# Optimistic Updates

## Objective

Make actions feel instant while still recovering cleanly when the server disagrees.

Apply the expected result of a mutation immediately, then reconcile with the server response or roll back.

## 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. Apply optimism only to actions that almost always succeed and are cheap to reverse: toggles, reordering, renaming, adding a comment.
2. Keep the pre-mutation value so a failure can restore exactly that, and no more.
3. Give newly created records a temporary client identifier and swap it for the server identifier when the response lands, updating anything already referencing it.
4. Show the pending item as pending. Instant does not mean pretending the write already reached the server.
5. Do not apply optimism to payments, deletions of anything expensive to recreate, or actions with side effects the user cannot see. Wait for the server and say so.

## 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

- A rollback must revert only its own mutation, leaving later successful changes to the same record intact.
- Responses arriving out of order must not overwrite a newer value with an older one. Sequence them.
- A permission or validation failure discovered on the server needs a real explanation, not a silent revert.
- The revert must not jump the scroll position or move the item the user is about to click.
- A mutation on an item the user has since navigated away from still has to report its failure somewhere.
- Repeated failures against the same record should stop being optimistic and fall back to waiting.

## 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

- [ ] Eligible actions render immediately and show a pending state.
- [ ] A failure reverts only that mutation and explains what happened.
- [ ] Temporary client IDs are reconciled with server IDs, including in referencing views.
- [ ] Out-of-order responses cannot restore stale values.
- [ ] Reverts do not shift layout or scroll position.
- [ ] Irreversible and side-effecting actions are excluded by design.
- [ ] 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.