# Functional Design Plan — U2 Data Durability

**Unit**: U2 Data Durability
**Round**: R1 (with U1 Hosting & Serving)
**Requirements**: FR-11, FR-12
**Components**: C-05, C-06, C-07, U2 portion of C-16

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## Step 1: Analyze unit context
- [x] Read the U2 definition from `unit-of-work.md`
- [x] Read the requirement assignment from `unit-of-work-story-map.md`
- [x] Read the carried-in design items — § 5.1 duplicate registration conflict, explicit application discriminator

## Step 2: Design the Data Protection key ring
- [x] Define the key-storage entity and its owning context
- [x] Define the application-discriminator source and stability guarantee
- [x] Define key encryption at rest
- [x] Define key lifetime and rotation behaviour
- [x] Define the registration-order rule that resolves the duplicate-registration conflict

## Step 3: Design startup migration behaviour
- [x] Define which contexts migrate and in what order
- [x] Define failure behaviour and what is logged before failing
- [x] Define behaviour when the database is temporarily unreachable at startup
- [x] Define behaviour when a migration is applied concurrently by two starting instances

## Step 4: Define business rules
- [x] Enumerate key-ring durability rules
- [x] Enumerate migration rules
- [x] Identify error and edge-case scenarios

## Step 5: Design verification approach
- [x] Define how "the persistent key store survives module registration" is asserted
- [x] Define how discriminator stability across a content-root change is asserted

## Step 6: Generate artifacts
- [x] Generate `business-logic-model.md`
- [x] Generate `business-rules.md`
- [x] Generate `domain-entities.md`
- [x] Validate all diagrams against the Mermaid standards
- [x] Verify Security Baseline compliance for this unit's design