L07 Oranizing Domain Logic

Lecture 07
Organizing Domain Logic

Agenda
 Layering
 The Three Principal Layers
 Domain layer Patterns
– Transaction Script
– Domain Model
– Table Module
– Service Layer
 From Problem to Pattern
 Building the domain

Reading
 Fowler Introduction
– Thinking about Performance (p. 6-9)
 Fowler 1: Layering
 Fowler 2: Organizing Domain Logic
 Fowler 9: Domain Logic Patterns

Layering
 Software systems can get complicated
– Abstractions are needed
 Layering provides abstraction by separating
computer systems in layers
– Higher layers use services from
lower layers
– Each layer has dedicated tasks
and hides complexity from upper
layers

Benefits of Layering
 You can understand a single layer as a
coherent whole without knowing much about
other layers
 You can substitute layers with alternative
implementation of the same basic service
 You minimize dependencies between layers
 Layers make good places for standardization
 Once you have a layer built, you can use it for
many higher-level services

Downsides
 Layers encapsulate some, but not all, things well
– Cascading changes
– For example adding a field in the UI requires changes
on each layer
 Extra layers can harm performance
– At every layer things typically need to be transformed
from one presentation to another
 Organizational problems
– If difference teams work on different layers, code
duplications and other problems emerge

The Three Layers
 Presentation
– User’s interface to the system
– User can be another system
– Accepts input, displays views
 Domain
– The Application of the system
– The “Business logic”
– Tends to creep into presentation and data source
 Data Source
– Connection to the database
– Also Persistence

Where is the Business Logic?
 “Business Logic”
– or “Complex business illogic”?
– Business Rules including all special cases
– It is easy to think of business logic but difficult to spot
– many corner cases
– Also referred to as Domain Logic
 Enterprise software has a lot of “noise”
– “Plumbing” code around the domain logic
– Is getCustomer domain logic?
– Application Logic – or workflow

How to define Domain Logic?
 Definition
– If all environment (noise) is taken way, what is left should
be domain logic
– Environment is any middleware (web, data, ejb)
 Domain Logic should be
– Simple classes and interfaces – POJOs
– Easy to test!
– Easy to move around
– Easy to access
 Limited dependencies
– Dependencies can be reduced by injection

The Three Layers
 Dependencies
– Presentation gets information from lower layers,
preferable Domain Layer
– Domain or Data Source Layers should not depend
on the Presentation Layer
– Domain Layer depends on Data Source Layer
– Data Source Layer could use some objects from the
Domain Layer

Logic that is not Domain Logic
 Logic in different places

Presentation Logic
 How to display items, how things look
– Which colours to choose
– Number and Date formats, zero padding
 The presentation should
– Have easy and unified access to the data
– Avoid having assumptions on the data
– Avoid the need to calculate based on the data
 Example view models
– HTML, Flash
– JSP, ASP

Data Source Logic
 How to query and store items
– Optimization
– Data manipulation for upper layer
 The Data Source Layer should
– Provide simple interface to the domain layer
– Maintain speed and integrity
– Hide the database layout from the object model
 Example data model
– DAO
– Stored Procedures

Application Logic
 Having to do with application responsibilities
– “Workflow” logic
– Noise
 Example:
– Notifying contract administrator
– Checking HTTP parameters and selecting which
controller to pass the responsibility to

Interfaces
 Users can be other programs or services
– Presentation is an external interface that you provide
as a service to others
 Data can be other systems
– Data source is an external interface of services that
you use

 Create a specific Layer for the Domain Logic
– Well know patterns like Domain Model
– An object model of the domain that incorporates both
behaviour and data
– Object-oriented programming!!!

 Flooding of logic...
 If you don’t provide a place for the Domain Logic
it will find a place – usually in the wrong!

Enterprise Web Layers
 Clients usually run on the user’s machine
– Separate tier
– Web Browser or Rich Internet Application (RIA)
– Can also be processes or other applications
 Web Components run on a Web Server
 Domain Components run on Application Server
– Web Server or EJB Server for EJBs
 Data Source Components run on the same
Application Server (as the domain)
 Database is usually on a separate tier

Domain Layer
 Provides the business logic of the application
– Not part of the Presentation Layer nor the Data
Source layer

Domain Layer Patterns
 Transaction Script
– Organizes business logic by procedures where each
procedure handles single request from the
presentation
 Domain Model
– An object model of the domain that incorporates both
data and behaviour
 Table Module
– One class that provides domain logic to table or view
in database

Transaction Scripts
Organizes business logic by procedures
where each procedure handles a single request
from the presentation
 Most business applications can be thought of as
a series of transactions
– A Transaction Script organizes all this logic primarily
as a single procedure

Transaction Scripts
 Works well if model is simple
– Small amount of logic
– No state needed
– Moving data between presentation and database
 Problems
– Code duplication between transactions
– Common code tends to be duplicated
– Since no state is used, and each transaction is
separate from any other, there might be too many
calls to database layer

Transaction Scripts
 Revenue recognitions example
– One script handles all the logic
– Uses a data gateway to access data

Domain Model
An object model of the domain that
incorporates both behavior and data
 Rules and logic describe many different cases
and slants of behavior
 Web of interconnected objects
– Where each object represents some meaningful entity
– Dependencies between objects
 How it works
– Object that represent data (value objects) and
business rules (behavior)

Domain Model
 Simple model
– Similar to the database design
– Simple to map to the database
 Rich model
– Different from the database
design
– Better for solving some
complex logic and doing
calculation

Domain Model
 Revenue recognitions example
– Multiple classes each with different responsibility
– Each class has data and logic to calculate

