Ch07 CE (Services)(1)

8/2/2019 Ch07 CE (Services)(1)

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Concepts of Database ManagementSeventh Edition

Chapter 7

DBMS Functions


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Objectives

Introduce the functions, or services, provided by aDBMS

Describe how a DBMS handles updating and

retrieving data Examine the catalog feature of a DBMS

Illustrate the concurrent update problem anddescribe how a DBMS handles this problem

Explain the data recovery process in a databaseenvironment

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Objectives (continued)

Describe the security services provided by a DBMS

Examine the data integrity features provided by aDBMS

Discuss the extent to which a DBMS achieves dataindependence

Define and describe data replication

Present the utility services provided by a DBMS

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Introduction

Some services provided by a DBMS

Data Access: Update and retrieve data

Support data independence

Data integrity features Concurrent update

Catalog

Security

Support data replication Data recovery

Utility services

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Access to Data: Client Server

User (or application) issues SQL commands to theDB server

5

Client /

Application

(e.g., ssms or

C++ app)

Database

ServerDatabase

Storage


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Update and Retrieve Data

Fundamental capability of a DBMS

Users or applications dont need to know how data

is stored or how to manipulate it

User (or program) makes a request that describeswhat needs to be accomplished

The DBMS interprets the request and does all thework to retrieve, add, update or delete data

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Making DB Requests

SQL Access (a non-procedural language)

QBE - Easy-to-use menu-driven interface

Access by non-DBMS software (usually

implemented with procedural languages) Intermixing SQL with procedural statements and

using a pre-compiler to translate to appropriateprocedure calls

Using an API (application program interface), usuallyimplemented as an object-oriented set of classesthat provide a procedural access to the database,incorporating SQL statements as strings as required.

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Microsoft Database APIs

9

Taken from

Wikipedia

MDAC

Supported

by many

platforms
http://en.wikipedia.org/wiki/File:MDAC_Architecture.svghttp://en.wikipedia.org/wiki/File:MDAC_Architecture.svg


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ADO Example Codehttp://www.timesheetsmts.com/adotutorial.htm

Private Sub Command1_Click()

declare ADO objectsDim conConnection As New ADODB.Connection

Dim cmdCommand As New ADODB.Command

Dim rstRecordSet As New ADODB.Recordset

open a connection

conConnection.ConnectionString = _

"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" & _

App.Path & "\" & "database.mdb;Mode=Read|Write"

conConnection.CursorLocation = adUseClientconConnection.Open

create a command

With cmdCommand Prepare SQL command

.ActiveConnection = conConnection

.CommandText = "SELECT * FROM tabTestTable;"

.CommandType = adCmdText

End With

use command to establish a record set

With rstRecordSet Use SQL to get cursor

.CursorType = adOpenStatic

.CursorLocation = adUseClient

.LockType = adLockOptimistic

.Open cmdCommand

End With

If rstRecordSet.EOF = False Then Use cursor

rstRecordSet.MoveFirst

10

Do See whats in the tableMsgBox "Record " & rstRecordSet.AbsolutePosition & " " & _

rstRecordSet.Fields(0).Name & "=" & rstRecordSet.Fields(0) & " " & _

rstRecordSet.Fields(1).Name & "=" & rstRecordSet.Fields(1)

rstRecordSet.MoveNext

Loop Until rstRecordSet.EOF = True

With rstRecordSet Add a row

.AddNew

.Fields(0) = "New"

.Fields(1) = "Record"

.Update

End With

rstRecordSet.MoveFirst Delete a row

rstRecordSet.Delete

rstRecordSet.Update

rstRecordSet.Close

Else

MsgBox "No records were returned using the query " & _

cmdCommand.CommandText

End If

reset objects

conConnection.Close

Set conConnection = Nothing

Set cmdCommand = Nothing

Set rstRecordSet = Nothing

End Sub


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Support Data Independence

Data independence: can change databasestructure without needing to change programs thataccess the database

Types of changes: Adding a field

Changing a field property (such as length)

Creating an index

Adding or changing a relationship

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Adding a Field

Only need to change programs that will use thenew field

Note: SQL SELECT * FROM command will present

the extra field Solution:

Many application program interfaces (API) make thistransparent, but for those that do not,

Instead of using *, list the required fields in any SQLSELECT commandthis is considered a bestpractice

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Changing the Length of a Field

Generally, dont need to change programs unless a

portion of a screen or report (or a variable used inthe program) that is set aside for the field is no

longer large enough to support new, larger datavalues.

