L08 Data Source Layer

Lectures
L01 Enterprise Application Architecture
Introduction, 1
L02 Software Design
L03 Design Patterns Introduction
L04 Base Patterns 18
L05 Frameworks
L06 Process Design
L07 Organizing the Domain Layer 2, 9

Lectures
L08 Mapping to Relational Databases 3, 10
L09 Behavioral Design 3, 11
L10 Web Presentation 4, 14
L11 Putting it all together 8
L12 Concurrent Programming 5, 16
L13 Session State and Distribution Strategies 6, 7,
15, 16
L14 Summary and Conclusions

Reading
 Fowler 3 Mapping to Relational Database
 Fowler 10 Data Source Architectural Patterns
– Table Data Gateway (144)
– Row Data Gateway (152)
– Active Record (160)
– Data Mapper (165)
 Fowler 15 Distribution Patterns
– Data Transfer Object (401)
 Fowler 18 Base Pattern
– Record set (508)

Agenda
 Design Objectives
 Data Source Patterns
– Data Transfer Object (401)
– Record set (508)

Relational Databases
 We are dealing with relational databases
– Wide-spread and well understood
– SQL based
– These are important in enterprise software
 Alternatives
– NoSQL databases
– O/R Mappers

Connecting to Data Sources
 Programs need to interface the Data Source
– Usually this means relational databases
 Database vendors usually supply drivers for
database
 Rational databases use SQL language
– Fairly standard
Program
Database
classes
Driver Database

Objectives
 Hide SQL from the Domain Layer
 Access to database needs to ensure
– Speed and data integrity
– Concurrent access of many clients
 Database independence
– It can be an objective to keep the system independent
of particular database technology
 Data Source Layer needs to be maintainable
– Database will change

The Legacy Problem
 In most cases the data model exists
– The schema already exists
– We cannot assume that we create the schema
 Data tends to stick where it lends
– Cannot assume that our application controls the
schema
– The schema will likely outlive the application

The Usability Problem
 The Database API determines the usability of
the data access
– Should be easy to use
 The programming model is important
– Does matter how efficient and good a persistence
framework is, if it is complex and cumbersome to use
 Tools may help, but should not be used to
conceal excessive complexity
– If tools are required to generate data access the
programming model is likely to be complex

Using Databases
 Programmers tend to want to solve all problems
in their domain
– Should we solve all problems in our object domain?
– Should we write everything in Java or C#?
 Databases are good at what they do
– But it’s necessary to let them do it in a natural way

Database Code
 Database programming can be very repetitive
– Opportunities for reusability
– JDBC is too low-level
 Code is Bad!
– Don’t write code unless you have to
– Try to write code for the business layer
 Persistence Frameworks are difficult to build
– Use the frameworks that exist

Which of these statements is false
A) Data tends to stick where it lends
B) Database programming tends to be low-level
C) Objects tend to map nicely to the database
D) Database programming tends to be repetitive
QUIZ
✔

Domain Layer Patterns Recap
 Transaction Script
– Organizes business logic by procedures where each
procedure handles a single request from the
presentation
 Domain Model
– An object model of the domain that incorporates both
behaviour and data
 Table Module
– A single instance that handles the business logic for
all rows in a database table or view

Good Design
 Separate database code from other code
– Provide database classes to access the database
– All SQL code in the same place
– Factories for each database
– Use of Connection Pools
 Error handling
– SQLException is isolated in the Data Source Layer
– Wrap in domain specific exceptions – use of runtime
exceptions

Design Example
 Domain Model uses gateways

Useful Patterns
 Data Transfer Object (401)
– An object that carries data between processes in
order to deduce the number of method calls
 Record Set (508)
– An in-memory representation of tabular data

Data Transfer Object (401)
An object that carries data between processes in order
to deduce the number of method calls
 Object that is used to transfer data between
layers
– Data Source returns data objects to web layer

 How it Works
– Similar to Value Object but is constructed to carry
data between layers
– Data source layer creates DTO for transfer
– DTOs holds data – get/set method
– Can be mutable or immutable
– Could have methods to transform data – for example
serialize the data or convert to XML
– Simple Domain Objects can be used as DTO
• Creates dependencies

 Assembling DTO from domain objects
– Assembler reduces dependencies
 When To Use It
– Whenever you need to transfer multiple items of data
between two processes in a single method call

Record Set (508)
 An in-memory representation of tabular data
 How It Works
– Contains the result of a database query
– Common in ADO.NET and JDBC
– One record is current, clients can traverse the set
– Usually provided by the database code
 When to Use It
– When returning data from a query

Data Source Patterns
 Table Data Gateway (144)
– Acts as a Gateway to a database table
 Row Data Gateway (152)
– Acts as a Gateway to a single record in a data source
 Active Record (160)
– Wraps a row in a database table or
view, encapsulates the database access, and adds
domain logic on that data
 Data Mapper (165)
– A layer of Mappers that moves data between objects
and database while keeping them independent

Data Source Layer
 Domain layer has influence on the patterns
 Transaction Script
– Table Data Gateway for single access to a table
– Row Data Gateway for single access to a row of a
table
 Domain Model
– Active Record or Row Data Gateway
– Data Mapper
 Table Module
– Table Data Gateway with Record Set

