Incremental Migration in Magento 2

Decomposing the Monolith

Feature Map

Current Monolith:
┌─────────────────────────────────────┐
│         Legacy Magento              │
├─────────┬─────────┬─────────┬──────┤
│ Catalog │ Orders  │ Payments│ Users │
│ - Products│ - Create│ - Stripe│ - Auth│
│ - Search │ - Status│ - PayPal│ - ACL │
│ - Rules  │ - Refund│ - Invoice│ - API │
└─────────┴─────────┴─────────┴──────┘

Target: Migrate features independently

Migration Order (by dependency):
1. User Authentication (no dependencies)
2. Product Catalog (depends on users)
3. Cart (depends on catalog)
4. Orders (depends on cart, payments)
5. Payments (depends on orders)

Migration Priority Matrix

Feature        | Business Value | Complexity | Dependencies | Priority
---------------|---------------|------------|-------------|----------
User Auth      | High          | Low        | None        | P1
Product Catalog| High          | Medium     | Users       | P1
Search         | Medium        | Medium     | Catalog     | P2
Cart           | High          | Medium     | Catalog     | P1
Checkout       | High          | High       | Cart, Pay   | P2
Orders         | High          | High       | Checkout    | P3
Payments       | High          | High       | Orders      | P3
Admin Panel    | Medium        | Medium     | All         | P4

Feature Migration Steps

For each feature:
1. Define interface (API contract)
2. Build new implementation
3. Write tests for both old and new
4. Implement data sync
5. Route traffic gradually
6. Verify consistency
7. Remove old code
8. Update documentation

Estimation per Feature

$featureEstimates = [
    'user_auth' => [
        'new_code' => 40,      // hours
        'tests' => 16,
        'data_sync' => 8,
        'integration' => 16,
        'total' => 80
    ],
    'product_catalog' => [
        'new_code' => 80,
        'tests' => 32,
        'data_sync' => 24,
        'integration' => 24,
        'total' => 160
    ],
];

Sync Strategies

Real-Time Sync

// Event-based synchronization
class OrderSyncService
{
    public function __construct(
        private EventManager $eventManager,
        private LegacyOrderRepository $legacyRepo,
        private NewOrderRepository $newRepo
    ) {
        $this->eventManager->dispatch('order_save_after', [
            'order' => $order,
            'sync_callback' => [$this, 'syncOrder']
        ]);
    }
    
    public function syncOrder(OrderInterface $order): void
    {
        $newOrder = $this->convertOrder($order);
        $this->newRepo->save($newOrder);
    }
}

Batch Sync

// Nightly batch synchronization
public function syncOrdersBatch(int $batchSize = 1000): void
{
    $lastId = $this->getLastSyncedId();
    
    do {
        $orders = $this->legacyRepo->getOrdersAfter($lastId, $batchSize);
        
        foreach ($orders as $order) {
            $this->syncOrder($order);
            $lastId = $order->getId();
        }
        
        $this->updateLastSyncedId($lastId);
        
    } while (count($orders) === $batchSize);
}

Conflict Resolution

// Handle sync conflicts
public function resolveConflict(
    OrderInterface $legacy,
    OrderInterface $new
): OrderInterface {
    // Strategy 1: Last write wins
    if ($legacy->getUpdatedAt() > $new->getUpdatedAt()) {
        return $this->convertToNew($legacy);
    }
    
    // Strategy 2: Legacy wins during migration
    if ($this->isLegacyPrimary()) {
        return $this->convertToNew($legacy);
    }
    
    // Strategy 3: Manual resolution
    $this->flagForReview($legacy, $new);
    return $new; // Default to new
}

Sync Monitoring

// Monitor sync health
public function getSyncStatus(): array
{
    return [
        'legacy_count' => $this->legacyRepo->count(),
        'new_count' => $this->newRepo->count(),
        'synced_count' => $this->getSyncedCount(),
        'pending_count' => $this->getPendingCount(),
        'error_count' => $this->getErrorCount(),
        'last_sync' => $this->getLastSyncTime(),
        'sync_lag' => $this->getSyncLag(),
    ];
}

Writing to Both Systems

Synchronous Dual-Write

// Write to both systems atomically
public function saveOrder(OrderInterface $order): void
{
    $transaction = $this->transactionFactory->create();
    
    try {
        // Write to new system
        $this->newOrderRepo->save($order);
        
        // Write to legacy system
        $legacyOrder = $this->convertToLegacy($order);
        $this->legacyOrderRepo->save($legacyOrder);
        
