# Structured Logging

## Objective

Turn print statements into searchable records with stable fields.

Every log line becomes a structured event with a fixed field schema, a real severity level, and redaction applied before it leaves the process.

## 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. Define the field schema once and write it down: timestamp, level, message, service, environment, release version, and the request or job context. Every log line carries the same field names, spelled the same way.
2. Use severity levels with an agreed meaning. If everything is 'info', the levels are decoration. Reserve error for things a human must look at, and warn for things that degraded but recovered.
3. Redact at the logging layer, not at the call sites. Maintain a deny list of field names — password, token, secret, authorization, card number, email where policy requires it — and truncate large payloads to a bounded size.
4. Do NOT put unbounded high-cardinality values in indexed fields. A user ID is fine; a full URL with query string, a raw SQL statement, or a UUID per line will make the log store slow and expensive.
5. Keep human-readable output in local development. Machine-readable JSON in production, pretty lines on a developer's terminal.

## 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

- Log lines emitted before the logger is configured must not be lost or crash the boot — buffer them or fall back to plain output.
- Multi-line content like stack traces must stay in one event, not fragment into a line per frame.
- Redaction must survive nesting. A token buried three levels down inside a serialized object is still a token.
- An object that fails to serialize must not raise inside the logger and take down the request. Log the failure and move on.
- Very high log volume must not become the bottleneck. Sample noisy repeated events rather than dropping them silently.
- Third-party libraries and the web server write their own logs in their own format — decide whether to adapt them or accept two formats, but do not pretend the problem does not exist.

## 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 log event carries the same core fields with identical names.
- [ ] Severity levels are documented and used consistently.
- [ ] Secrets and personal data are redacted at the logging layer, including inside nested objects.
- [ ] No indexed field carries unbounded high-cardinality values.
- [ ] Development output is human-readable; production output is machine-parseable.
- [ ] A serialization failure inside the logger never breaks the request.
- [ ] 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.