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IT420: Database Management and Organization
Managing Multi-user Databases29 March 2006
Adina Crăiniceanuwww.cs.usna.edu/~adina
Kroenke, Database Processing 2
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
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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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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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 WithoutAtomic Transaction
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Errors Prevented WithAtomic 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) Commit T2: R(item) 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 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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Statement Level Consistency
UPDATE CUSTOMER
SET AreaCode = ‘425’
WHERE ZipCode = ‘21666’
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Transaction Level Consistency
Start transactionUPDATE CUSTOMERSET AreaCode = ‘425’WHERE ZipCode = ‘21666’….other transaction workUPDATE CUSTOMERSET Discount = 0.25WHERE 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: 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