# Record Locking

## Objective

Reserve a record while you edit it so nobody else can start editing at the same time.

Pessimistic locks on high-risk records: one editor at a time, with expiry and a supervised takeover path.

## 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. Acquire the lock on the server when a user opens the edit view, storing the holder, the acquisition time, and an expiry.
2. Keep the lock alive with a heartbeat from the open editor and release it on save, cancel, or navigation away.
3. Enforce the lock on every write path, not just in the UI. A client-side lock is a courtesy, not a guarantee — the server must reject writes from anyone who does not hold it.
4. Let an authorized user force-release a lock they do not own, and record who broke which lock and when.
5. Do NOT lock reading. Viewers must always get through; only the edit action is gated.

## 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 crashed browser or closed laptop leaves a lock with no owner. Expire on missed heartbeats, not only on an absolute timeout.
- The same user opening a second tab should take over their own lock, not be blocked by themselves.
- Show who holds the lock and since when. 'Locked by another user' with no name is a support ticket.
- A force-release must notify the displaced editor while they still have their unsaved work on screen.
- The lock holder must be warned before their lock expires mid-edit, with a way to extend it.
- Deleting or archiving a locked record has to release the lock rather than strand it.

## 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

- [ ] Opening the editor acquires a server-side lock and closing it releases one.
- [ ] Writes from a non-holder are rejected by the server.
- [ ] Locks expire after missed heartbeats without manual cleanup.
- [ ] A user's own second session takes over rather than being blocked.
- [ ] The lock holder's identity and start time are shown to anyone blocked.
- [ ] Force-releases are permission-checked and recorded.
- [ ] 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.