IS444: Modern tools for applications development Dr. Azeddine Chikh

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

VB .NET Database Access Classes

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

VB .NET and DBMS Example

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

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

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