Migration Strategist

Migration Strategist Protocol

This skill focuses on taking a database from State A to State B without causing downtime, locks, or data loss in production. Direct ALTER TABLE operations on large tables are dangerous and often require a multi-step rollout.

Core assumption: You cannot lock the table. You cannot break the old version of the application while the new version is deploying.

---

1. The Zero-Downtime Pipeline

When asked "How do I rename this column?" or "How do I split this table?", standard ALTER TABLE RENAME breaks the app. Always enforce a backward-compatible migration strategy:

The Expand & Contract Pattern (4 Steps)

For breaking changes (e.g., renaming a column name to full_name):

1. Add (Expand): Add the new column full_name (nullable initially).
2. Dual Write: Deploy application code that writes to BOTH name and full_name, but reads from name.
3. Backfill: Write a script or background job to populate full_name using name for old rows.
4. Switch & Drop (Contract): Change the app to read/write ONLY full_name. After successful deployment, run a final migration to drop name.

2. Output Format (The Plan)

Provide a numbered timeline mapping Application Code states vs Database changes.

Required Outputs (Must write BOTH to docs/database-report/):

1. Human-Readable Markdown (docs/database-report/migration-strategist-report.md)

### 🕰️ Step 1: Database Expand (Migration 1)
- Add `full_name` column.
- SQL: `ALTER TABLE users ADD COLUMN full_name VARCHAR(255);`

### 💻 Step 2: Application Release (v1.1)
- Code updates to write to BOTH `name` and `full_name`.
- Read from `name`.

### 🔄 Step 3: Data Backfill Script
- SQL: `UPDATE users SET full_name = name WHERE full_name IS NULL;`
- Action: Run this out-of-band in batches of 10,000 to avoid long locks.

### 💻 Step 4: Application Release (v1.2)
- Code updates to read/write ONLY to `full_name`.

### ✂️ Step 5: Database Contract (Migration 2)
- SQL: `ALTER TABLE users DROP COLUMN name;`
- Action: Execute only when v1.1 is fully retired from traffic.

2. Machine-Readable JSON (docs/database-report/migration-strategist-output.json)

{
  "skill": "migration-strategist",
  "steps": [
    {"phase": "expand", "target_table": "users", "action": "add_column", "details": "full_name"},
    {"phase": "app_release_1", "action": "dual_write"},
    {"phase": "backfill", "query": "UPDATE users SET full_name = name"},
    {"phase": "app_release_2", "action": "read_write_new_only"},
    {"phase": "contract", "target_table": "users", "action": "drop_column", "details": "name"}
  ]
}

3. Rollback Strategy

Every Migration plan must include a clear rollback path if Step 2 or Step 4 fails. How do we reverse it?

• "If v1.2 fails, rollback to v1.1. Data is still dual-written to name, so reverting the code is safe."

---

Guardrails

• No DEFAULT on new columns for large tables: In some older SQL versions, adding a column with a default value locks and rewrites the entire table. Use nullable + background backfill.
• Data Type Casting: Be explicitly careful about data truncation passing from State A to State B.
• Constraints Last: Add NOT NULL, UNIQUE, or Foreign Keys only at the very end of the backfill process.