1 IT420: Database Management and Organization Managing Multi-user Databases 29 March 2006 Adina...

IT420: Database Management and Organization

Managing Multi-user Databases29 March 2006

Adina Crăiniceanuwww.cs.usna.edu/~adina

Kroenke, Database Processing 2

Managing multi-user databases

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

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.

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

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>’;}

PHP – Delete Session Variables

unset($_SESSION[‘myvar’]) Delete session variable myvar

$_SESSION = array(); Delete ALL session variables

Overview

Miscellaneous Managing multi-user databases

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)

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

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

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

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

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

Errors Introduced WithoutAtomic Transaction

Errors Prevented WithAtomic Transaction

Make changes permanent

Undo changes

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.

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

Concurrent Transaction Processing

Lost-Update Problem

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

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

Resource Locking

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

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?

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

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

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

Deadlock

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

Optimistic Locking

Pessimistic Locking

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

Marking Transaction Boundaries

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

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

Statement Level Consistency

UPDATE CUSTOMER

SET AreaCode = ‘425’

WHERE ZipCode = ‘21666’

Transaction Level Consistency

Start transactionUPDATE CUSTOMERSET AreaCode = ‘425’WHERE ZipCode = ‘21666’….other transaction workUPDATE CUSTOMERSET Discount = 0.25WHERE AreaCode = ‘425’End Transaction

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

Transaction Isolation Level

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

Cursor Summary

1 IT420: Database Management and Organization Managing Multi-user Databases 29 March 2006 Adina...

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