Table Module
A single instance that handles the business logic for
all rows in a database table or view
 Organizes domain logic with one class per table in
the database
– Single instance of the class contains various procedures
that will act on the data
– One object per table handle
 How it works
– One class that provides domain logic to table or view in
database
– A Domain Model will have one order object per order while
a Table Module will have one object to handle all orders

Table Module
 When to use it
– Useful when application is centered on data
– Objects in the model are similar to the database

Making a Choice
 Pattern depends on
– Complexity of the domain logic and
– How the domain maps to the database

Service Layer
Defines an application’s boundary with a layer of
services that establishes a set of available
opertaions and coordinates the application’s
response in each operation
 Defines an application's boundary
– Provides a set of available operations from the
perspective of interfacing client layers
– Encapsulates the application's business logic

Service Layer
 Domain façade
– Provides thin interface
to the Domain Model
– Does not implement
any business logic
 Operation Script
– Implementation of
application logic
– Use the Domain Model
for domain logic

I want to structure my domain layer and use a pattern that applies to each
situation:
A) The logic is rather extensive
B) The logic is simple and procedure based
C) The logic is moderate and is centered around the database
EXERCISE

From Problem to Pattern
How do I structure my domain layer?
A) The logic is rather extensive
Domain Model

Domain Model
An object model of the domain that
incorporates both behaviour and data
 Rules and logic describe many different cases
and slants of behaviour
 Web of interconnected objects
– Where each object represents some meaningful entity
– Dependencies between objects
 How it works
– Object that represent data (value objects) and
business rules (behaviour)

B) The logic is simple and procedure
based
Transaction Script

C) The logic is moderate and is
centered around the database
Table Module

How do I give my domain logic
distinct API?
Service Layer

Service Layer
Defines an application’s boundary with a layer of
services that establishes a set of available
operations and coordinates the application’s
response in each operation
 Defines an application's boundary
– Provides a set of available operations from the
perspective of interfacing client layers
– Encapsulates the application's business logic

Problem – Ru Movie DB (rumdb)

 We need to build a solution
– We know the problem – sort of
 To get to the solution
– What patterns to use?
Rumdb
The RU Movie Database
SolutionProblem

Solution?
 How to we get to this?

Building the Domain
 Objects – the nouns
– Movie, Ratings, Channel, …
 User Stories
– What you do with the objects
 Example
– As a Operator, I want to add new movie
– As a User, I want to get all movies for a specific
channel
– As a User, I want to rate specific movie

Movie Database Example
 The Service Layer
– MovieService Provides simple and clean interface for
manipulation of Movies
– External services can be injected into the Service
Layer

 MovieService
– Provides an interface to manipulate Movies
public class MovieService extends ApplicationService
{
private MovieDataGateway movieDataGateway;
public void addMovie(Movie movie) throws ServiceException
{
movie.initialize();
if(!movie.validate())
{
throw new ServiceException("Movie is invalid");
}
movieDataGateway.addMovie(movie);
getMailGateway().SendMessage("netfang@ru.is", "netfang@ru.is",
"New Movie", "New Movie was added!");
}
...

 ApplicationService
– Layered Supertype for the Service Layer
– Provides support for all service classes
public class ApplicationService
{
private MailGateway mailGateway;
public MailGateway getMailGateway()
{
return mailGateway;
}
public void setMailGateway(MailGateway mailGateway)
{
this.mailGateway = mailGateway;
}
}

 The Domain Model
– POJOs with attributes and operations

 Movie
– Object for describing and manipulating contents
– Has a Rating object which keeps count of views and
rates
public class Movie implements Comparable
{
private int id;
private String title;
private String link;
private String description;
private Date release;
private String director;
private Rating rating = new Rating();
private List<Validator> validators = new ArrayList<Validator>();

 Methods in Movie
public void initialize()
{
rating.reset();
clearValidators();
addValidator(new DefaultMovieValidator(this));
}
public boolean validate()
{
for(Validator v : validators)
{
if(!v.validate())
return false;
}
return true;
}

– Has validators – easily extendable according to the
Open-Close Principle
public void clearValidators()
{
validators.clear();
}
public void addValidator(Validator validator)
{
validators.add(validator);
}

public void view()
{
rating.incrementView();
}
public void rate(int rate)
{
rating.incrementRate(rate);
}
public double getAverageRate()
{
return rating.getAverageRate();
}

 Rating
public class Rating
{
private int views; // Number of requests
private int rates; // Number of rates
private int score; // Combined values of scores
public void incrementView()
{
views++;
}
public void incrementRate(int rate)
{
rates++;
score += rate;
}
public double getAverageRate()
{
return score/rates;
}

 TestSimple
public TestSimple()
{
MovieService movieService = new MovieService();
movieService.setMovieDataGateway(new MovieDataGatewayStub());
movieService.setMailGateway(new MailServerStub());
try {
movieService.addMovie(new Movie(1, "Movie 1”, "http1", "", new Date(), ""));
movieService.addMovie(new Movie(1, "Movie 2", "http2", "", new Date(), ""));
}
catch (ServiceException sex) { ... }
List<Movie> list = movieService.getMovies();
for (Movie movie : list)
{
System.out.println(movie);
}

Summary
 Layering
 Three Principal Layers
 Domain layer Patterns to choose from
– Transaction Script for simple business logic
– Domain Model for complex business logic
– Table Module for moderate business logic when you
have good tools around Record Set
– Service Layer to provide API
 Building the domain
 From Problem to Pattern

L07 Oranizing Domain Logic

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