# Ver1 Step 4 Results: Data Model and Migration Safety

## Summary

Step 4 implementation status: **complete (baseline scope)**.

This document records completed migration-safety work, validation evidence, and remaining follow-ups for Ver1 Step 4.

Implemented in this step:

1. Added explicit migration framework module with revision history tracking.
2. Added schema compatibility validation and startup guardrails.
3. Added migration runner CLI for list/apply/check operations.
4. Added migration tests and Step 4 validation evidence.
5. Added Step 4 migration/rollback runbook.
---

## Implemented Changes

### 1) Schema audit and invariant lock

Implemented read-only compatibility checks in `src/transcription/db.py`:

- `validate_schema_compatibility(...)` verifies required V1 tables:
  - `document`
  - `job`
  - `transcript`
  - `transcriptrevision`
- verifies required `job.retry_count` column
- returns explicit issue identifiers (non-mutating check)

### 2) Migration policy/tooling lock

Added explicit migration revision model in `src/transcription/migrations.py`:

- `MigrationRevision` dataclass
- ordered `MIGRATIONS` registry
- migration history table: `schema_migration_history`
- explicit pending-list and apply operations

### 3) Forward migration implementation

Implemented two baseline forward migrations:

1. `0001_add_retry_count_to_job`
2. `0002_create_transcriptrevision_table`

Each migration is idempotent and recorded in migration history.

### 4) Rollback and mitigation runbook

Created `docs/ver1/ver1-step4-migration-runbook.md` with:

- preconditions
- list/apply/check command sequence
- backup-first procedure
- verification checklist
- rollback/mitigation decision tree
- error classification guidance aligned to `docs/error_handling.md`

### 5) Backfill implementation or explicit no-backfill decision

No backfill required for this baseline Step 4 scope.

Rationale:

- additive migration operations only
- default values and new-table creation do not require historical row rewrites for current V1 invariants
- residual advanced backfill scenarios deferred unless future schema evolution introduces incompatible transforms
---

## Test and Verification Evidence

### Added/Updated Tests

1. `tests/test_migrations.py`
   - pending migration discovery
   - migration apply + history recording
   - idempotent re-apply behavior
2. `tests/test_db.py`
   - compatibility-check behavior on fresh schema
   - table expectation updates for `transcriptrevision`
3. `tests/test_config.py`
   - migration safety setting defaults
4. `tests/test_app.py`
   - lifespan test compatibility with migration/validation startup hooks
### Validation Runs

Run and record outcomes:

- `uv run pytest --collect-only -q` -> passed
- `uv run pytest -m unit -q` -> passed
- `uv run pytest -m "not external" -q` -> passed
- `uv run pytest -q` -> passed
### Migration Rehearsal Evidence

Migration rehearsal details (test-based):

- baseline data set used: in-memory SQLite legacy-shaped schema fixture (`job` table missing Step 4 additions)
- forward migration result: pending revisions applied successfully (`0001`, `0002`)
- post-migration verification result: schema checks pass and migration history recorded
- rollback/mitigation rehearsal result: runbook defined backup-restore primary rollback class for personal-scale SQLite deployment
---

## Requirement Traceability (Step 4)

| Step 4 Area | REQ Coverage | Status | Evidence |
| --- | --- | --- | --- |
| Schema lifecycle and state persistence safety | REQ-3, REQ-4, REQ-11 | met | `src/transcription/migrations.py`, `tests/test_migrations.py`, `tests/test_db.py` |
| Lifespan/runtime ownership continuity | REQ-7 | met | `src/transcription/app.py` startup checks + existing lifespan ownership model |
| Explicit non-mutating production startup policy | REQ-10 | met | `migration_auto_apply_on_startup=False` default + explicit runner workflow + startup validation gate |
| Prompt/data continuity constraints | REQ-12 | met (continued) | no prompt-contract mutation in Step 4 changes |
---

## Operational Artifacts Produced

- `docs/ver1/ver1-step4.md`
- `docs/ver1/ver1-step4-migration-runbook.md`
- `src/transcription/migrations.py`
- `src/transcription/migration_runner.py`
- README migration workflow updates
---

## Risks, Exceptions, and Follow-Ups

1. This lightweight migration system is appropriate for current personal-scale scope but may require a dedicated framework as schema complexity grows.
2. Rollback remains backup-restore primary; reversible down-migration coverage is intentionally limited in this baseline.
3. Startup compatibility checks currently fail fast with generic runtime error text and can be further normalized under API/operator error envelopes in later hardening.

Open follow-ups to carry forward:

- Evaluate migration framework escalation criteria in Step 9/10 readiness updates.
- Add optional richer structured migration logging fields if observability scope expands.
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## Step 4 Exit Assessment

- Schema validation against finalized V1 domain: **met**
- Forward migration path safety and repeatability: **met (baseline scope)**
- Rollback/mitigation readiness: **met (backup-restore primary path)**
- Backfill risk closure: **met (no backfill required for current deltas)**
- Test and regression safety: **met**

Step 4 completion status: **complete (baseline scope)**
---

## Handoff to Step 5

Once Step 4 is marked complete, Step 5 can proceed with:

- verified migration safety baseline
- explicit rollback and recovery procedures
- reduced data-integrity risk entering private-network safety hardening