System Architecture

Design patterns, architectural principles, and scalability concepts for building robust software systems.

Core Principles

SOLID

Principle	Meaning
S — Single Responsibility	A class/module should have one reason to change
O — Open/Closed	Open for extension, closed for modification
L — Liskov Substitution	Subtypes must be substitutable for their base types
I — Interface Segregation	Many specific interfaces over one general one
D — Dependency Inversion	Depend on abstractions, not concretions

Common Patterns

Layered Architecture


Presentation Layer  →  UI, API endpoints
Application Layer   →  Use cases, orchestration
Domain Layer        →  Business logic, entities
Infrastructure      →  Database, external services

Event-Driven Architecture

Events as the primary communication mechanism
Producers emit events; consumers subscribe
Enables loose coupling and scalability

CQRS (Command Query Responsibility Segregation)

Separate models for reading (Query) and writing (Command)
Useful for complex domains with high read/write asymmetry

Scalability Concepts

Pattern	Purpose
Horizontal scaling	Add more instances (scale out)
Load balancing	Distribute traffic across instances
Caching (Redis, CDN)	Reduce database and compute load
Message queues	Decouple async workloads
Database sharding	Partition data across multiple DBs

References

Martin, R.C. Clean Architecture. Prentice Hall, 2017.
Fowler, M. Patterns of Enterprise Application Architecture. Addison-Wesley, 2002.
Richardson, C. Microservices Patterns. Manning, 2018.