Design principles to modularise
a monolith codebase
- Prashant Kalkar

Agenda
How modules can help?
Reusable module boundaries
Principles of Cohesion
Module dependencies (Coupling principles)
Breaking up Dependencies

How modules can help?

Big ball of Mud (Class coupling)
Reference: Neal Ford: "Building Evolutionary Architectures"

Module 1 Module 2
Module breaks dependencies (disallow)

Modules - What am I talking about
● Modules allowed by build tools like Maven, Gradle or others.
● In Repo, modules (multi-module project)

Opposing requirements for a Reusable code

Reusable module boundaries

Where are the module boundaries?

How changes affect clients
Change Option 1 Option 2 Option3
Changes In Switch or
Interface
Yes Yes Yes
Changes in Light
Implementation
Yes Yes Only Clients using
Light
Changes in Fan
Implementation
Yes Yes Only Clients using
Fan

Checkout Example

Need to support Card Payments

Payment as plugin/extension

Support more payments without module changes

ISP and OCP
ISP - “Make fine grained interfaces that are client specific.”
Interface consumer and Interfaces are named and packaged together.
OCP - “Software entities (classes, modules, functions, etc.) should be open for
extension but closed for modification.”
Abstraction modules allows to satisfy Opposing requirements for a Reusable
code.

Real life examples
JPA - Hibernate
JDBC - MySQL client, PostgreSQL clients etc.
Slf4j - Log4j, Logback etc.
Kubernetes - CNI, CRI, CSI, CPI etc

Principles of Cohesion

CCP - “Classes that change together are packaged together.”
“ When we make a change we want to be able to jump to a single clear point in
the system and make the change.”
- Refactoring: Improving the Design of Existing Code, Ch 03. Bad
Smells. Divergent Change
For, Layered architecture one change per layer.
The Common Closure Principle

How to apply Common Closure Principle

Module dependencies (Coupling principles)

Which module / class is more stable? A or B?

What about now?

Fan In, Fan Out
Instability = Fan Out / (Fan In + Fan Out)
Depend on things which are stable than you

Dependency cycles

Dependency cycles

Modularity
Dependency cycle prevent module creation
Maven modules will not work.
Java 9 modules will not work.

Dependency direction towards Instability

Ideal dependency graph
● Dependency flow downwards
● Depency direction flows toward stability
● No cycles.

Ideal dependency graph

Breaking up Dependencies

Example Application codebase
Orchestration framework which uses the DAG as inputs. Different type of nodes
are supported like REST API services, or message passing brokers (like Kafka
etc).

IntelliJ Idea DSM

Move classes

Move classes (Cont)

Dependency Inversion Principle (DIP)

Architectural Tests

References
Agile Software Development, Principles, Patterns, and Practices
Clean Architecture: A Craftsman's Guide to Software Structure and Design
Refactoring: Improving the Design of Existing Code
Building Evolutionary Architectures, 2nd Edition

Thank You!