Designing backend application programming interfaces specifically for mobile clients, addressing mobile device constraints, network variability, and cross platform compatibility. Core concerns include bandwidth optimization techniques such as pagination, selective field queries, delta responses, efficient serialization formats, and compression; response shape and protocol trade offs including representational state transfer versus GraphQL; versioning and backward compatibility strategies for long lived mobile applications; error handling, retry semantics, and user friendly error surfaces appropriate for intermittent connectivity; rate limiting, throttling, and quota management; real time synchronization patterns including polling, web sockets, and server sent events; caching, offline support, data synchronization, conflict resolution, and optimistic updates; authentication and token refresh strategies, security and privacy considerations; and monitoring, telemetry, testing, and deployment practices that minimize breaking changes for mobile clients.
EasyTechnical
50 practiced
Describe an offline-first strategy for a mobile app that must remain usable with intermittent connectivity: how to queue outgoing operations, implement optimistic UI updates, run background sync when network is available, detect conflicts, and surface sync status to the user. Discuss data/model layering and battery/data usage trade-offs.
HardTechnical
52 practiced
Design telemetry and monitoring to quickly detect regressions impacting mobile users. Specify key metrics (p95/p99 API latency broken down by OS/version/device, error rates by endpoint and region, 4xx/5xx spikes), logs with structured request IDs, distributed tracing for slow paths, synthetic mobile network tests, RUM-style metrics in apps, dashboards, and alerting thresholds and runbook actions for mobile-impacting incidents.
MediumTechnical
49 practiced
Describe how to securely store and manage tokens on iOS and Android clients. Cover secure enclave/Keychain and Android Keystore usage, hardware-backed keys, biometric protection, storage access patterns, mitigation for rooted/jailbroken devices, and complementary server-side controls (refresh token rotation, device attestation, revocation).
HardSystem Design
48 practiced
Design a throttling and quota enforcement system for a high-throughput mobile API that supports dynamic quotas by plan, burst handling, per-device limits, and minimizes false positives. Discuss enforcement locations (edge/proxy vs service), approximate vs exact counters, data structures and storage (Redis, token buckets), how to communicate quotas to clients, and reconciliation strategies for quota drift in multi-region deployments.
HardTechnical
45 practiced
For a collaborative document editor with mobile clients and offline edits, design a synchronization protocol that handles concurrent edits, minimizes bandwidth, and is battery-friendly. Compare CRDTs vs Operational Transformation (OT): their metadata overhead, merge semantics, and compaction strategies. Describe how to store operation logs on mobile, perform compaction/snapshots, and implement undo/redo.
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