# Functional Design Plan — U4 Observability Integration

**Unit**: U4 Observability Integration
**Round**: R2 (with U3 Security Headers & CSP)
**Requirements**: FR-13, FR-14, FR-15, FR-16, FR-19
**Components**: C-08, C-09, C-14, C-15, U4 portion of C-16

**Scope boundary**: this stage designs *behaviour* — what is reported, when, and what happens when a service is absent or unreachable. The *pattern* decisions (structured-log shape, correlation-ID mechanism per OPEN-01, alert-rule design) belong to this unit's NFR Design, which follows.

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## Step 1: Analyze unit context
- [x] Read the U4 definition from `unit-of-work.md`
- [x] Read FR-13 through FR-16 and FR-19 from `requirements.md`
- [x] Read the reference project's Sentry and Umami implementation for the frontend
- [x] Note the coupling to U3: the origins this unit introduces must be permitted by U3's CSP

## Step 2: Design backend error and log reporting
- [x] Define behaviour when no Sentry DSN is configured
- [x] Define behaviour when Sentry is configured but unreachable
- [x] Define what is sent and what is deliberately withheld — see Question 2
- [x] Define the production log level — see Question 3
- [x] Define environment and release tagging

## Step 3: Design security event emission
- [x] Define which events are emitted for alerting — see Question 4
- [x] Define what context each event carries, and what it must never carry
- [x] Confirm no event carries a password, token or PII

## Step 4: Design frontend observability
- [x] Define Sentry initialisation and the absent-DSN path
- [x] Define the transport route to Sentry — see Question 1
- [x] Define Umami inclusion behaviour and its absent-configuration path — see Question 5
- [x] Define behaviour in local development

## Step 5: Design the same-origin API base URL
- [x] Define resolution when `VITE_API_BASE_URL` is absent or empty
- [x] Define resolution when an explicit absolute URL is supplied
- [x] Define validation behaviour for a malformed value
- [x] Confirm the local master and slave development setups keep working

## Step 6: Define business rules
- [x] Enumerate reporting rules
- [x] Enumerate degradation rules
- [x] Enumerate configuration-resolution rules
- [x] Identify error and edge-case scenarios

## Step 7: Generate artifacts
- [x] Generate `business-logic-model.md`
- [x] Generate `business-rules.md`
- [x] Generate `domain-entities.md`
- [x] Generate `frontend-components.md`
- [x] Validate all diagrams against the Mermaid standards
- [x] Verify Security Baseline compliance for this unit's design