moves from one consistent state to another.

Transactions are important is several areas of database theory (including concurrency).

BACS 485—Database ManagementConcurrency Control

Transactions Good transactions have the following

characteristics: Atomic – performed totally or not at all Consistency preservation – consistent state is

maintained before and after the transaction is executed.

Permanence – Changes that are “committed” are permanent. They cannot be lost.

Isolation – Transaction updates should not be visible outside of the transaction.

Serializability – Running interleaved transactions should produce the same result as running them in some serial order.

BACS 485—Database ManagementConcurrency Control

Levels of Control The concurrency mechanisms that will

be discussed in this slide set apply to multiple levels of the database.

Specifically, they can apply to: Data-item level Tuple level Table level Extent level Full database

BACS 485—Database ManagementConcurrency Control

Uncontrolled Concurrency Uncontrolled concurrency implies

that the transactions are interleaved randomly.

This causes 3 classic problems: Lost update problem Temporary update problem Incorrect summary problem

BACS 485—Database ManagementConcurrency Control

Lost Update Problem

T1 | T2 _____________________________________|____________________________

READ checking-balance |Balance = Balance - 50 |

| READ checking-balance | Balance = Balance + 100

WRITE checking-balance |READ savings-balance |

| WRITE checking-balance <-T1 updateBalance = Balance + 50 | lostWRITE savings-balance |

BACS 485—Database ManagementConcurrency Control

Temporary Update Problem

T1 | T2 _________________________________|____________________________

READ checking-balance |Balance = Balance - 50 | WRITE checking-balance |

| READ checking-balance | Balance = Balance + 100 | WRITE checking-balance

READ savings-balance |

T1 fails and must be restored -->

BACS 485—Database ManagementConcurrency Control

Incorrect Summary Problem

T1 | T2 ________________________________|____________________________

| Sum = 0| READ check-balance-1| Sum = Sum + Balance| ...

READ checking-balance-5 |Balance = Balance - 50 |WRITE check-balance-5 |

| READ check-balance-5| Sum = Sum + Balance| READ check-balance-6| Sum = Sum + Balance

READ checking-balance-6 |Balance = Balance + 50 |WRITE check-balance-6 |

How do you avoid these problems?

You can avoid the problems of uncontrolled concurrency by generating “serializable schedules.”

A “schedule” is a plan for transaction execution.

A “serial schedule” is one where there is no concurrency (one transaction after another).

A serializable schedule is one that allows instruction interleaving while still giving serial schedule results.

BACS 486—Advanced Database Management

Concurrency Control

Schedules assume… Transactions are individually correct If database consistent (i.e., correct)

before a transaction, it will be consistent after

Schedules preserve instruction order A serial execution of several

transactions is “correct” by definition Interleaved execution is “correct” it is

equivalent to any serial execution (called a “serializable schedule”)BACS 486—Advanced Database

ManagementConcurrency Control

Schedule ExampleExample: T1 transfers $50 from checking to savings while T2 takes 10% of checking and puts it into savings. Initial value checking $1000, savings $2000. 

T1 T2READ checking-balance READ checking-balanceBalance = Balance - 50 Temp = Balance * 10%WRITE checking-balance Balance = Balance - TempREAD savings-balance WRITE checking-balanceBalance = Balance + 50 READ savings-balanceWRITE savings-balance Balance = Balance + Temp

WRITE savings-balance

BACS 486—Advanced Database Management

Concurrency Control

Schedule ExampleSCHEDULE 1:

T1 | T2

________________________|____________________________READ checking-balance |Balance = Balance - 50 |WRITE checking-balance |READ savings-balance |Balance = Balance + 50 |WRITE savings-balance |

| READ checking-balance| Temp = Balance * 10% | Balance = Balance - Temp| WRITE checking-balance| READ savings-balance| Balance = Balance + Temp| WRITE savings-balance

Checking Value = $855 Savings Value = $2145

BACS 486—Advanced Database Management

Concurrency Control

Schedule ExampleSCHEDULE 2:

T1 | T2________________________|____________________________

| READ checking-balance| Temp = Balance * 10%| Balance = Balance - Temp| WRITE checking-balance| READ savings-balance| Balance = Balance + Temp| WRITE savings-balance

READ checking-balance | Balance = Balance - 50 |WRITE checking-balance |READ savings-balance |Balance = Balance + 50 |WRITE savings-balance |

 Checking Value = $850 Savings Value = $2150

BACS 486—Advanced Database Management

Concurrency Control

Locks and Protocols Types of simple Locks

Exclusive Locks Shared Locks (read locks) Update Locks

Lock Problems Deadlock Livelock

BACS 486—Advanced Database Management

Concurrency Control