# Request Correlation IDs

## Objective

Follow one user action through every request, job, and downstream call.

A single ID generated or accepted at the edge and carried through logs, background jobs, outbound calls, and error reports, so one user action can be reconstructed end to end.

## 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. Generate a correlation ID at the entry point of every inbound request. If the caller supplied one, validate it before trusting it — bounded length, a restricted character set, no newlines — and generate a fresh one if it fails.
2. Store the ID in a context that follows the unit of work, not in a global variable. Concurrent requests must never share one.
3. Propagate it explicitly: attach it to every log line, enqueue it with every background job, send it as a header on every outbound HTTP call, and attach it to error-tracker reports.
4. Show the ID on user-facing error pages so a support ticket can be matched to the exact failure. Show only the ID — never the exception, the stack, or the internal service that failed.
5. Do NOT derive the ID from user data, session tokens, or sequential counters. It must be unguessable and it must reveal nothing about who the user is.
6. Field names, severity levels, and redaction rules are owned by Structured Logging; extend that schema with the correlation field rather than defining a second logging convention 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

- A job that spawns further jobs must pass the ID down the chain, and a retry must reuse the original ID rather than minting a new one.
- Scheduled and cron-triggered work has no inbound request. Generate an ID at the start of the run so those events are still traceable.
- An untrusted inbound header must never be echoed back unvalidated — that is a header injection and log-forging path.
- Async work that outlives the request must not lose the context when the request completes.
- The ID must appear in the response headers so a client or proxy can record it without reading the page.
- Do not let a missing ID be an error. If context is unavailable for any reason, log with a placeholder and continue serving.

## 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

- [ ] Every inbound request has a correlation ID, either validated from the caller or freshly generated.
- [ ] The ID appears on every log line produced during that request.
- [ ] Background jobs, retries, and outbound HTTP calls carry the same ID.
- [ ] Error-tracker reports are tagged with the ID and user-facing error pages display it.
- [ ] The ID is returned in a response header.
- [ ] IDs contain no user-identifying or sequential information.
- [ ] 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.