Design Cross-Platform Sync

Section 01 - Cross-Platform Sync in understanding

In Section 01: understanding for Cross-Platform Sync (sec-001), clarify actors, trust boundaries, and what “done” means for Cross-Platform Sync. Name one metric (p95 latency, durability, or cost) and how you degrade gracefully when the system is under 2× load.

Why interviewers care

Cross-Platform Sync interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

The failure that defines the design

The outage to narrate is the divergent device. Your phone shows a message your laptop never received, or a conversation you read on the laptop still shows unread on the phone — because a sync delta was lost and the device's cursor advanced past it, so the gap is silently skipped forever. The fix is the spine of the design: a per-device cursor over a monotonic per-user update log that is the source of truth, with deltas applied idempotently by sequence and the cursor advanced only over a contiguous, gap-free run — so a detected gap triggers a re-sync from the last good cursor rather than a permanent hole. State per-device-cursor-over-an-authoritative-log with gap-free apply up front, because cross-platform sync is a convergence problem, and the disaster is two of a user's devices disagreeing about what happened.