System Design: Scenario-Based Speaking Practice for a Multilingual Learning App

Goal

Design a scenario-based speaking practice feature where users select real-life scenarios (e.g., ordering food, job interviews) and practice spoken dialogs. Address both product and engineering aspects for a production-scale system.

Requirements

1. User flows: scenario discovery, selection, in-session experience, feedback, and review.
2. Content modeling: scenarios, roles (user/agent), dialog turns, prompts, evaluation rubrics, and metadata.
3. Session orchestration: turn-taking, real-time feedback, error recovery, and session state.
4. Audio pipeline: capture, streaming, ASR/NLU, TTS/LLM, latency targets, barge-in, and offline fallback.
5. Personalization: level selection, adaptive difficulty, and content recommendations.
6. Progress tracking: learning metrics, mastery signals, and reporting.
7. Content localization: multi-language support, regional variants, and voice selection.
8. Architecture: services, storage choices, APIs, rate limiting, and cost-aware scaling to millions of sessions.
9. Privacy and security: data handling, compliance, abuse prevention.
10. Experimentation and rollout: adding new scenarios safely and measuring impact.