Software Architecture Patterns: Layered Architecture

The most common architecture pattern is the layered architecture
pattern, otherwise known as the n-tier architecture pattern. This
pattern is the de facto standard for most Java EE applications and
therefore is widely known by most architects, designers, and devel‐
opers. The layered architecture pattern closely matches the tradi‐
tional IT communication and organizational structures found in
most companies, making it a natural choice for most business appli‐
cation development efforts.

Pattern Description

Components within the layered architecture pattern are organized
into horizontal layers, each layer performing a specific role within
the application (e.g., presentation logic or business logic). Although
the layered architecture pattern does not specify the number and
types of layers that must exist in the pattern, most layered architec‐
tures consist of four standard layers: presentation, business, persis‐
tence, and database (Figure 1-1). In some cases, the business layer
and persistence layer are combined into a single business layer, par‐
ticularly when the persistence logic (e.g., SQL or HSQL) is embed‐
ded within the business layer components. Thus, smaller
applications may have only three layers, whereas larger and more
complex business applications may contain five or more layers.

Each layer of the layered architecture pattern has a specific role and
responsibility within the application. For example, a presentation
layer would be responsible for handling all user interface and

browser communication logic, whereas a business layer would be
responsible for executing specific business rules associated with the
request. Each layer in the architecture forms an abstraction around
the work that needs to be done to satisfy a particular business
request. For example, the presentation layer doesn’t need to know
or worry about how to get customer data; it only needs to display
that information on a screen in particular format. Similarly, the
business layer doesn’t need to be concerned about how to format
customer data for display on a screen or even where the customer
data is coming from; it only needs to get the data from the persis‐
tence layer, perform business logic against the data (e.g., calculate
values or aggregate data), and pass that information up to the pre‐
sentation layer.

One of the powerful features of the layered architecture pattern is
the separation of concerns among components. Components within
a specific layer deal only with logic that pertains to that layer. For
example, components in the presentation layer deal only with pre‐
sentation logic, whereas components residing in the business layer
deal only with business logic. This type of component classification
makes it easy to build effective roles and responsibility models into
your architecture, and also makes it easy to develop, test, govern,
and maintain applications using this architecture pattern due to
well-defined component interfaces and limited component scope.

Key Concepts

Notice in Figure 1-2 that each of the layers in the architecture is
marked as being closed. This is a very important concept in the lay‐
ered architecture pattern. A closed layer means that as a request
moves from layer to layer, it must go through the layer right below it
to get to the next layer below that one. For example, a request origi‐
nating from the presentation layer must first go through the busi‐
ness layer and then to the persistence layer before finally hitting the
database layer.

So why not allow the presentation layer direct access to either the
persistence layer or database layer? After all, direct database access
from the presentation layer is much faster than going through a
bunch of unnecessary layers just to retrieve or save database infor‐
mation. The answer to this question lies in a key concept known
as layers of isolation.
The layers of isolation concept means that changes made in one
layer of the architecture generally don’t impact or affect components
in other layers: the change is isolated to the components within that
layer, and possibly another associated layer (such as a persistence
layer containing SQL). If you allow the presentation layer direct
access to the persistence layer, then changes made to SQL within the

persistence layer would impact both the business layer and the pre‐
sentation layer, thereby producing a very tightly coupled application
with lots of interdependencies between components. This type of
architecture then becomes very hard and expensive to change.
The layers of isolation concept also means that each layer is inde‐
pendent of the other layers, thereby having little or no knowledge of
the inner workings of other layers in the architecture. To understand
the power and importance of this concept, consider a large refactor‐
ing effort to convert the presentation framework from JSP (Java
Server Pages) to JSF (Java Server Faces). Assuming that the contracts
(e.g., model) used between the presentation layer and the business
layer remain the same, the business layer is not affected by the refac‐
toring and remains completely independent of the type of userinterface framework used by the presentation layer.

While closed layers facilitate layers of isolation and therefore help
isolate change within the architecture, there are times when it makes
sense for certain layers to be open. For example, suppose you want
to add a shared-services layer to an architecture containing com‐
mon service components accessed by components within the busi‐
ness layer (e.g., data and string utility classes or auditing and logging
classes). Creating a services layer is usually a good idea in this case
because architecturally it restricts access to the shared services to the
business layer (and not the presentation layer). Without a separate
layer, there is nothing architecturally that restricts the presentation
layer from accessing these common services, making it difficult to
govern this access restriction.
In this example, the new services layer would likely reside below the
business layer to indicate that components in this services layer are
not accessible from the presentation layer. However, this presents a
problem in that the business layer is now required to go through the
services layer to get to the persistence layer, which makes no sense at
all. This is an age-old problem with the layered architecture, and is
solved by creating open layers within the architecture.

As illustrated in Figure 1-3, the services layer in this case is marked
as open, meaning requests are allowed to bypass this open layer and
go directly to the layer below it. In the following example, since the
services layer is open, the business layer is now allowed to bypass it
and go directly to the persistence layer, which makes perfect sense.

Leveraging the concept of open and closed layers helps define the
relationship between architecture layers and request flows and also
provides designers and developers with the necessary information to
understand the various layer access restrictions within the architec‐
ture. Failure to document or properly communicate which layers in
the architecture are open and closed (and why) usually results in
tightly coupled and brittle architectures that are very difficult to test,
maintain, and deploy.

Pattern Example

To illustrate how the layered architecture works, consider a request
from a business user to retrieve customer information for a particu‐
lar individual as illustrated in Figure 1-4. The black arrows show
the request flowing down to the database to retrieve the customer
data, and the red arrows show the response flowing back up to the
screen to display the data. In this example, the customer informa‐
tion consists of both customer data and order data (orders placed by
the customer).

