Microservices vs Monolith: Making the Right Architecture Decision

┌─────────────────────────────┐
│          Monolith            │
│  ┌─────┐ ┌─────┐ ┌─────┐   │
│  │Users│ │Orders│ │Notify│  │
│  └─────┘ └─────┘ └─────┘   │
│         Shared DB            │
└─────────────────────────────┘

┌────────┐  ┌────────┐  ┌────────┐
│User Svc│  │Order Svc│  │Notify  │
│  + DB  │  │  + DB   │  │  + DB  │
└────────┘  └────────┘  └────────┘
     ↕           ↕           ↕
         Message Queue / API Gateway

✅ Simple development and debugging
✅ Easy local development (one repo, one command)
✅ ACID transactions across the entire system
✅ No network latency between components
✅ Straightforward deployment
✅ Lower infrastructure costs

✅ Independent deployment and scaling
✅ Team autonomy and ownership
✅ Technology flexibility per service
✅ Fault isolation (one service failing doesn't crash everything)
✅ Organizational alignment (Conway's Law)

┌─────────────────────────────────┐
│        Modular Monolith          │
│  ┌────────┐ ┌────────┐ ┌─────┐ │
│  │ Users  │ │ Orders │ │Notif│ │
│  │ Module │ │ Module │ │Mod  │ │
│  └───┬────┘ └───┬────┘ └──┬──┘ │
│      │          │          │     │
│  ┌───┴──────────┴──────────┴──┐ │
│  │       Shared Database      │ │
│  └────────────────────────────┘ │
└─────────────────────────────────┘

Monolith → Modular Monolith → Selective Microservices