        $transaction->commit();
    } catch (\Exception $e) {
        $transaction->rollBack();
        throw $e;
    }
}

Asynchronous Dual-Write

// Write to primary, queue secondary
public function saveOrder(OrderInterface $order): void
{
    // Write to primary (new system)
    $this->newOrderRepo->save($order);
    
    // Queue for legacy sync
    $this->messageQueue->publish('order.sync', [
        'order_id' => $order->getId(),
        'action' => 'save',
        'timestamp' => time()
    ]);
}

// Consumer process
public function processSyncMessage(array $message): void
{
    $order = $this->newOrderRepo->get($message['order_id']);
    $legacyOrder = $this->convertToLegacy($order);
    $this->legacyOrderRepo->save($legacyOrder);
}

Switching Primary System

// Gradually switch which system is primary
public function saveOrder(OrderInterface $order): void
{
    if ($this->isNewSystemPrimary()) {
        // New is primary, write to legacy async
        $this->newOrderRepo->save($order);
        $this->queueLegacySync($order);
    } else {
        // Legacy is primary, write to new async
        $this->legacyOrderRepo->save($order);
        $this->queueNewSync($order);
    }
}

Error Handling

// Handle dual-write failures
public function saveOrderWithRetry(OrderInterface $order, int $maxRetries = 3): void
{
    for ($i = 0; $i < $maxRetries; $i++) {
        try {
            $this->saveOrder($order);
            return;
        } catch (\Exception $e) {
            $this->logger->warning('Dual-write failed', [
                'attempt' => $i + 1,
                'error' => $e->getMessage()
            ]);
            
            if ($i === $maxRetries - 1) {
                $this->flagForManualReview($order, $e);
                throw $e;
            }
            
            sleep(pow(2, $i)); // Exponential backoff
        }
    }
}

Consistency Verification

Automated Validation

// Verify data consistency between systems
public function validateOrderConsistency(int $orderId): ValidationResult
{
    $legacy = $this->legacyRepo->get($orderId);
    $new = $this->newRepo->get($orderId);
    
    $diffs = [];
    
    // Compare fields
    $fields = ['grand_total', 'status', 'customer_email', 'shipping_method'];
    
    foreach ($fields as $field) {
        $legacyValue = $legacy->getData($field);
        $newValue = $new->getData($field);
        
        if ($legacyValue != $newValue) {
            $diffs[] = [
                'field' => $field,
                'legacy' => $legacyValue,
                'new' => $newValue
            ];
        }
    }
    
    return new ValidationResult(
        count($diffs) === 0,
        $diffs
    );
}

Bulk Validation

// Validate all synced orders
public function validateAllSynced(int $limit = 10000): array
{
    $results = [
        'total' => 0,
        'consistent' => 0,
        'inconsistent' => 0,
        'missing_legacy' => 0,
        'missing_new' => 0
    ];
    
    $orderIds = $this->getSyncedOrderIds($limit);
    
    foreach ($orderIds as $orderId) {
        $results['total']++;
        
        $legacy = $this->legacyRepo->get($orderId);
        $new = $this->newRepo->get($orderId);
        
        if (!$legacy) {
            $results['missing_legacy']++;
            continue;
        }
        
        if (!$new) {
            $results['missing_new']++;
            continue;
        }
        
        if ($this->validateOrderConsistency($orderId)->isConsistent()) {
            $results['consistent']++;
        } else {
            $results['inconsistent']++
        }
    }
    
    return $results;
}

Reconciliation Reports

// Generate reconciliation report
public function generateReport(): string
{
    $report = "Data Reconciliation Report\n";
    $report .= str_repeat('=', 50) . "\n\n";
    
    $results = $this->validateAllSynced();
    
    $report .= "Total Orders Validated: {$results['total']}\n";
    $report .= "Consistent: {$results['consistent']}\n";
    $report .= "Inconsistent: {$results['inconsistent']}\n";
    $report .= "Missing in Legacy: {$results['missing_legacy']}\n";
    $report .= "Missing in New: {$results['missing_new']}\n\n";
    
    $consistencyRate = ($results['consistent'] / $results['total']) * 100;
    $report .= sprintf("Consistency Rate: %.2f%%\n", $consistencyRate);
    
    if ($results['inconsistent'] > 0) {
        $report .= "\nInconsistent Orders:\n";
        $inconsistent = $this->getInconsistentOrders();
        foreach ($inconsistent as $order) {
            $report .= "- Order #{$order['id']}: {$order['diff_count']} differences\n";
        }
    }
    
    return $report;
}