How Shopify Handled 30TB per Minute With a Modular Monolith Architecture 🔥

A modular monolith is a software architecture that combines the simplicity of a monolithic system with the organizational benefits of modular design. It structures the application as a single deployable unit while enforcing strict boundaries between modules, improving maintainability and scalability compared to traditional monoliths. This approach is ideal for teams transitioning from monoliths to microservices or those needing better internal structure without distributed system complexity.

Core Technical Concepts/Technologies

• Modular Monolith
• Bounded Contexts (Domain-Driven Design)
• Layered Architecture
• Intra-Module Communication (in-process calls, events)
• Database Schema Isolation
• Deployment Strategies

Main Points

• Definition & Benefits

  • Single deployable unit with well-defined modules.
  • Easier debugging and testing than distributed systems.
  • Reduced operational overhead compared to microservices.

• Key Design Principles

  • Module Independence: Clear boundaries via interfaces/contracts.
  • Domain-Driven Design (DDD): Modules align with business domains.
  • Database Isolation: Schemas per module or shared with access control.

• Communication Patterns

  • Synchronous: Direct method calls within the process.
  • Asynchronous: In-memory events or message buses for decoupling.

• Deployment & Scalability

  • Deployed as a single artifact (e.g., WAR, JAR, or container).
  • Scales vertically; modules can be split into microservices later if needed.

• When to Use

  • Projects needing structured code but not yet ready for microservices.
  • Teams valuing simplicity over distributed system complexity.

Technical Specifications & Implementation

• Code Organization: Modules as separate packages/namespaces (e.g., Java 9+ modules, .NET assemblies).
• Database: Shared tables with schema prefixes or separate databases.
• Example Communication:

  // Synchronous call between modules  
  OrderService orderService = ModuleA.getOrderService();  
  orderService.placeOrder(request);  
  
  // Asynchronous event  
  EventBus.publish(new OrderPlacedEvent(orderId));  
  

Key Takeaways

1. Balanced Approach: Offers maintainability of modular design without microservices' operational costs.
2. Evolutionary Path: Can be refactored into microservices incrementally.
3. Simplified Testing: End-to-end tests are easier than in distributed systems.
4. Team Alignment: Modules can mirror team structure (e.g., one team per domain).

Limitations & Considerations

• Scalability: Limited to vertical scaling; may require eventual migration to microservices.
• Tight Coupling Risks: Poorly enforced boundaries can lead to a "big ball of mud."
• Not for Distributed Needs: Unsuitable if modules must scale independently or use different tech stacks.

Further Exploration

• Transition Strategies: Patterns for splitting modules into microservices later.
• Tooling: Frameworks like Spring Modulith or modular monolith-specific libraries.
• Case Studies: Real-world examples of successful modular monoliths.