IT420: Database Management and
Organization
Managing Multi-user Databases
29 March 2006
Adina Crăiniceanu
www.cs.usna.edu/~adina
1
Goals
 Managing multi-user databases
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Reminders/Announcements
 SAVE YOUR WORK TO X: DRIVE
 Project 2
 Individual project
 MySQL DBMS
 Part 1 due next Tuesday
 SQL queries
 Available today
 Due next Wednesday
 Exam next week, Friday
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PHP Miscellaneous
 int mysql_insert_id()
 Retrieves the ID generated for an
AUTO_INCREMENT column by the previous
INSERT query
 Return value:
 The ID generated for an AUTO_INCREMENT
column by the previous INSERT query on success
 0 if the previous query does not generate an
AUTO_INCREMENT value
 FALSE if no MySQL connection was established.
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PHP Miscellaneous
 array mysql_fetch_assoc(resource res)
 Returns associative array
 array mysql_fetch_row(resource res)
 Returns enumerated array
 array mysql_fetch_array(resource res)
 Return enumerated and associative array
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Example: Arrays
$result = mysql_query(“Select Name From
Users”);
while ($row = mysql_fetch_assoc($result)){
echo $row[‘Name’]. ‘</br>’;
}
while ($row = mysql_fetch_row($result)){
echo $row[0]. ‘</br>’;
}
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PHP – Delete Session Variables
 unset($_SESSION[‘myvar’])
 Delete session variable myvar
 $_SESSION = array();
 Delete ALL session variables
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Overview
 Miscellaneous
 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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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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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 - COMMIT
 Or none of them do - ABORT
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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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Transactions Examples
 Reserve an airline seat. Buy an airline
ticket.
 Withdraw money from an ATM.
 Verify a credit card sale.
 Order an item from an Internet retailer.
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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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DBMS’s View
 User A: Read item 100
Set count 5 Write item 100
 User B:
Read item 100
Set
count 7 Write item 100
 T1: R(item)
W(item)
 T2:
R(item)
Commit
W(item) Commit
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Inconsistent-Read Problem
 Dirty reads – read uncommitted data
 T1: R(A), W(A),
R(B), W(B), Abort
 T2:
R(A), W(A), Commit
 Unrepeatable reads
 T1: R(A),
R(A), W(A), Commit
 T2:
R(A), W(A), Commit
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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
 Exclusive lock (X)- prohibits other users from reading the
locked resource
 Shared lock (S) - allows other users to read the locked
resource, but they cannot update it
 When would you use exclusive vs. shared?
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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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Statement Level Consistency
UPDATE CUSTOMER
SET
AreaCode = ‘425’
WHERE ZipCode = ‘21666’
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Transaction Level Consistency
Start transaction
UPDATE CUSTOMER
SET
AreaCode = ‘425’
WHERE ZipCode = ‘21666’
….other transaction work
UPDATE CUSTOMER
SET
Discount = 0.25
WHERE AreaCode = ‘425’
End Transaction
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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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Cursor Type
 A cursor is a pointer into a set of records
 It can be defined using SELECT statements
 Four cursor types
 Forward only: the application can only move forward through
the recordset
 Scrollable cursors can be scrolled forward and backward
through the recordset
 Static: processes a snapshot of the relation that was taken when
the cursor was opened
 Keyset: combines some features of static cursors with some
features of dynamic cursors
 Dynamic: a fully featured cursor
 Choosing appropriate isolation levels and cursor types
is critical to database design
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Cursor
Summary
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