System Design: A Developer API Platform for SMS and Voice Calls Like Twilio

Design a cloud communications platform like Twilio, which lets developers add text messaging and phone calls to their own applications through an API. ```hint Follow one text message Trace a single outbound text from the developer's API call to the recipient's phone and back as a delivery report, and ask what can fail at each hop, especially the hops you do not control. ``` ### Clarifying Questions - Which channels are in scope: SMS only, or voice calls too, and other channels such as email or chat apps? - Do we connect to mobile carriers directly, or route through existing messaging aggregators? - What traffic should we design for: time-critical one-time passcodes, bulk marketing campaigns, or both on the same system? - Who is responsible for the consent and opt-out rules that carriers enforce: the platform or each customer? ### What a Strong Answer Covers - Functional and non-functional requirements for developers sending and receiving messages and calls, with a stated scale estimate. - The developer-facing API and webhooks: sending, inbound messages, delivery status callbacks, phone number provisioning, authentication and idempotency. - The outbound and inbound message paths, including queuing, carrier routing, per-sender throughput limits and delivery receipts. - Reliability, fairness between customers, and failure handling when a carrier or a customer's webhook is down. - Compliance, abuse prevention, billing and observability, with the product trade-offs a PM would own. ### Follow-up Questions - One customer starts a large marketing blast and other customers' login codes begin arriving late. What do you change? - A carrier connection silently stops returning delivery receipts. How would you detect it, and what do customers see in the meantime? - How would you price messages, and how does pricing feed back into how you route them?