Low-Level Design Explained: Classes, Interfaces, State Machines, and Testable Code

Low-Level Design Explained: Classes, Interfaces, State Machines, and Testable Code

Low-Level Design (LLD) turns a feature or component into concrete code structures: classes, interfaces, methods, state, validation rules, errors, and collaboration between objects.

It answers how this component should be implemented, not how the entire distributed system is deployed.

Start With Invariants and Responsibilities

Before drawing classes, write the rules that must always remain true.

For an order component:

paid orders cannot return to pending
cancelled orders cannot be shipped
refund cannot exceed captured amount

Then place each rule with the object/service that owns it.

Keep Dependencies Explicit

Prefer constructors/functions that clearly declare required collaborators:

OrderService(
  orderRepository,
  paymentGateway,
  clock
)

Explicit dependencies make code easier to test and change than hidden global state or hard-coded infrastructure clients.

Depend on Useful Interfaces

Use an abstraction when multiple implementations, isolation, or testability genuinely benefit from it.

PaymentGateway
  ├── StripeGateway
  └── FakeGatewayForTests

Do not create an interface for every class purely to follow a pattern.

Model State Explicitly

Workflows with real states should use a state machine or clear transition rules instead of many unrelated booleans.

This makes invalid transitions testable and visible.

Design Failure Behavior

Define expected error categories and which layer owns recovery. Avoid catch-all exceptions that erase the difference between validation, conflict, dependency timeout, and internal failure.

For concurrent updates, define whether the component uses optimistic version checks, locking, or idempotency rather than discovering race conditions later.

Use Patterns for Problems, Not Decoration

Factories, strategies, adapters, state machines, repositories, and other patterns are useful when they solve recurring coupling or behavior problems.

A design pattern is not evidence of good design by itself. Prefer the simplest code shape that keeps responsibilities and dependencies clear.

Final Takeaway

Good LLD makes business rules, state, dependencies, errors, and extension points explicit. If the implementation can be understood and tested without knowing every infrastructure detail around it, the low-level design is doing its job.

References

• Microsoft: Architectural Principles
• Azure Architecture Center: Design for Evolution