# Migration Implementation Guide

## Pre-Migration Assessment

### Current State Analysis

```bash
set -euo pipefail
# Document current implementation
find . -name "*.ts" -o -name "*.py" | xargs grep -l "retellai" > retellai-files.txt

# Count integration points
wc -l retellai-files.txt

# Identify dependencies
npm list | grep retellai
pip freeze | grep retellai
```

### Data Inventory

```typescript
interface MigrationInventory {
  dataTypes: string[];
  recordCounts: Record<string, number>;
  dependencies: string[];
  integrationPoints: string[];
  customizations: string[];
}

async function assessRetellAIMigration(): Promise<MigrationInventory> {
  return {
    dataTypes: await getDataTypes(),
    recordCounts: await getRecordCounts(),
    dependencies: await analyzeDependencies(),
    integrationPoints: await findIntegrationPoints(),
    customizations: await documentCustomizations(),
  };
}
```

## Strangler Fig Pattern Diagram

```
Phase 1: Parallel Run
┌─────────────┐     ┌─────────────┐
│   Old       │     │   New       │
│   System    │ ──▶ │  Retell AI   │
│   (100%)    │     │   (0%)      │
└─────────────┘     └─────────────┘

Phase 2: Gradual Shift
┌─────────────┐     ┌─────────────┐
│   Old       │     │   New       │
│   (50%)     │ ──▶ │   (50%)     │
└─────────────┘     └─────────────┘

Phase 3: Complete
┌─────────────┐     ┌─────────────┐
│   Old       │     │   New       │
│   (0%)      │ ──▶ │   (100%)    │
└─────────────┘     └─────────────┘
```

## Phase-by-Phase Implementation

### Phase 1: Setup (Week 1-2)

```bash
set -euo pipefail
# Install Retell AI SDK
npm install @retellai/sdk

# Configure credentials
cp .env.example .env.retellai
# Edit with new credentials

# Verify connectivity
node -e "require('@retellai/sdk').ping()"
```

### Phase 2: Adapter Layer (Week 3-4)

```typescript
// src/adapters/retellai.ts
interface ServiceAdapter {
  create(data: CreateInput): Promise<Resource>;
  read(id: string): Promise<Resource>;
  update(id: string, data: UpdateInput): Promise<Resource>;
  delete(id: string): Promise<void>;
}

class RetellAIAdapter implements ServiceAdapter {
  async create(data: CreateInput): Promise<Resource> {
    const retellaiData = this.transform(data);
    return retellaiClient.create(retellaiData);
  }

  private transform(data: CreateInput): RetellAIInput {
    // Map from old format to Retell AI format
  }
}
```

### Phase 3: Data Migration (Week 5-6)

```typescript
async function migrateRetellAIData(): Promise<MigrationResult> {
  const batchSize = 100;
  let processed = 0;
  let errors: MigrationError[] = [];

  for await (const batch of oldSystem.iterateBatches(batchSize)) {
    try {
      const transformed = batch.map(transform);
      await retellaiClient.batchCreate(transformed);
      processed += batch.length;
    } catch (error) {
      errors.push({ batch, error });
    }

    // Progress update
    console.log(`Migrated ${processed} records`);
  }

  return { processed, errors };
}
```

### Phase 4: Traffic Shift (Week 7-8)

```typescript
// Feature flag controlled traffic split
function getServiceAdapter(): ServiceAdapter {
  const retellaiPercentage = getFeatureFlag('retellai_migration_percentage');

  if (Math.random() * 100 < retellaiPercentage) {
    return new RetellAIAdapter();
  }

  return new LegacyAdapter();
}
```

## Rollback Plan

```bash
set -euo pipefail
# Immediate rollback
kubectl set env deployment/app RETELLAI_ENABLED=false
kubectl rollout restart deployment/app

# Data rollback (if needed)
./scripts/restore-from-backup.sh --date YYYY-MM-DD

# Verify rollback
curl https://app.yourcompany.com/health | jq '.services.retellai'
```

## Post-Migration Validation

```typescript
async function validateRetellAIMigration(): Promise<ValidationReport> {
  const checks = [
    { name: 'Data count match', fn: checkDataCounts },
    { name: 'API functionality', fn: checkApiFunctionality },
    { name: 'Performance baseline', fn: checkPerformance },
    { name: 'Error rates', fn: checkErrorRates },
  ];

  const results = await Promise.all(
    checks.map(async c => ({ name: c.name, result: await c.fn() }))
  );

  return { checks: results, passed: results.every(r => r.result.success) };
}
```