Table Data Gateway (114)
An object that acts as a Gateway (446) to a
database table. One instance handles all the
rows in the table.
 Also called Data Access Objects – DAO
 How It Works
– Simple interface to a table with several find methods
and methods for maintaining data
– CRUD methods (Create, Read, Update, Delete)
– Acts as a gateway to a table
– One gateway for each table
– Finders return Collection of DTOs or Record Set

Table Data Gateway (114)
 When to Use It
– Works for Table Module (125) since it is based on
Record Set (509)
– Useful for web application where Domain Model (116)
is used

Row Data Gateway (152)
An object that acts as a Gateway (446) to a single
record in a data source. There is only one
instance per row.
 How It Works
– Object that is exactly one
single record
– Each table column is a
field in the object
– Do not have logic
– Finder object
– Can be generated

Row Data Gateway (152)
 When to Use It
– Works well for simple domain layer for example
Transaction Script

Active Record (160)
An object that wraps a row in a database table or
view, encapsulates the database access, and adds
domain logic on that data
 How It Works
– Each object can read and
store itself
– Contain domain logic
 When to Use It
– When Domain Logic is not
too complex
– When using Transaction Script
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Data Mapper (165)
A layer of Mappers that moves data between
objects and a database while keeping them
independent of each other and the mapper itself
 Sparates the in-memory objects from the
databse

Data Mapper
 How It works
– Simple Data Mappers map in-memory object to table on a
field-to-field basis
– Others need to map more complicated object hierarchies
to multiple tables
– Mapper uses Identity Map to see if object is already
loaded
 For insert and updates
– The mapper must know what objects have changed, which
are new, and which must be destroyed
– Unit of Work pattern

Data Mapper
 When to Use It
– Database and object model must be independent
– Data Mappers are useful with Domain Model
– For simple Domain Model an Active Record could
be used, but as it becomes more complicated some
mapping is needed
 O/R mapping solutions can provide the mappers
– For example Hibernate

Data Mapper
 Simple example
– Loading from the database

Data Mapper
 Simple example
– Updating data
– Client asks the mapper to save a domain object
– The mapper pulls the data out of the domain object
and saves to the database

Data Source class maps nicely to the rows in a table and
contains some useful methods
A) Row Data Gateway
B) Table Data Gateway
C) Active Record
D) Data Mapper
QUIZ
✔

Spring JDBC
 Spring JDBC packages
– org.springframework.jdbc
 datasource
– Classes for connecting to the database
 core
– Base classes for accessing the database –
JdbcTemplate
 object
– Classes that support updating, inserting and deleting
data from the database
 support
– Utility classes

Loading the DataSource
 Code
 Configuration – data.xml
Resource res = new FileSystemResource("data.xml");
XmlBeanFactory factory = new XmlBeanFactory(res);
DataSource ds = (DataSource)factory.getBean("dataSource");
<beans>
<bean id="dataSource“ class=
"org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName">
<value>net.sourceforge.jtds.jdbc.Driver</value>
</property>
<property name="url”>
<value>jdbc:jtds:sqlserver://honn.ru.is:1433</value>
</property>
<property name="username"><value>andri</value></property>
<property name="password"><value>abc123</value></property>
</bean>
</beans>

JdbcTemplate
 Main class of core package
– Simplifies queries
 Template Method pattern
– JdbcTemplate handles the
processing and calls our code
– Dependency Injection

JdbcTemplate Example
ParameterizedRowMapper<String> rm = new ParameterizedRowMapper<String>()
{
public String mapRow(ResultSet rs, int rowNum)
throws SQLException
{
return rs.getString("title");
}
};
JdbcTemplate tpl = new JdbcTemplate(getDataSource());
Collection<String> col = tpl.query("select * from contents", rm);
for(String s : col)
{
System.out.println(s);
}
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Collecting Data
 Spring Interface RowMapper
– An interface used by JdbcTemplate for mapping
returned result sets
– Class that implements this interface can be used to
collect data
public interface ParameterizedRowMapper<T> extends RowMapper<T>
{
Object mapRow(ResultSet rs, int rowNum) throws SQLException;
}

ContentRowMapper
public class ContentRowMapper implements
ParameterizedRowMapper<Content>
{
public Content mapRow(ResultSet rs, int rowNum) throws SQLException
{
Content content = new Content (rs.getInt (1), // id
rs.getString (2), // title
rs.getString (3), // link
rs.getString (4), // description
rs.getDate (5), // pubdate
rs.getString(6)); // author
return content;
}
}

Using ContentRowMapper
 JdbcTemplate method query takes
RowMapper interface as parameter
ContentRowMapper crm = new ContentRowMapper();
JdbcTemplate tpl = new JdbcTemplate(ds);
List l = tpl.query("select * from contents", crm);
Iterator i = l.iterator();
Content cont;
while (i.hasNext())
{
cont = (Content) i.next();
System.out.println(cont);
}