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Creating an Index

To create an index, enter a simple SQL commandor select a few options

DBMSs use the new index transparently, as

appropriate, for best performance For some DBMSs, may need to make minor

changes in already existing programs in order totake advantage of new indexes

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Adding or Changing a Relationship

May need to modify applications

A better solution, though, is to provide for this sortof change in advance by creating views

incorporating tables whose business requirementscan change then, change the view at the sametime you change the relationshipthe change isthen transparent to any programs that use the

view.

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Provide Data Integrity Features

Business requirements dictate constraints related todata accuracy and consistency

Constraints may relate to tables (e.g., a column must beunique, or may contain only certain values), or

They may relate to relationships (e.g., a tuple in one tablecannot exist without a corresponding tuple in another), or

They may relate to contents of several tables, possiblynot even in a relationship (e.g., the inventory table mustbe updated whenever an order is generated)

A good DBMS enforces a wide variety of constraints,but programming may be required for complexconstraints.

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Provide Data Integrity Features(review)

Constraints enforced by good databases include

Key integrity Primary key

Foreign key

Data integrity Legal values

Data type

Domain / Format (Check)

Null allowed

Default

Uniqueness

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Provide Data Integrity Features

Four ways to handle integrity constraints:

1. Ignore it

2. Place responsibility on users

3. Place responsibility on programmers4. Place responsibility on DBMS

Number 4 is preferred; number 3 has issues, but is

better than not enforcing at all (essentially,

numbers 1 and 2)

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Support Concurrent Update

Concurrent update:multiple users make updatesto the same database at the same time

DBMS manages complex update scenarios

Ensures accuracy when several users updatedatabase at the same time

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The Concurrent Update Problem

FIGURE 7-4: Scenario 1Ryan and Elenas updates dont overlap (OK)

- continued on next slide - 20

Th C U d P bl


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The Concurrent Update Problem(continued)

FIGURE 7-5: Scenario 1 (continued) no overlapno problems

21

Ryan is

done with

his update

Th C t U d t P bl


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FIGURE 7-6: Scenario 2Ryans and Elenas updates overlap 22

Ryan and

Elena are

updating

at the

same time

Ryan adds

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FIGURE 7-6: Scenario 2 (continued)Ryans and Elenas updates overlap -

result in a lost update

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Elena

subtracts

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Avoiding the Lost Update Problem

Need to assure that updates dont overlap

inappropriately.

One approach: Batch processing

All updates done through a special program runperiodically

Problem: data becomes stale and may causeincorrect decisions regarding further transactions

Does not work in situations that require data to becurrent

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Batch Processing

FIGURE 7-7: Delaying updates to the Premiere Products database to avoid the

lost update problem

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DB Transaction

Transaction:set of steps completed by a DBMS toaccomplish a user task

A sequence of operations (e.g., SQL statements)

Brings the database from one consistentstate toanother (but partial completion would leave thedatabase in an inconsistent state)

Must be executed completelyor undone completely

SQL Server places the statements between BEGINTRANSACTION and {COMMIT | ROLLBACK}TRANSACTION (Oracle has implicit BEGIN onlyspecify COMMIT or ROLLBACK)

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Locking

Locking:deny other users access to data whileone users updates are being processed

The DBMS determines what locks are necessary

Generally at least two levels: read, write a read

lock allows other read locks (but not write locks), awrite lock allows neither a read nor a write lock.

Locks have granularity: database, table, block,record, field

The DBMS choses the lowest appropriate level of lock preferring table, block or record

If a user holds a number of locks at a given level, theDBMS may choose to promote it (if that doesnt

conflict with other users locks).

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Locking (continued)

FIGURE 7-8: The DBMS uses a locking scheme to apply Ryans and Elenas

updates to the database

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Locking (continued)


updates to the database (continued)

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Locking (continued)


updates to the database (continued)30

Ryan now

cannot

access the

record.


