Purpose of this guide
This guide is intended to give the principles for the
application design. They should be implemented as a good practice.
Layering
When layering an application, you should consider:
-
Action layer for processing data significant
regarding to the business
-
Model interface for carrying data significant to
the business
-
Transverse component for processing data that
are not significant for the business
An application shall have three important action layer
-
Data
-
Services
-
Exposure : HMI – WS - Reporting
Each of the interface between those layers is done by a set
of classes named model interfaces.
-
Data model: is the separation between DAO and
Service
-
Data Transfer Object model implements the model
for exchanging with exposure layer. You can have HMIDTO, WSDTO and Reporting
DTO. Actually, you can have as many
Apart from those two major class, you may have the
transverse layer
-
Utility classes, for sharing common routines in
classes
-
Formatting, internationalization and conversion
classes
-
System interface classes
Action layer
For the interface between action layers you shall use models
Data Layer
The data layer is typically in charge of communicating with
the lower layer. DAO communicates with services using the object model.
Service Layer
The service layer contains the actions that make sense for
the business. They are named upon there objective. For instance, the service
named EmployeeService will group the methods applying to an employee.
In large project, it is a good practice to have one service
for each Exposure Layer. For instance, you would have:
-
EmployeeHMIService for performing task linked to
HMI
-
EmployeeWSService for performing task linked to
the Web Service Apis
However, this rule maybe unworthy on small projects.
Services communicates with the exposure service using DTO
(data transfer objects)
This service separation is done thanks to packages
Model interfaces
Data Access Model
Data access model is something very close from the structure
in the database.
It is a good principle not to reuse the data model interface
for the exchange with the exposure layer in order to guarantee a good layer
separation.
Data Transfer Model
The data transfer model is used to connect with the exposure
layer. It only contains the data necessary to the upper level.
It has not to be uniquely bound to a same concept. For
instance, if you have a table displaying a list of employees with only name and
first name. You may have a bean with only two fields. While, if you want to
produce a screen with the employee detail, you may create a DTO with all the
employee fields.
It is a good practice to closely to transform the Data
Access Model with some specific methods of the service.
For instance, you would have two methods in the employee
service
-
EmployeeDetailHMIDTO transformEmployee2EmployeeDetailHMI(Employee)
-
EmployeeListHMIDTO transformEmployee2EmployeeListHMI(Employee)
