Tutorials System Design Tutorial
Object-Oriented Design for Services — Complete Guide
Object-Oriented Design for Services — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of System Design Tutorial on Toolliyo Academy.
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System Design Tutorial · Lesson 74 of 100
Object-Oriented Design for Services
Basics ✓ → Scale ✓ → Interview
Interview · 3 — Case studies · ~10 min · Module 8: Low-Level Design
What is this?
OOD for services applies encapsulation, clear domain objects, and boundaries so service code models the business, not just HTTP handlers.
Why should you care?
ShopNest Order as anemic bags of fields leads to logic scattered across controllers.
See it live — copy this example
Sketch the architecture on paper. These lessons focus on concepts and trade-offs.
Domain: Order.place(items), Order.cancel(reason)
Application: PlaceOrderHandler orchestration
Infra: OrderRepositorySql, HttpPaymentClient
Controller stays thinRun Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Put business rules in domain types.
- Handlers orchestrate.
- Infra implements ports.
- Controllers translate HTTP.
Practice next
- Move a ShopNest rule into a domain method.
- Keep controllers thin.
- Isolate SQL in repositories.
- Forbid controller→SqlConnection direct use.
- Add domain unit tests for cancel rules.
Remember
Rich domain where it helps. Thin edges. Infra behind ports.
Order.cancel rules
ShopNest cancellation policy lives on Order.
Outcome: API and worker share the same rule.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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