Apple Interview Guide: Process, Pair Programming & Design Focus

Design Approach: This is a classic Apple pair-programming question — it starts simple and layers in complexity. Step 1 — Basic Operations: • Use a hash map for the main store • Implement get(key), set(key, value), delete(key) Step 2 — Transaction Support: • Use a stack of hash maps (transaction stack) • begin() pushes a new empty map onto the stack • set/delete within a transaction writes to the top of the stack • get traverses the stack top-down for the most recent value Step 3 — Commit & Rollback: • commit() merges the top transaction into the one below (or the main store) • rollback() simply pops the top transaction, discarding changes Edge Cases to Address: • Commit/rollback with no active transaction (throw error) • Deleting a key that only exists in a parent transaction • Nested transactions (begin within begin) Apple-Specific Tip: They'll watch how you evolve the design incrementally. Don't try to build the full solution upfront — start with the basics and refactor as requirements are added. This mirrors Apple's iterative design philosophy.

API Design First: Apple cares deeply about how you design interfaces before implementation. Clean Interface: • RateLimiter protocol/interface with a single method: shouldAllow(clientId: string) -> boolean • Strategy pattern: each algorithm implements the same interface • Factory method: createLimiter(strategy, maxRequests, windowSize) Implementation — Fixed Window: • Hash map of clientId -> {count, windowStart} • If current time > windowStart + windowSize, reset count • Simple but has the boundary burst problem Implementation — Sliding Window Log: • Hash map of clientId -> sorted list of timestamps • On each request, remove timestamps older than windowSize • Check if remaining count < maxRequests • More accurate but higher memory usage Implementation — Token Bucket: • Hash map of clientId -> {tokens, lastRefill} • Refill tokens based on elapsed time since lastRefill • Deduct one token per request; reject if tokens = 0 • Allows controlled bursting Design Principles Apple Values: • Protocol-oriented design (especially for Swift roles) • Clean separation of concerns between strategy and storage • Thread safety considerations (mention mutex or concurrent data structures) • Testability — each strategy is independently testable

Why Apple Asks This: Apple's culture centers on saying 'no' to good ideas to focus on great ones. They want engineers who are user advocates, not just ticket completers. Strong Answer Structure: • Context: 'Our PM wanted to ship a settings page with 15 toggle options for a notification feature. The design was functional but overwhelming.' • Your Push-back: 'I prototyped two alternatives: one with smart defaults that eliminated 10 toggles, and one with a progressive disclosure pattern (basic/advanced). I tested both with 5 colleagues and presented the data.' • Collaboration: 'The designer and I iterated together on the progressive disclosure approach, refining the categorization of basic vs advanced settings.' • Outcome: 'We shipped with 3 visible settings and an advanced section. Support tickets about notification confusion dropped 40%. The PM later thanked me for pushing back.' Signals Apple Looks For: • You have taste and an opinion about quality • You use data and prototypes, not just opinions • You collaborate rather than dictate • The user experience improved measurably