Tutorials System Design Tutorial
Dependency Injection at Scale — Complete Guide
Dependency Injection at Scale — 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 77 of 100
Dependency Injection at Scale
Basics ✓ → Scale ✓ → Interview
Interview · 3 — Case studies · ~10 min · Module 8: Low-Level Design
What is this?
DI supplies dependencies from the outside so types do not construct their own infrastructure graph.
Why should you care?
ShopNest services need test doubles and swappable implementations without editing constructors everywhere by hand.
See it live — copy this example
Sketch the architecture on paper. These lessons focus on concepts and trade-offs.
services.AddScoped<IOrderRepository, SqlOrderRepository>()
services.AddHttpClient<IPaymentClient, PaymentClient>()
Handler ctor(IOrderRepository, IPaymentClient)
Tests: replace with fakesRun Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Composition roots wire the graph.
- Prefer constructor injection.
- Watch scopes (singleton capturing scoped DB contexts).
Practice next
- Register ShopNest ports in the composition root.
- Inject via constructors.
- Add a fake payment client for tests.
- Typed HttpClient for Payment.
- Validate scopes in tests.
Remember
Inject dependencies. Wire at composition root. Mind lifetimes.
Testable OrderHandler
ShopNest swaps payment client in tests via DI.
Outcome: Fast unit tests without real PSP calls.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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