Enterprise Architecture styles
Before we look into J2EE architecture,
it is necessary to consider the architectural styles of contemporary
distributed applications – 2-tier, 3-tier, and n-tier architecture. The purpose
of this section is mainly to enable
the reader to recognize the scope of and place
for these patterns.
Typically client-server systems are
based on the 2-tier architecture, where there is a clear separation between the data and the
presentation/business logic. Such systems are generally data-driven with the
server most of time being database server, and the client being a graphical user interface to operate on data. While this
approach allows us to share data
across the enterprise, it does have many drawbacks.
Single-Tier Architecture
Simple software applications are
written to run on a single computer. User inputs its verification, business
logic and data accessed all these could be found together on single computer,
this kind of architecture is called
Single Tier Architecture.
Because of all logic application
services, the presentation, business rules and
the data access
layers exist in a single computing layer.
Advantage:
Single tier system is relatively easy
to manage and data consistency is simple because data is stored at only one single location.
Disadvantage:
Single storage location is not
sufficient because of the changing business needs. With the single tier
application we cannot share the data in the large amount; it can also not
handle multiple users.
Two-Tier Architecture
§ Application
which is divided
into two separate tires, client
machine and database server machine is called as Two – Tier architecture application.
§ The application includes the presentation and
business logic. Data
is accessed by
connecting client machine to a database
which is lying on another machine.
Thin Client :
With the two tier architecture, if the
presentation manager resides only with the client tier then the client is
called as thin client. Other presentation logic, application logic, business
logic, data logic and database manager reside with the server side.
Thick Client :
|
Application
|
|
Business Logic
|
|
Database Logic
|
|
Database Manager
|
|
With the two tiers architecture if
the presentation manager, presentation
logic, application logic reside with the client tier then the client is called
as thick client. Others like business logic, data logic and database
manager reside
Normal Client :
With the two tires architecture if the
presentation manager and presentation logic reside with the client tier then
the client is called as normal client. Others like application logic, business
logic,
data logic and database manager reside with the
server side.
Advantages :
Any changes made in data access logic
will not affect the presentation and
business logic. With the two tier architecture it is easy to develop an application.
Disadvantages :
One of the disadvantages of Two – tier
architecture is that the application is expected to support a limited number of
users. The reason is that each client requires its own connection and
each connection requires CPU and
memory. As the number of connections increases, the database performance degrades.
§ Another typical problem with a 2-tired approach is
that of maintenance
§ Even the smallest of changes to an application
might involve a complete rollout to the entire user base.
§ Even if it’s
possible to automate the process, you are still faced with updating every
single client installation.
Three Tier Architecture
§ In 3-tier Architecture an application is broken up
into three separate logical layers, each with a well-defined set of interfaces.
§ The first tier is referred to as presentation
layer and typically consists of a
graphical user interface of some kind.
§ The middle tier, or business layer consists of the
application or business logic.
§ And the third
tier – the data layer
– contains the data that is needed for the application
§ The middle tier
(application logic ) is basically the code
that the user
calls upon to
retrieve the desired data.
§ The presentation
layer then receives the data and formats it for display.
§ This separation of application logic from the user
interface adds enormous flexibility to the design of the application.
§ Multiple user
interfaces can be built and deployed without ever changing the application
logic, provided the application logic presents a clearly defined interface to
the presentation layer.
§ J2EE provides
several abstractions to meet the needs of each of these tiers.
§ The third tier
contains the data that is needed for the application.
§ This data can
consist of any source of information,
including an enterprise
database such as
oracle or Sybase,
a set of XML documents,
or even a directory service.
§ In addition to
the traditional relational database storage mechanism, there are many different
sources of enterprise data that your applications can access.
Advantages :
It improves scalability since the
application servers can be deployed
on many machines.
The database no longer requires a
connection from every client – it only requires
connections from a smaller number of application servers. It
provides better reusability because client
does not have direct access to the database.
Disadvantage :
It increases the complexity because to
develop the three tier application is more difficult than developing a two tier
applications.
N-Tier Architecture
As the title suggests, there is no
hard and
fast way to define the application layers for an n-tier system.
In fact, this kind of system can
support a number of different configurations.
In an n-tier architecture the application
logic is logically divided by function, rather than physically.
§ A user
interface that handles
the user’s interaction with the
application – this can be
a web browser running
through a firewall,
a heavier desktop application, or
even a wireless device.
§ Presentation logic that defines what the user
interface displays and how a user’s requests are handled. Depending on what
user interfaces are supported, you may need
to have slightly
different versions of the presentation logic to handle the client appropriately.
§ Business logic that models the application’s
business rules, often through interaction with the application’s data.
§ Infrastructure
services that provide additional functionality required by the application
components, such as messaging, transactional support.
§ The data layer where the enterprise’s data resides.
Advantages :
§ Separation of
user interface logic
and business logic is done. Business
logic resides on small number of
centralized machines (may be just one).
§ Easy to
maintain, to manage, to scale, loosely coupled etc.
§ Additional
features can be easily added.
Disadvantages :
§ It is having
more complex structures and difficult to setup and maintain all the separated
layers.
