1 IT420: Database Management and Organization Session Control Managing Multi-user Databases 24 March...

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IT420: Database Management and Organization

Session ControlManaging Multi-user Databases

24 March 2006

Adina Crăiniceanuwww.cs.usna.edu/~adina

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Goals

Session control

Managing multi-user databases

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Session Control

HTTP – no built-in way to maintain state between two transactions

Want: Track a user during a single session on a website – remember state Show content personalized to user Implement shopping carts

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PHP Session Control

Session ID – cryptographically random number Generated for each session Stored on client site

Cookie URL

Session variables Created by PHP script Stored on the server site If session id visible (cookie or URL), session variables

can be accessed by all scripts

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Implementing Sessions in PHP

Start a session – session_start() Register session variables

$_SESSION[‘myvar’] = ‘some value’ Use session variables

session_start() if ( isset($_SESSION[‘myvar’] ) ) { //OK code}

Deregister variables unset($_SESSION[‘myvar’])

Destroy session session_destroy()

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Session Demo

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sesstart.php

<?php //Create session session_start(); //Create session variable - Save user name $_SESSION['login'] = $_POST[login]; //Display session variable echo 'Content of $_SESSION[\'login\'] is '.

$_SESSION['login']."<BR>"; echo '<br /><a href="ss1.php">page 1</a>'; ?>

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ss1.php

<?php // Use session variable session_start(); echo 'Content of $_SESSION[\'login\'] is '.

$_SESSION['login']." <BR>"; echo '<br /><a href="ss2.php">page

2</a>'; ?>

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ss2.php – Use, Unset

<?php // Use session variable session_start(); echo 'Content of $_SESSION[\'login\'] is '.

$_SESSION['login']." <BR>"; // Unset session variable- should not be visible

anymore unset($_SESSION['login']); echo 'We unset the session varible</br>'; echo '<br /><a href="ss3.php">page 3</a>'; ?>

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ss3.php – Cannot Use Session Var

<?php //Try use session variable session_start(); if (empty($_SESSION['login'])) echo 'Empty <br>'; else echo 'Not Empty <br>'; echo 'Content of $_SESSION[\'login\'] is '.

$_SESSION['login']." <BR>"; //Destroy session session_destroy(); ?>

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Class Exercise

Write PHP to implement db authentication Login page: get user info First page: check user Next pages: display only if user logged in Logout page

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Overview

Session control Managing multi-user databases

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Database Administration

All large and small databases need database administration

NCLCA database (small DB) Both “user” and “administrator” Easy to change and manage

What about large, multi-user DB? Much more difficult to manage May require a staff to manage (if large enough)

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DBA Tasks

Managing database structure Controlling concurrent processing Managing processing rights and responsibilities Developing database security Providing for database recovery Managing the DBMS Maintaining the data repository

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Managing Database Structure

Participate in database and application development Assist in requirements stage and data model creation Play an active role in database design and creation

Facilitate changes to database structure Seek community-wide solutions Assess impact on all users Provide configuration control forum Be prepared for problems after changes are made Maintain documentation

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Concurrency Control

Concurrency control: ensure that one user’s work does not inappropriately influence another user’s work No single concurrency control technique is

ideal for all circumstances Trade-offs need to be made between level of

protection and throughput

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Atomic Transactions

A transaction, or logical unit of work (LUW), is a series of actions taken against the database that occurs as an atomic unit Either all actions in a transaction occur or none of

them do

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Errors Introduced WithoutAtomic Transaction

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Errors Prevented WithAtomic Transaction

Make changes permanent

Undo changes

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Concurrent Transaction

Concurrent transactions: transactions that appear to users as they are being processed at the same time

In reality, CPU can execute only one instruction at a time Transactions are interleaved

Concurrency problems Lost updates Inconsistent reads

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Concurrent Transaction Processing

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Lost-Update Problem

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Inconsistent-Read Problem

Dirty reads – read uncommitted data

Unrepeatable reads

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Resource Locking

Resource locking prevents multiple applications from obtaining copies of the same record when the record is about to be changed

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Lock Terminology

Implicit locks are locks placed by the DBMS Explicit locks are issued by the application program Lock granularity refers to size of a locked resource

Rows, page, table, and database level

Large granularity is easy to manage but frequently causes conflicts

Types of lock An exclusive lock prohibits other users from reading the locked

resource A shared lock allows other users to read the locked resource,

but they cannot update it

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Concurrent Processingwith Explicit Locks

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Serializable Transactions

Serializable transactions refer to two transactions that run concurrently and generate results that are consistent with the results that would have occurred if they had run separately

Two-phased locking is one of the techniques used to achieve serializability

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Two-phased Locking

Two-phase locking Transactions are allowed to obtain locks as necessary

(growing phase) Once the first lock is released (shrinking phase), no

other lock can be obtained Strict two-phase locking

All locks are released at the end of transaction (COMMIT or ROLLBACK)

More restrictive but easier to implement than two-phase locking

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Deadlock

Deadlock: two transactions are each waiting on a resource that the other transaction holds

Preventing deadlock Allow users to issue all lock requests at one time Require all application programs to lock resources in the same

order Breaking deadlock

Almost every DBMS has algorithms for detecting deadlock When deadlock occurs, DBMS aborts one of the transactions

and rollbacks partially completed work

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Deadlock

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Optimistic versus PessimisticLocking Optimistic locking assumes that no transaction conflict

will occur: DBMS processes a transaction; checks whether conflict

occurred: If not, the transaction is finished If so, the transaction is repeated until there is no conflict

Pessimistic locking assumes that conflict will occur: Locks are issued before a transaction is processed, and then the

locks are released

Optimistic locking is preferred for the Internet and for many intranet applications

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Optimistic Locking

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Pessimistic Locking

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Declaring Lock Characteristics

Most application programs do not explicitly declare locks due to its complication

Instead, they mark transaction boundaries and declare locking behavior they want the DBMS to use Transaction boundary markers: BEGIN, COMMIT, and

ROLLBACK TRANSACTION

Advantage If the locking behavior needs to be changed, only the lock

declaration need be changed, not the application program

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Marking Transaction Boundaries

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ACID Transactions

Acronym ACID transaction is one that is Atomic, Consistent, Isolated, and Durable

Atomic means either all or none of the database actions occur

Durable means database committed changes are permanent

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ACID Transactions

Consistency means either statement level or transaction level consistency Statement level consistency: each statement

independently processes rows consistently Transaction level consistency: all rows impacted by

either of the SQL statements are protected from changes during the entire transaction With transaction level consistency, a transaction may not see

its own changes

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ACID Transactions

Isolation means application programmers are able to declare the type of isolation level and to have the DBMS manage locks so as to achieve that level of isolation

SQL-92 defines four transaction isolation levels: Read uncommitted Read committed Repeatable read Serializable

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Transaction Isolation Level