LLD — Senior · Resources

1Designing for extension

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Senior LLD is judged on whether the design survives new requirements without rewrites. Show OCP thinking with real seams.

Build for change

Plugin / strategy-heavy designs so new behaviour is added, not edited in.

Refactoring Guru — Strategy.

Dependency injection & inversion of control for testability and swappable pieces.

Fowler — Dependency Injection.

Composition, interfaces, and the law of Demeter to keep coupling low.

Law of Demeter.

Anti-corruption layers / adapters at boundaries with external systems.

Refactoring Guru — Adapter.

Open/closed in practice: a registry of handlers vs a growing switch.

OCP.

2Concurrency in designs

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The senior differentiator: your objects are correct under concurrent access, and you can reason about locks and races.

Thread-safe by design

Where shared mutable state lives; making it safe (locks, atomics, immutability).

Jenkov — Java concurrency.

Thread-safe singleton (double-checked locking, holder idiom) and why naive versions break.

Refactoring Guru — Singleton.

Producer–consumer with a bounded blocking queue; avoiding deadlock.

Jenkov — Producer/consumer.

Optimistic vs pessimistic locking on a shared resource (seat booking).

Optimistic concurrency.

Read-write locks, semaphores, and thread pools at a glance.

Jenkov — Read/write locks.

3Harder machine-coding prompts

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Larger builds where structure and concurrency both matter. Get a core running fast, then extend under time pressure.

Design + build these

LRU / LFU cache — hashmap + doubly linked list; O(1) get/put; thread-safety.

LeetCode — LRU Cache.

Rate limiter (OO) — token bucket / sliding window as swappable strategies.

awesome-low-level-design.

In-memory key-value store with TTL and eviction.

awesome-low-level-design.

Logging framework — levels, appenders, formatters; observer + strategy.

awesome-low-level-design — Logger.

Notification service — channels (email/SMS/push) via strategy + factory; retries.

awesome-low-level-design — Notification.

Tinyurl (OO) — encoder strategy, storage abstraction, collision handling.

awesome-low-level-design.

4API & component design

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Seniors design components others depend on. Ergonomics and clear contracts are graded.

Design good interfaces

Make the API hard to misuse: clear names, sensible defaults, fail fast.

Bloch — How to design a good API.

Error-handling contract: exceptions vs result types; what callers must handle.

Fowler — error handling.

Immutability and value objects to reduce bugs.

Fowler — Value object.

Designing for testability so consumers can mock your boundary.

Fowler — Mocks aren't stubs.

5Refactoring & smells

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Seniors improve existing designs safely and can name why code is bad.

Improve safely

Common code smells (god class, feature envy, long method) and their refactors.

Refactoring Guru — Smells.

Refactoring behind tests; small safe steps.

Fowler — Refactoring.

Replacing conditionals with polymorphism / strategy.

Refactoring Guru — Replace conditional.