The customer screen is responsible for accepting the request and dis‐
playing the customer information. It does not know where the data
is, how it is retrieved, or how many database tables must be queries
to get the data. Once the customer screen receives a request to get
customer information for a particular individual, it then forwards
that request onto the customer delegate module. This module is
responsible for knowing which modules in the business layer can
process that request and also how to get to that module and what
data it needs (the contract). The customer object in the business layer
is responsible for aggregating all of the information needed by the
business request (in this case to get customer information). This
module calls out to the customer dao (data access object) module in
the persistence layer to get customer data, and also the order dao
module to get order information. These modules in turn execute
SQL statements to retrieve the corresponding data and pass it back
up to the customer object in the business layer. Once the customer
object receives the data, it aggregates the data and passes that infor‐
mation back up to the customer delegate, which then passes that
data to the customer screen to be presented to the user.

From a technology perspective, there are literally dozens of ways
these modules can be implemented. For example, in the Java plat‐
form, the customer screen can be a (JSF) Java Server Faces screen

coupled with the customer delegate as the managed bean compo‐
nent. The customer object in the business layer can be a local Spring
bean or a remote EJB3 bean. The data access objects illustrated in
the previous example can be implemented as simple POJO’s (Plain
Old Java Objects), MyBatis XML Mapper files, or even objects
encapsulating raw JDBC calls or Hibernate queries. From a Micro‐
soft platform perspective, the customer screen can be an ASP (active
server pages) module using the .NET framework to access C# mod‐
ules in the business layer, with the customer and order data access
modules implemented as ADO (ActiveX Data Objects).

Considerations

The layered architecture pattern is a solid general-purpose pattern,
making it a good starting point for most applications, particularly
when you are not sure what architecture pattern is best suited for
your application. However, there are a couple of things to consider
from an architecture standpoint when choosing this pattern.
The first thing to watch out for is what is known as the architecture
sinkhole anti-pattern. This anti-pattern describes the situation where
requests flow through multiple layers of the architecture as simple
pass-through processing with little or no logic performed within
each layer. For example, assume the presentation layer responds to a
request from the user to retrieve customer data. The presentation
layer passes the request to the business layer, which simply passes
the request to the persistence layer, which then makes a simple SQL
call to the database layer to retrieve the customer data. The data is
then passed all the way back up the stack with no additional pro‐
cessing or logic to aggregate, calculate, or transform the data.

Every layered architecture will have at least some scenarios that fall
into the architecture sinkhole anti-pattern. The key, however, is to
analyze the percentage of requests that fall into this category. The
80-20 rule is usually a good practice to follow to determine whether
or not you are experiencing the architecture sinkhole anti-pattern. It
is typical to have around 20 percent of the requests as simple passthrough processing and 80 percent of the requests having some
business logic associated with the request. However, if you find that
this ratio is reversed and a majority of your requests are simple passthrough processing, you might want to consider making some of the

architecture layers open, keeping in mind that it will be more diffi‐
cult to control change due to the lack of layer isolation.
Another consideration with the layered architecture pattern is that it
tends to lend itself toward monolithic applications, even if you split
the presentation layer and business layers into separate deployable
units. While this may not be a concern for some applications, it does
pose some potential issues in terms of deployment, general robust‐
ness and reliability, performance, and scalability.

Pattern Analysis

The following table contains a rating and analysis of the common
architecture characteristics for the layered architecture pattern. The
rating for each characteristic is based on the natural tendency
for that characteristic as a capability based on a typical implementa‐
tion of the pattern, as well as what the pattern is generally known
for. For a side-by-side comparison of how this pattern relates to
other patterns in this report, please refer to Appendix A at the end
of this report.

Overall agility

Rating: Low
Analysis: Overall agility is the ability to respond quickly to a
constantly changing environment. While change can be isolated
through the layers of isolation feature of this pattern, it is still
cumbersome and time-consuming to make changes in this
architecture pattern because of the monolithic nature of most
implementations as well as the tight coupling of components
usually found with this pattern.

Ease of deployment

Rating: Low
Analysis: Depending on how you implement this pattern,
deployment can become an issue, particularly for larger applica‐
tions. One small change to a component can require a
redeployment of the entire application (or a large portion of the
application), resulting in deployments that need to be planned,
scheduled, and executed during off-hours or on weekends.
As such, this pattern does not easily lend itself toward a contin‐
uous delivery pipeline, further reducing the overall rating for
deployment.

Testability

Rating: High
Analysis: Because components belong to specific layers in the
architecture, other layers can be mocked or stubbed, making
this pattern is relatively easy to test. A developer can mock a
presentation component or screen to isolate testing within a
business component, as well as mock the business layer to test
certain screen functionality.

Performance

Rating: Low
Analysis: While it is true some layered architectures can per‐
form well, the pattern does not lend itself to high-performance
applications due to the inefficiencies of having to go through
multiple layers of the architecture to fulfill a business request.

Scalability

Rating: Low
Analysis: Because of the trend toward tightly coupled and mon‐
olithic implementations of this pattern, applications build using
this architecture pattern are generally difficult to scale. You can
scale a layered architecture by splitting the layers into separate
physical deployments or replicating the entire application into
multiple nodes, but overall the granularity is too broad, making
it expensive to scale.

Ease of development

Rating: High
Analysis: Ease of development gets a relatively high score,
mostly because this pattern is so well known and is not overly
complex to implement. Because most companies develop appli‐
cations by separating skill sets by layers (presentation, business,
database), this pattern becomes a natural choice for most
business-application development. The connection between a
company’s communication and organization structure and the
way it develops software is outlined is what is called Conway’s
law. You can Google “Conway’s law" to get more information
about this fascinating correlation.