• The goal?
‣ Component principles.
• Decoupling layers.
• Application-independent business rules. Entities.
• Application-specific business rules. Use cases.
• Interface adapters.
• Gateways. Frameworks and drivers.
• The clean architecture.
• The ultimate detail.
• A typical scenario.
3. THE GOAL?
The goal of software architecture is to minimize the human resources required to
build and maintain the required system.
Software was invented to be “soft.” It was intended to be a way to easily change the
behavior of machines. If we’d wanted the behavior of machines to be hard to
change, we would have called it hardware.
Good software systems begin with clean code. On the one hand, if the bricks aren’t
well made, the architecture of the building doesn’t matter much. On the other hand,
you can make a substantial mess with well-made bricks.
into classes, and how those classes should be interconnected.
The architecture of a software system is the shape given to that system by those who
build it. The form of that shape is in the division of that system into components, the
arrangement of those components, and the ways in which those components
communicate with each other.
A good architecture must support:
• The use cases and operation of the system.
• The maintenance of the system.
• The development of the system.
• The deployment of the system.
5. COMPONENT PRINCIPLES
A component is a grouping of related functionality behind a nice clean interface, which
resides inside an execution environment like an application.
If the SOLID principles tell us how to arrange the bricks into walls and rooms, then the
component principles tell us how to arrange the rooms into buildings. Large software
systems, like large buildings, are built out of smaller components.
• REP: The Reuse/Release Equivalence Principle. The granule of reuse is the granule
of release. This means that the classes and modules that are formed into a
component must belong to a cohesive group.
• CCP: The Common Closure Principle. Gather into components those classes that
change for the same reasons and at the same times. Separate into different
components those classes that change at different times and for different reasons.
• CRP: The Common Reuse Principle. Don’t force users of a component to depend on
things they don’t need.
The SOLID principles tell us how to arrange our functions and data structures into classes, and
how those classes should be interconnected.
SRP: The Single Responsibility Principle. Each software module has one, and only one, reason
to change. (What do we mean by the word “module”? The simplest definition is just a source
OCP: The Open-Closed Principle. For software systems to be easy to change, they must be
designed to allow the behavior of those system to be changed by adding new code, rather than
changing existing code.
LSP: The Liskov Substitution Principle. To build software systems from interchangeable parts,
those parts must adhere to a contract that allows those parts to be substituted one for another.
ISP: The Interface Segregation Principle. This principle advises software designers to avoid
depending on things that they don’t use.
DIP: The Dependency Inversion Principle. The code that implements high-level policy should
not depend on the code that implements low-level details. Rather, details should depend on
7. DECOUPLING LAYERS
The architect can employ the Single Responsibility Principle and the Common
Closure Principle to separate those things that change for different reasons, and to
collect those things that change for the same reasons—given the context of the
intent of the system.
What changes for different reasons?
User interfaces change for reasons that have nothing to do with business rules.
Business rules themselves may be closely tied to the application, or they may be
more general. The database, the query language, and even the schema are
technical details that have nothing to do with the business rules or the UI.
Thus we find the system divided into decoupled horizontal layers— the application-
independent business rules, Application-specific business rules, UI, and the
8. APPLICATION-INDEPENDENT BUSINESS RULES
Application-independent business rules are rules or procedures that make or save the
business money. Irrespective of whether they were implemented on a computer, they
would make or save money even if they were executed manually.
An Entity is an object within our computer system in a single and separate module
that embodies a small set of critical business rules operating on Critical Business
Application-independent business rules
9. APPLICATION-SPECIFIC BUSINESS RULES
Entities have no knowledge of the use cases that control
The use cases of the system will be plainly visible within the
structure of that system. Those elements will be classes or
functions or modules that have prominent positions within
the architecture, and they will have names that clearly
describe their function.
From the use case, it is impossible to tell whether the
application is delivered on the web, or on a thick client, or on
a console, or is a pure service.
A use case is a description of the way that an automated system is used. It specifies the
input to be provided by the user, the output to be returned to the user, and the
processing steps involved in producing that output. A use case describes application-
specific business rules as opposed to the Critical Business Rules within the Entities.
10. INTERFACE ADAPTERS
The software in the interface adapters
layer is a set of adapters that convert
data from the format most convenient for
the use cases and entities, to the format
most convenient for some external
agency such as the database or the
The presenters, views, and controllers
all belong in the interface adapters layer.
No code inward of this circle should
know anything at all about the database.
11. The frameworks and drivers layer is
where all the details go. The web is a
detail. The database is a detail. We
keep these things on the outside
where they can do little harm.
Between the use case interactors and
the database are the database
gateways.These gateways are
polymorphic interfaces that contain
methods for every create, read,
update, or delete operation that can
be performed by the application on
FRAMEWORKS AND DRIVERS
13. THE ULTIMATE DETAIL
The Main component is the ultimate detail—the lowest-level policy. It is the initial entry
point of the system. Nothing, other than the operating system, depends on it. It is in
this Main component that dependencies should be injected by a Dependency
Injection framework. Main is a dirty low-level module in the outermost circle of the
clean architecture. Think of Main as a plugin to the application—a plugin that sets up
the initial conditions and configurations, gathers all the outside resources, and then
hands control over to the high-level policy of the application.
When Main is released, it has utterly no effect on any of the other components in the
system. They don’t know about Main, and they don’t care when it changes.
14. A TYPICAL SCENARIO
If the application wants to display money on the screen, it might pass a Currency
object to the Presenter. The Presenter will format that object with the appropriate
decimal places and currency markers, creating a string that it can place in the View
Model. If that currency value should be turned red if it is negative, then a simple
boolean flag in the View model will be set appropriately.
Anything and everything that appears on the screen, and that the application has
some kind of control over, is represented in the View Model as a string, or a boolean,
or an enum. Nothing is left for the View to do other than to load the data from the
View Model into the screen.
If the application needs to know the last names of all the users who logged in
yesterday, then the UserGateway interface will have a method named
getLastNamesOfUsersWhoLoggedInAfter that takes a Date as its argument and
returns a list of last names.
15. A TYPICAL SCENARIO
Typically the data that crosses the boundaries consists of simple data structures. You
can use basic structs or simple data transfer objects if you like. Or the data can simply
be arguments in function calls. Or you can pack it into a hash-map, or construct it into
Warning! Many database frameworks return a convenient data format in response to
a query. We might call this a “row structure.” We don’t want to pass that row structure
inward across a boundary. Doing so would violate the Dependency Rule because it
would force an inner circle to know something about an outer circle.
BUT WHAT ABOUT DUPLICATION?
Architects often fall into a trap—a trap that hinges on their fear of duplication.
When code is truly duplicated, we are honour-bound as professionals to reduce and
But there are different kinds of duplication.
There is true duplication, in which every change to one instance necessitates the
same change to every duplicate of that instance.
If two apparently duplicated sections of code evolve along different paths—if they
change at different rates, and for different reasons—then they are false or accidental
will help you keep the layers properly decoupled.
When you are vertically separating use cases from one another, you will run into this
issue too, and your temptation will be to couple the use cases because they have
similar screen structures, or similar algorithms, or similar database queries and/or
schemas. Be careful. Resist the temptation to commit the sin of knee-jerk elimination
of duplication. Make sure the duplication is real.
By the same token, when you are separating layers horizontally, you might notice that
the data structure of a particular database record is very similar to the data structure
of a particular screen view. You may be tempted to simply pass the database record
up to the UI, rather than to create a view model that looks the same and copy the
elements across. Be careful: This duplication is almost certainly accidental. Creating
the separate view model is not a lot of effort, and it will help you keep the layers