# Sync Conflict Resolution

## Objective

Decide what happens when your app and an external system disagree.

Detection and human resolution of divergence between a local record and its counterpart in a synced external system.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Write down the source-of-truth policy per field or per object before building anything: app wins, external wins, or a human decides. An implicit policy becomes silent data loss.
2. Detect divergence by comparing stored versions or change timestamps from both systems against the last synced snapshot. Last-write-wins on arrival order is not detection; it is overwriting.
3. When a human must decide, show both values side by side and state the consequence of each choice, including what will be pushed to the external system.
4. Support field-level resolution where the object allows it, so choosing a local phone number does not also revert an externally-updated address.
5. Stamp each resolution with the versions it settled, and make later sync jobs carrying older versions no-ops. Do NOT let a queued stale job reopen a conflict a user already resolved.
6. Two users saving the same record in your app is a different problem, owned by Edit Conflict Detection, which limits itself to the moment of writing. Do not re-implement its version-token mechanics here.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- The external system may not expose a version or reliable modified timestamp. Fall back to hashing the fields you care about and say so in the policy.
- Clock skew between systems makes timestamp comparison unsafe near the boundary. Prefer versions; when using time, require a meaningful margin.
- A record deleted on one side and edited on the other is a conflict, not a delete. Never propagate a destructive action as the automatic resolution.
- Conflicts arrive in bulk after an outage. Offer a reviewed bulk resolution by policy rather than making someone click through hundreds.
- Resolution must be auditable — who chose which side, when, and what the discarded value was. Someone will ask.
- While a conflict is unresolved, the record must be visibly flagged, and syncing that field must pause instead of quietly picking a winner.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] A source-of-truth policy exists per synced object or field and is documented in the app.
- [ ] Conflicts are detected by version or timestamp comparison against a last-synced snapshot.
- [ ] The resolution UI shows both values and states what each choice will do.
- [ ] Field-level resolution is possible where the object supports it.
- [ ] A stale sync job cannot reopen or reverse a resolved conflict.
- [ ] Every resolution records the chooser, the time, and the discarded value.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.