This felt like a product question at first and I went down the wrong path talking about UX for username conflicts.
Clarify that Gmail usernames are globally unique, so the core challenge is ensuring atomicity and consistency across distributed data centers. Propose a design that uses a globally consistent, low-latency service (e.g., a distributed lock or consensus-based registry) to serialize username registrations, with fallback strategies for high availability and conflict resolution.
Pro tip: Acknowledge the CAP theorem trade-off: you can't have perfect consistency and availability during a network partition, so discuss how you'd prioritize consistency for username uniqueness while minimizing user impact (e.g., short-lived locks, retries with backoff).
Confirm that usernames must be globally unique and that simultaneous registrations from different regions must be handled without conflicts. Discuss expected scale, latency requirements, and consistency needs.
Suggest a centralized or globally distributed service (e.g., Google Spanner, Chubby, or a custom Paxos-based system) that provides linearizable operations for username reservation. Mention the need for a unique index and atomic compare-and-set.
Describe how a user request is routed to the nearest data center, which then forwards the username reservation to the global service. The service attempts to insert the username; if it already exists, it returns a conflict error.
Explain how to handle network partitions, service outages, and high latency: use timeouts, retries with exponential backoff, and possibly a two-phase commit or lease-based approach to avoid deadlocks.
Compare strong consistency (e.g., Spanner) vs. eventual consistency with conflict resolution (e.g., last-write-wins with timestamps). Highlight why strong consistency is preferable for username uniqueness.
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