End-to-End Feature Design and Development Questions

The integrated 'build a complete feature' walkthrough: taking one concrete feature (a follow button, a like button, a comments thread, a create-post form, a profile editor, a large file upload) from the user-facing flow through the API contract, backend logic, data model and storage in a single coherent answer. Includes optimistic UI and the backend idempotency it needs, concurrent and repeated requests, denormalized counts that must stay correct, client and server validation with errors mapped back to the right field, a contract shared by web and mobile clients, and how the feature is tested, rolled out behind a flag and observed end to end. Tests whether the candidate connects the layers and makes consistent trade-offs across them rather than depth in any one. Deep dives in a single layer (UI state, API style, schema, caching, pipelines, observability, security and authentication, rate limiting), payments and notification delivery, feature-flag platforms, migrations and deployment pipelines, and standalone large-scale system design are covered elsewhere.