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Two-Phase Locking

Transactions usually require multiple locks

All the locks need to be held until the end of thetransaction to assure integrity

The DBMS accomplishes this using two-phaselocking:

Growing phase: DBMS requests new locks withoutreleasing those already held

Shrinking phase: DBMS releases locks withoutacquiring any new ones

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Deadlock

Deadlock or deadly embrace

Two users hold locks and require a lock on a resource that theother already has neither can procede

To minimize occurrence, the DBMS could try to lock records in

a consistent order applicable to all users, but it has no way topredict what data the user will access, let alone the order.

The DBMS must detect and break deadlocks when theyoccur

DBMS chooses one user to be the victim DBMS rolls back victims transaction, freeing up its locks for

other users transactions this usually requires the victimssoftware to detect the failure and reattempt the transaction.

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Deadlock (continued)

FIGURE 7-9: Two users experiencing deadlockchoose one as the victim

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Locking on PC-Based DBMSs

Usually more limited than locking facilities onenterprise DBMSs

Programs can lock an entire table or an individual

row within a table, but only one or the other Programs can release any or all of the locks they

currently hold

Programs can inquire whether a given row or table

is locked

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Timestamping

Time stamping is an alternative to locking DBMS assigns each database update a unique time

(timestamp) when the update started

Compares the stamp to decide victim

Advantages Avoids need to lock rows

Eliminates processing time needed to apply andrelease locks and to detect and resolve deadlocks

Disadvantages

Additional disk and memory space

Extra processing time

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ACID Propertiesto guarantee transaction reliability

Atomicity Transaction is all or nothing

Consistency Transaction brings DB from one consistent state to

another

Isolation No transaction can interfere with another

Durability Once committed, it will remain so no matter what

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Catalog Services

Metadata: data about data

Catalog stores metadata and makes it accessibleto users

A data dictionary is a view of the catalog,providing information about database objects.

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Security Services

Security: prevention of unauthorized access, eitherintentional or accidental, to a database

Most common security features used by DBMSs:

Authentication Authorization

Views

Encryption

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Authentication

Authentication:techniques for identifying the personattempting to access the DBMS

Password:string of characters assigned by DBA to auser that must be entered for access

Note that more secure options are available; e.g., integration with

Windows security is preferred for SQL Server.

Biometrics:identify users by physical characteristicssuch as fingerprints, voiceprints, handwritten signatures,and facial characteristics

Smart cards:small plastic cards with built-in circuitscontaining processing logic to identify the cardholder

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Authorization

DBA can use authorization rules to specify whichusers have what type of access to which data

Workgroups: groups (classes) of users defined to

simplify the authorization process Permissions:specify what kind of access the user

has to objects in the databasewe talked aboutthe GRANT command earlier in the course.

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Views

View: snapshot of certain data in the database at agiven moment in time

For most users or groups, the DBA grants access

to views, rather than the underlying tables, in orderto provide appropriate data security.

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Privacy

Privacy:right of individuals to have certaininformation about them kept confidential

Laws and regulations dictate some privacy rules

Companies institute additional privacy rules Privacy is enforced by views.

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Encryption

Encryption:converts data to a format indecipherable toother programs, and stores it in that form when writingor updating.

Decryption:reverses the encryption to get clear data.

DBMS uses encryption in several contexts: Passwords encryption or one-way hash

Data stored on disk - only DBMS may decrypt

prevents back-door access

transparent to legitimate users

Data transmitted between user and DBMS onlyendpoints see data in the clear

prevents snoopers from seeing data

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Two Types of EncryptionSymmetric

Symmetric uses a key

Key is relatively short (


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Two Types of EncryptionAsymmetric (aka. Public Key)

Asymmetric uses a key pair Two keys one private another public public key is

published (e.g., in a certificate) private key must beprotected (often in a separate device that does theactual encryption / decryption)

To encrypt (and send) data, encrypt with other partys

public key only that party can read it.

To sign (and publish) data, encrypt with private key anyone can use public key to decrypt (knowing its

from you)

Relatively slow Keys / blocks are large (>= 1KB)

Generally used to transmit symmetric keys, or digitalsignatures (an encrypted hash of the signed content)

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Two Types of Encryption

Symmetric and asymmetric are often used incombination

E.g., when using a https site, the browser chooses asymmetric key, encrypts it using the servers public

key, and sends it to the server (who is the only onethat can decrypt it); data is encrypted both directionsusing the symmetric key

Because symmetric keys are easier to break, the twosides may agree to change the symmetric key fromtime to time

Some secure transmissions require both sides tohave a public key pair smart cards provide onesecure mechanism to support this

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Data Replication

Replication: providing redundant copies (replicas) of

data often at different sites

for performance (at remote sites)

and/or recovery (of central site)

Synchronization: DBMS exchanges all updated databetween master database and a replica

Depending on requirements and cost, synchronization can bebatch or real time (or in between)

The complexity of synchronization depends on whether data

can be updated at more than one site.

