What is the difference between monolith and modular monolith?

Question

Accepted Answer

**Monolith** — single codebase. **Modular Monolith** — well-structured modules within monolith.

**Monolith:**
- Single deployable unit.
- Tight coupling between parts.
- Hard to maintain at scale.
- Simple initially.

```
Monolith:
[User][Order][Payment][Auth][Notification]
  All tightly coupled, shared database
  Difficult to extract services
```

**Modular Monolith:**
- Clear module boundaries.
- Modules communicate via interfaces.
- Easier to extract services later.
- Maintainable at scale.

```
Modular Monolith:
[User Module] ←interface→ [Order Module]
  Each module has own logic
  Can extract to microservice later
```

**Key differences:**
- **Monolith** — tight coupling.
- **Modular Monolith** — clear boundaries.
- **Monolith** — hard to split.
- **Modular Monolith** — easy to extract.

Systems Analyst: What is the difference between monolith and modular monolith?