Phase 1: Deploy new code (disabled)
Phase 2: Enable for 1% traffic
Phase 3: Ramp to 10%, 50%, 100%
Phase 4: Remove old code

Phase 1: Write to both old and new
Phase 2: Backfill old → new
Phase 3: Read from new (write both)
Phase 4: Stop writing to old
Phase 5: Decommission old

Blue (current) → Green (new)
Switch traffic: Blue → Green
Rollback available: Green → Blue

# Migration Plan: MySQL → PostgreSQL

## Overview

**What:** Migrate user database from MySQL to PostgreSQL
**Why:** Better JSON support, improved performance
**When:** Q1 2024
**Owner:** Database Team
**Risk Level:** HIGH

## Current State

- MySQL 8.0
- 500GB data
- 100K users
- 1000 writes/min
- 10,000 reads/min

## Target State

- PostgreSQL 15
- Same data model
- No downtime
- Data validation 100% match

## Phases

### Phase 1: Dual Write (Week 1-2)

**Goal:** Write to both databases

**Steps:**

1. Deploy PostgreSQL cluster
2. Create schema in PostgreSQL
3. Deploy dual-write code
4. Enable dual writes (MySQL primary, PostgreSQL secondary)

**Code:**

```typescript
async function createUser(data: CreateUserDto) {
  // Write to MySQL (primary)
  const mysqlUser = await mysql.users.create(data);

  // Write to PostgreSQL (secondary, fire and forget)
  postgres.users.create(data).catch((err) => {
    logger.error("PostgreSQL write failed", err);
  });

  return mysqlUser; // Still trust MySQL
}
```

def backfill():
    last_id = get_last_migrated_id()
    batch_size = 1000

    while True:
        users = mysql.query(
            "SELECT * FROM users WHERE id > %s LIMIT %s",
            [last_id, batch_size]
        )

        if not users:
            break

        postgres.bulk_insert(users)
        last_id = users[-1]['id']
        save_checkpoint(last_id)

        time.sleep(0.1)  # Rate limit

async function getUser(id: string) {
  const mysqlUser = await mysql.users.findById(id);

  // Shadow read from PostgreSQL
  postgres.users.findById(id).then((pgUser) => {
    if (!deepEqual(mysqlUser, pgUser)) {
      logger.warn("Data mismatch", { id, mysqlUser, pgUser });
      metrics.increment("migration.mismatch");
    }
  });

  return mysqlUser; // Still trust MySQL
}

async function getUser(id: string) {
  if (featureFlags.readFromPostgres) {
    return postgres.users.findById(id);
  }
  return mysql.users.findById(id);
}

async function createUser(data: CreateUserDto) {
  return postgres.users.create(data);
  // No longer writing to MySQL
}

def validate_migration():
    # Row counts
    mysql_count = mysql.query("SELECT COUNT(*) FROM users")[0]
    pg_count = postgres.query("SELECT COUNT(*) FROM users")[0]
    assert mysql_count == pg_count

    # Random sampling
    sample = mysql.query("SELECT * FROM users ORDER BY RAND() LIMIT 1000")
    for row in sample:
        pg_row = postgres.query("SELECT * FROM users WHERE id = %s", [row['id']])
        assert row == pg_row

    # Checksums
    mysql_checksum = mysql.query("SELECT MD5(GROUP_CONCAT(id, email)) FROM users")
    pg_checksum = postgres.query("SELECT MD5(STRING_AGG(id::text || email, '')) FROM users")
    assert mysql_checksum == pg_checksum

**Week 0:** Migration announced
**Week 2:** Phase 1 complete (dual writes)
**Week 4:** Backfill complete
**Week 6:** Traffic shifted to PostgreSQL
**Week 8:** Migration complete