IT420: Database Management and
Organization
Session Control
Managing Multi-user Databases
24 March 2006
Adina Crăiniceanu
www.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
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<?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
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<?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
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<?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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?>
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ss3.php – Cannot Use Session Var
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<?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
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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
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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
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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 Without
Atomic Transaction
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Errors Prevented With
Atomic 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 Processing
with 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 twophase 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 Pessimistic
Locking
 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:
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Read uncommitted
Read committed
Repeatable read
Serializable
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Transaction Isolation Level
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