1. IS444: Modern tools for applications development
Dr. Azeddine Chikh

2. Chapter 5: Data Access classes and persistence
Part 2. VB.NET reminder
Chapter 5: Data Access classes and persistence

3. Objectives In this lesson, you will: Make objects persistent
Design a Data Access class
Communicate with a data access class
Use a relational database with VB .NET

4. Making Objects Persistent
Object persistence
The capacity to store and retrieve information from files
Two approaches to achieving persistence
Attribute storage
Object storage

5. Making Objects Persistent
Attribute storage
Involves
Retrieving attribute values from the instance
Writing attribute values to a file
Disadvantage
It is necessary to re-create the object

6. Making Objects Persistent
Options for storing and retrieving objects:
Attribute storage and retrieval using the StreamWriter and StreamReader
Object serialization
Attribute storage and retrieval using databases

7. Making Objects Persistent
Object serialization
An easy way to store and retrieve objects
Serialization process
Transforms an object into a stream that can be saved to a sequential file
Deserialization
Transforms the stream from the file back to the object state before the object was stored
Advantage
The object can be retrieved intact
It is not necessary to re-create the object

8. Making Objects Persistent
Databases
One or more files organized to help make queries
Can be used to make objects persistent
Organize data into tables that can be related to each other
Each table column represents an attribute
Each row represents a record
Structured Query Language (SQL)
A standard set of keywords and statements used to access relational databases

9. Designing a Data Access Class
Purpose of a DA class
To provide methods that store and retrieve data
Make instances of a PD class persistent
Reasons for placing data storage and retrieval tasks in a DA class
Data input and output code are isolated from other classes
Can dramatically reduce maintenance
Separate classes for each tier make deployment easier in a client-server environment
GUI, PD, and DA functions may reside on multiple machines at various sites

10. Designing a Data Access Class
DA methods can be invoked only by the PD class
Services provided by the DA class appear to be provided by the PD class
Hides the DA class from all other classes

11. Data Access Methods A separate DA class is written for each PD class
Example:
CourseDA
Provides data storage and retrieval services for courses
Methods can only be invoked by the PD class

12. Data Access Methods Four basic tasks provided by CourseDA and Course:
Retrieve a course
Store a course
Change a course’s data
Remove a course

13. Data Access Methods CourseDA class Find method AddNew method
Functionality implemented: retrieve a course
AddNew method
Functionality implemented: store a course
Update method
Functionality implemented: update a course’s data
Delete method
Functionality implemented: remove a course

14. Data Access Methods CourseDA class (continued) Additional methods
Initialize
Terminate
GetAll
Exception handler classes
NotFoundException
DuplicateException

15. Communicating with a Data Access Class
Course class
Invokes methods in CourseDA 
Methods
Find
AddNew
Update
Delete
Initialize
Terminate
GetAll

16. Finding a Course The PD Find method Invokes the DA Find method
Public Shared Function Find(ByVal CourseId As String) As Course
Return CourseDA.Find(CourseId)
End Function

17. Adding a Course The PD AddNew method
Invokes the AddNew method in the DA class to store a new course instance
Public Sub AddNew()
CourseDA.AddNew(Me)
End Sub

18. Changing a Course The PD Update method
Invokes the Update method in the DA class
Public Sub Update()
CourseDA.Update(Me)
End Sub

19. Deleting a Course The PD Delete method
Invokes the Delete method in the DA class
Public Sub Delete()
CourseDA.Delete(Me)
End Sub

20. Additional Problem Domain Methods
Additional PD methods
Initialize
Public Shared Sub Initialize()
CourseDA.Initialize()
End Sub
Terminate
Public Shared Sub Terminate()
CourseDA.Terminate()

21. Additional Problem Domain Methods
Additional PD methods (continued)
GetAll
Public Shared Function GetAll() As ArrayList
Return CourseDA.GetAll
End Function

22. Using Relational Databases with VB .NET
Referred to as database management systems (DBMSs)
Provide tools for organizing data into tables
In a DBMS
Each column represents a field
Each row represents a record
Primary key: a field used to uniquely identify a record

23. Structured Query Language
Structured Query Language (SQL)
A standardized language used to manage and query relational databases
Used by DA methods
Find method uses the SQL SELECT statement to
Retrieve a specific course’s record from the database
AddNew method uses the SQL INSERT INTO statement to
Add a new course’s record to the database

24. Structured Query Language
Uses of SQL by DA methods (continued)
Update method uses the SQL UPDATE statement to
Change the contents of one or more fields in a course’s record
Delete method executes the SQL DELETE statement
SQL DELETE statement specifies the key value of the course to be deleted

25. Accessing a DBMS with VB .NET
VB .NET options for accessing data
ADO (Active-X Data Objects)
Maintains a constant connection to the server
Data is kept and processed in a recordset
ADO .NET
Only connects to the server when
Data is needed from the data store
Updates back to the data store are required
Relies on the concept of datasets as the basis for processing data

26. VB .NET Database Access Classes
VB .NET managed providers for accessing data:
The OleDb data provider
Designed for accessing Microsoft Access databases and other non-SQLServer data sources
Namespace: OleDb
The SQLServer data provider
Designed for accessing Microsoft SQLServer databases
Namespace: SQLClient
Object database connectivity (ODBC)
Can be used to access most other databases

27. VB .NET Database Access Classes

28. VB .NET and DBMS Example Example
Project involves working with data from a relational database
Selecting, adding, updating, and deleting rows from a table
The GUI includes
A data grid
Four buttons
Add Record
Update Record
Delete Record
Find

29. VB .NET and DBMS Example

30. Implementing Object Persistence with a Database
To implement object persistence with a relational database
Initialize method establishes a connection to the database
Find, AddNew, Update, Delete, and GetAll methods access the database directly
Terminate method closes the database connection

31. Summary Object persistence: storing instance data for future retrieval
Persistence can be achieved by storing either attribute values or entire instances
A data access (DA) class provides methods that store and retrieve data
Methods of a DA class are invoked only by the methods of the corresponding PD class
Methods of a DA class: Find, AddNew, Update, Delete, GetAll, Initialize, and Terminate

32. Summary
VB .NET’s stream approach to I/O views data input and output as a flow of bytes
Persistence can be achieved using sequential files, object serialization, or relational databases
A relational database is used to organize data into tables
In a relational database, each column represents a field and each row represents a record
SQL is a popular, standard language used to access relational databases