Migration Risk Analyzer

## Migration Risk Analysis

### Summary
- **Operations:** {N} DDL, {M} DML
- **Tables affected:** {list with row counts}
- **Overall risk:** {High | Medium | Low}
- **Estimated duration:** {range}
- **Requires downtime:** {Yes | No}

### Operation Risk Table

| # | Operation | Risk | Lock Type | Est. Duration | Reversible |
|---|-----------|------|-----------|---------------|------------|
| 1 | {SQL operation} | {High/Med/Low} | {lock type} | {time} | {Yes/No} |

### Lock Analysis
- **Exclusive locks:** {list of operations that block all access}
- **Maximum lock duration:** {estimated time}
- **Affected queries:** {types of queries that will be blocked}

### Rollback Strategy

#### Reversible Operations
```sql
-- Rollback for operation 1: {description}
{rollback SQL}

-- Verify structural changes
{validation queries}

-- Verify data integrity
{integrity checks}

## Calibration Rules

1. **Assume large tables.** If table size is unknown, assume it's large enough for
   locks to matter. Overestimating risk is safer than underestimating.
2. **Engine-specific analysis.** PostgreSQL and MySQL handle DDL very differently.
   PostgreSQL can add nullable columns without table rewrite; MySQL often cannot.
   Always target the specific engine.
3. **Irreversible means irreversible.** DROP COLUMN destroys data. No amount of
   rollback scripting recovers it. Flag every irreversible operation prominently.
4. **Test the rollback.** Rollback scripts must be tested in staging before the
   migration runs in production. Untested rollback is no rollback.
5. **Sequence matters.** The order of operations affects lock duration. Adding a
   column then backfilling then adding NOT NULL is safer than adding a NOT NULL
   column with a default.

## Error Handling

| Problem | Resolution |
|---------|------------|
| Database engine not specified | Ask. Lock behavior differs significantly between engines. |
| Table sizes unknown | Analyze without duration estimates. Flag that estimates require row counts. |
| ORM migration format (not raw SQL) | Parse the ORM migration file. Translate operations to SQL equivalents for analysis. |
| Migration has data-dependent logic | Flag conditional operations. Risk depends on data state at migration time. |
| Multiple migrations in sequence | Analyze each independently and as a group. Cross-migration lock accumulation is a risk. |

## When NOT to Analyze

Push back if:
- The migration is for a development/staging database — risk analysis is for production
- The migration only creates new tables (no ALTER, no existing data) — low risk by definition
- The user wants migration execution, not analysis — this skill assesses risk, it doesn't run migrations

## Rationalizations

| Rationalization | Reality |
|---|---|
| "It's backwards compatible" | Backwards compatible at the schema level doesn't mean backwards compatible at the application level — query plans, ORM mappings, and application code all interact |
| "We can roll back" | Rollback is not free — data written after migration may not survive rollback; DROP COLUMN has no rollback without backup |
| "It's a small table" | Table size is one factor — lock duration, concurrent write rate, and replication lag matter more than row count |
| "We've run this migration type before" | Past success doesn't predict future success — different data distribution, different load, different constraints |
| "Downtime window is long enough" | Estimate based on dev data, not production — migration on 10k rows takes seconds; on 50M rows with indexes, it takes hours |
| "The ORM handles it" | ORMs generate SQL, they don't guarantee safety — `ALTER TABLE` locking behavior is engine-specific and ORM-opaque |

## Red Flags

- No estimate of migration duration based on production data volume
- No rollback plan or rollback plan that doesn't account for data written post-migration
- Analyzing migration SQL without checking the current table size and write rate
- No consideration of replication lag in multi-replica setups
- Assuming zero downtime without verifying lock behavior for the specific DDL operation
- Skipping index analysis — adding an index on a large table can lock writes for minutes to hours

## Verification

- [ ] Production table sizes and row counts documented for all affected tables
- [ ] Lock behavior identified for each DDL statement (exclusive lock, no lock, etc.)
- [ ] Migration duration estimated using production-scale data, not dev fixtures
- [ ] Rollback plan documented with specific steps and data preservation guarantees
- [ ] Concurrent write impact assessed — what happens to in-flight transactions during migration
- [ ] Replication lag impact assessed for multi-replica configurations