Offline-first

Use this when

• Your users have intermittent connectivity (2G/3G, patchy Wi‑Fi, captive portals).
• Power cuts are normal and people lose sessions mid-task.
• You can’t assume “refresh fixes it”.

Checklist

Data model

• Decide what “offline” means for your product (read-only, capture-only, full CRUD).
• Define conflict strategy (last-write-wins, merge, manual resolution, append-only).
• Make IDs client-generated (UUID/ULID), not server-only.

Sync & reliability

• Treat sync as a queue, not a background mystery.
• Make sync idempotent, retries should be safe.
• Handle clock skew (don’t trust client timestamps for ordering).
• Show sync state in the UI (pending, synced, conflict).

UX under failure

• Never block a primary action on the network unless you absolutely must.
• Save locally first, sync second.
• Design for “resume later”: drafts, partial forms, and autosave.
• Make errors actionable: “Try again” is not actionable.

Performance & payload

• Budget your JS and images, set targets and enforce them.
• Lazy load everything non-critical.
• Avoid third-party embeds in critical flows.

Ops & support

• Log failures locally and upload when online (with user consent).
• Add a “report a problem” flow that works offline (queue it).
• Document a manual fallback (paper flow, SMS, phone call). Real life needs it.

Definition of done

• A user can complete the core task with airplane mode on.
• The app recovers cleanly after power loss or app kill.
• You can explain exactly how conflicts are handled, in one paragraph.