Insert
 SimpleJdbcInsert
– Class for inserts
public int add(Content content)
{
SimpleJdbcInsert insertContent =
new SimpleJdbcInsert(getDataSource())
.withTableName("contents")
.usingGeneratedKeyColumns("id");
Map<String, Object> parameters = new HashMap<String, Object>(5);
parameters.put("title", content.getTitle());
parameters.put("link", content.getLink());
parameters.put("description", content.getDescription());
parameters.put("pubdate", content.getPubDate());
parameters.put("author", content.getAuthor());
return insertContent.executeAndReturnKey(parameters).intValue();
}

RU Data Framework
Content Example

Content Example
 Table contents
– Contains content information
CREATE TABLE contents
(
id int Identity (1, 1) primary key NOT NULL,
title varchar(128),
link varchar(512) unique,
description text,
pubDate datetime,
author varchar(128),
)

Content Example
 Gateway class for the contents table
– ContentDataGateway interface contains the CRUD
operations
– Class ContentData implements the gateway and
provides the JDBC code
– Content is simple
JavaBean – acts as
Data Transfer
Object

RU Data Framework
 Classes and interfaces for accessing the
database
– Implementation of the Data Gateway
 For Table Data Gateway
– Each table has an Gateway interface
– Implementation in Data classes
– Factory pattern returns the implementation for each
Date Gateway

RuDataAccessFactory
 Factory for creating Gateway interfaces
– Reads information about the DataSource
– Spring Bean Definition file: data.xml
– Uses Spring Bean Factory
– RuDataAccessFactory reads information on each
gateway interface and which classes to use as
implementation
– Code using the gateway interface calls
getDataAccess in the factory class
factory = RuDataAccessFactory.getInstance("data.xml");
contentDataGateway = (ContentDataGateway)
factory.getDataAccess("contentDataAccess");

Data Definition
 File data.xml
<beans>

<bean id="dataSource"
class="org.springframework.jdbc.datasource.DriverManagerDataSource" >
<property name="driv erClassName">
<value>net.sourceforge.jtds.jdbc.Driver</value></property>
<property name="url">
<value>jdbc:jtds:sqlserver://honn.ru.is:1433</value></property>
<property name="username"><value>andri</value></property>
<property name="password"><value>abc123</value></property>
</bean>
<!– Content Gateway Interface -->
<bean id="contentGateway"
class="is.ru.honn.tube.data.content.ContentData"/>
</bean>
</beans>

DataSource
 DataSource is a connection to a database
– Driver Class Name (net.sourceforge.jtds.jdbc.Driver)
– URL (jdbc:jtds:sqlserver://honn.ru.is:1433)
– username (andri)
– password (abc123)
 RU framework uses Spring
– Load the DataSource information
– DriverManagerDataSource
extends
AbstractDataSource
which implements
DataSource

DataSource in Ru Framework
 RU Framework uses Spring to get DataSource

RuDataAccess
 RuDataAccess is base interface for Data
gateway interfaces
– All gateway interfaces extend RuDataAccess
– Has methods to set and get DataSource

RuData
 RuData is a class implementing RuDataAccess
– Handles DataSource
– Data classes extend this class

ContentDataGateway
 Contains all the method that are needed to
manage contents
– Gateway to the contents table
– Pattern Table Data Gateway
public interface ContentDataGateway extends RuDataAccess
{
public int add(Content content);
public List<Content> getContents();
}

ContentData
public class ContentData extends RuData implements ContentDataGateway
{
public int add(Content content)
{
SimpleJdbcInsert insertContent =
new SimpleJdbcInsert(getDataSource())
.withTableName("contents")
.usingGeneratedKeyColumns("id");
Map<String, Object> parameters = new HashMap<String, Object>(5);
parameters.put("title", content.getTitle());
parameters.put("link", content.getLink());
parameters.put("description", content.getDescription());
parameters.put("pubdate", content.getPubDate());
parameters.put("author", content.getAuthor());
return insertContent.executeAndReturnKey(parameters).intValue();
}

ContentData
public List getContents()
{
JdbcTemplate queryContent = new JdbcTemplate(getDataSource());
List<Content> contents = queryContent.query
("select * from contents", new ContentRowMapper());
return contents;
}

Usage
public class ContentServiceData implements ContentService {
private ContentDataGateway contentDataGateway = null;
public ContentServiceData()
{
RuDataAccessFactory factory = null;
try {
factory = RuDataAccessFactory.getInstance("data.xml");
} catch (RuException e) { ... }
contentDataGateway = (ContentDataGateway)
factory.getDataAccess("contentDataGateway");
}
public void addContent(Content content)
{
contentDataGateway.add(content);
}
public List<Content> getContents()
{
return contentDataGateway.getContents();
}
}

Contents
 Use ContentServiceData instead of a Service
Stub
– Change one line in the app.xml file
<bean id="contentService"
class="is.ru.honn.tube.service.ContentServiceData">
</bean>

Contents
 Use ContentServiceData instead of a Service
Stub
– Change one line in the app.xml file

Summary
 Design Objectives
– Object-relational impedance mismatch
– The Usability Problem
– The Legacy Problem
 Patterns
– Data Transfer Object (401) and Record set (508)

L08 Data Source Layer

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