Design a keyboard and mouse input system
Company: Amazon
Role: Software Engineer
Category: System Design
Difficulty: hard
Interview Round: Technical Screen
##### Question
Design an input processing system that captures keyboard and mouse events and can reliably reconstruct **what the user typed** (answering "what did the user input?"). The system must support multiple input devices, preserve a total ordering of events across them, persist an event log, and allow replay to derive the final text. Cover the following:
1. **Event capture and modeling** — handle key down/up, key repeat (auto-repeat), modifier keys, backspace/delete, and text composition. Define the data models for low-level events, normalized/canonical events, derived high-level operations, and sessions.
2. **Mouse and selection** — handle mouse clicks, movement, scroll, double-click (and word/line selection), drag-based text selection, and cursor movement.
3. **Clipboard and editing** — support copy, cut, and paste, plus undo and redo with sensible transaction grouping.
4. **Text reconstruction and IME** — reconstruct user-visible text from the text-input pipeline (TextInput / composition commit), correctly handling IME composition sequences (CompositionStart/Update/End), dead keys, and Unicode grapheme clusters (emoji, combining marks). Do **not** infer text from raw key codes.
5. **Ordering, timestamps, and concurrency** — maintain monotonic timestamps and a total order across multiple devices; discuss event queues, buffering/back-pressure, and thread-safety.
6. **Robustness** — handle debouncing (key/button bounce, mouse-move coalescing), double-click detection, and lost or duplicated events.
7. **APIs** — expose an interface such as `recordEvent(event)`, `getCurrentText()`, `getSelectionRange()`, `undo()`, `redo()`, plus session control and replay (`startSession`, `endSession`, `replay`).
8. **Persistence, recovery, and replay** — persist an append-only event log, support deterministic replay to rebuild final text, and recover correctly after a crash (snapshots + write-ahead log).
9. **Quality** — describe the high-level components, latency/throughput/memory targets, a testing strategy, and failure handling.
Quick Answer: An Amazon software-engineer system-design screen: design a system that captures keyboard and mouse events and reconstructs exactly what the user typed. It tests low-latency event capture, total ordering across multiple devices, IME/composition and grapheme-safe text reconstruction, mouse selection and clipboard/undo-redo, plus durable persistence with deterministic crash-safe replay.
