Modular Monolithic

Modular Monolithic Architecture is a type of system design where the application is deployed as a single unit (monolith) but is logically divided into well-defined, independent modules. Each module encapsulates a specific business capability and communicates with other modules via function calls, not network calls.

It combines the simplicity of monolithic deployment with the separation of concerns and code organization benefits of modularization.

Characteristics of Modular Monolithic

Aspect	Description
Deployment	Single deployable unit (e.g., a WAR, JAR, or binary).
Modularity	Code is divided into self-contained modules.
Communication	Modules interact via in-memory calls.
Database	Often a single database shared across modules.
Scaling	Cannot scale modules independently—must scale the entire monolith.
Code Management	Encourages clean architecture patterns, such as domain-driven design (DDD).

Structure of a Modular Monolith

Application (Monolith)
│
├── Module: User Management
│   ├── Controller
│   ├── Service
│   └── Repository
│
├── Module: Order Management
│   ├── Controller
│   ├── Service
│   └── Repository
│
├── Module: Inventory
│   ├── Controller
│   ├── Service
│   └── Repository
│
└── Shared Utilities (Logging, Error Handling, etc.)

Benefits of a Modular Monolith:

• Single deployment pipeline – easier to deploy and test.
• Code organization – better maintainability with clear module boundaries.
• Refactor-friendly – ideal for teams considering future migration to microservices.
• Performance – no network latency between modules.

Limitations of a Modular Monolith:

• Cannot scale parts independently – whole application must scale as one.
• Module boundaries are not enforced at runtime – developers can bypass boundaries if not disciplined.
• Large codebase over time – can become difficult to manage without good practices.

Example

Let’s say you’re building an e-commerce system.

Modules:

1. User Management Module: Handles registration, login, profile updates.
2. Product Catalog Module: Manages products, categories, search.
3. Order Management Module: Handles shopping cart, checkout, order tracking.
4. Payment Module: Manages payments, refunds, invoices.

Each of these modules:

• Has its own domain logic.
• Has a controller, service, and data layer.
• Shares the same database (but different tables or schemas).
• Communicates with other modules internally via method calls.

Deployment:

• A single deployable .jar or .war file contains all modules.
• CI/CD pipeline builds, tests, and deploys it as one application.

Comparison with Other Architectures

Feature	Modular Monolith	Monolith (Non-Modular)	Microservices
Deployment	Single unit	Single unit	Multiple services
Modularity	Logical modules	Mixed code	Separate services
Communication	In-memory	In-memory	API/Network
Scalability	Whole app	Whole app	Per service
Complexity	Medium	Low	High

Suitable for

• You're starting small but expect future growth.
• You want to maintain structure without microservices complexity.
• You prefer simpler operations and deployment.