This is often done by third-party software.

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Support Data Replication (continued)

FIGURE 7-18: DBMS synchronizes two databases in a replica set

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Recover Data

Recovery: returning database to a correct statefrom an incorrect state

This is usually required after a database failure(e.g., due to power failure, disaster, or malware

attack)

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Backup / Restore

Simplest recovery involves using backupsor replicas

Backup or save: copy of database

A full backup is often made on the order of once amonth (depending on the size and activity of thedatabase)

Partial backups are made at least daily theyinclude changes since the last full backup

Backups are always done, but are used onlyas a last resort since restoration of a largedatabase may take days (or even weeks)

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Recovery

Recovery: returning database to a correct state froman incorrect state

This is required after a database failure (e.g., due topower failure, disaster, or malware attack)

Simplest recovery involves using backups or replicas

Backups are always done, but recovery from backups is slow.

Replicas (often at a different site) lag the original, but are nottoo far behindprovides a hot standby for faster resumptionof service

No matter which is used, there is still a need to do someadjustments to assure integrity and to capture recenttransactions.

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Logging

Logging (aka Journalling): maintaining a log

(journal) of all updates to the database

Logs are usually replicated offsite and availableeven if database is destroyed

The DBMS writes to the log before writing changesto data.

The DBA must keep transaction logs goingback to the most recent backup (or at least to

what is known to be replicated at a hot backupsite)

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Logging

Information is logged for each transaction:

Each log entry includes:

Transaction ID

Date and time of each update

Log events include:

Start of a transaction

Before image(values of relevant data before a change)

After image (values of relevant data after a change)

Successful completion (commit) of a transaction

Checkpoint (i.e., periodic events where the DBMS stopsall activity and writes all information in cache to thedatabase, bringing the log in sync with the database)

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Logging

FIGURE 7-10: Four sample transactions

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Using Logs Logs can be used for recovery in two ways:

1. After a power failure, the DBMS uses the log to rollback transactions that were not committed, usingbefore-images.

2. After a recovery (from backups or using areplication site), the DBMS uses the log to rollforward transactions that were completed but arenot present in the recovered database (e.g., werenot on the most recent backup or not yet

replicated), using after-images.3. Recovery often requires a rollback phase followed

by a recovery phase since a replicated databasemay have partially completed transactions.

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Forward Recovery (continued)

FIGURE 7-12: Forward recovery

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Backward Recovery (continued)

FIGURE 7-13: Backward recovery

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Recovery on PC-Based DBMSs

Sophisticated recovery features not available onPC-based DBMSs

Regularly make backup copies using DBMS

Use most recent backup for recovery Systems with large number of updates between

backups

Recovery features not supplied by DBMS need to be

included in application programs

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Utility Services

Utility services assist in general databasemaintenance

Change database structure

Add new indexes and delete indexes Use services available from operating system

Export and import data

Support for easy-to-use edit and query capabilities,screen generators, report generators, etc.

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Summary

DBMS allows users to update and retrieve data in adatabase without needing to know how data isstructured on disk or manipulated

DBMS must store metadata (data about the data)

and make this data accessible to users DBMS must support concurrent update

Locking denies access by other users to data while

DBMS processes one users updates

During deadlockand deadly embrace, two or moreusers are waiting for the other user to release alock before they can proceed

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Summary (continued)

In timestamping, DBMS processes updates to adatabase in timestamp order

DBMS must provide methods to recover a

database in the event the database is damaged DBMSs provide facilities for periodically making a

backup copy of the database

Enterprise DBMSs maintain a log or journal of all

database updates since the last backup; log isused in recovery process

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Summary (continued)

DBMSs provide security features (encryption,authentication, authorizations, and views) toprevent unauthorized access to a database

DBMS must follow rules or integrity constraints (key

integrity constraints and data integrity constraints)so that it updates data accurately and consistently

DBMS must support data independence

DBMS must have facility to handle data replication DBMS must provide utility services that assist in

general maintenance of a database

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