LLD — Staff · Resources

1Domain modelling at scale

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Staff LLD is about modelling a whole domain cleanly so many teams can build on it without stepping on each other.

Model the domain

DDD: entities, value objects, aggregates, and aggregate boundaries.

Fowler — DDD Aggregate.

Bounded contexts & context mapping; where one model ends and another begins.

Fowler — Bounded Context.

Ubiquitous language — naming the whole org shares, from code to product.

Fowler — Ubiquitous Language.

Modelling invariants so illegal states are unrepresentable.

Illegal states unrepresentable.

Anemic vs rich domain models.

Fowler — Anemic domain model.

2Framework & platform design

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You design the thing other engineers build on. The bar is API quality, extensibility, and not breaking consumers.

Design for others

SDK/library API design: minimal surface, clear extension points, hard to misuse.

Bloch — Good API design.

Extension mechanisms: hooks, plugins, DI, inversion of control.

Fowler — Plugin.

Backward compatibility & deprecation: semver, expand/contract, migration windows.

Semantic Versioning.

Designing for testability so consumers can mock your boundary.

Fowler — Mocks aren't stubs.

3Large-system OO architecture

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Prompts get big ("design a rules engine / workflow engine"). Frame it, pick patterns, and set boundaries.

Design these end to end

Rules / workflow engine — interpreter + strategy + composite; data-driven behaviour.

Refactoring Guru — Interpreter.

Feature-flag / experimentation system — targeting, rollout, safe defaults.

Fowler — Feature Toggles.

Plugin architecture — a stable core + third-party extensions without core edits.

Fowler — Plugin.

Event sourcing & CQRS at a glance — when the log is the source of truth.

Fowler — Event Sourcing.

Domain events & how modules react without tight coupling.

Fowler — Domain Event.

4Setting standards

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Staff engineers raise the whole team's design quality — through reviews, patterns, and written guidance.

Raise the bar

Establishing design review + a lightweight RFC process for non-trivial designs.

Design docs at Google.

Codifying patterns/anti-patterns so teams converge without you in the room.

Refactoring Guru — patterns.

Managing large refactors/migrations safely (strangler fig, incremental cutover).

Strangler Fig.

Fitness functions / lint rules to keep architecture from drifting.

Fitness functions.