# Horizontal Scaling Strategies

## Overview

Changemaker Lite V2 can scale horizontally to handle increased traffic and data volume. This guide covers strategies for scaling each component.

**When to Scale:**
- API response time >500ms (P95)
- CPU usage >70% sustained
- Memory usage >80% sustained
- Database connection pool exhausted
- Job queue backing up (>100 jobs waiting)

---

## Database Scaling

### Read Replicas

**PostgreSQL streaming replication** for read-heavy workloads.

**Setup** (docker-compose.yml):
```yaml
v2-postgres-replica:
  image: postgres:16-alpine
  container_name: changemaker-v2-postgres-replica
  environment:
    POSTGRES_USER: replicator
    POSTGRES_PASSWORD: ${REPLICA_PASSWORD}
  command: |
    postgres -c wal_level=replica
             -c hot_standby=on
             -c max_wal_senders=3
             -c hot_standby_feedback=on
  volumes:
    - v2-postgres-replica-data:/var/lib/postgresql/data
```

**Primary config** (postgresql.conf):
```ini
wal_level = replica
max_wal_senders = 3
wal_keep_size = 64MB
```

**Replication user**:
```sql
CREATE ROLE replicator WITH REPLICATION LOGIN PASSWORD 'replica-password';
```

**Prisma read replica** (planned feature):
```typescript
// Future: Prisma read replicas
const prisma = new PrismaClient({
  datasources: {
    db: {
      url: process.env.DATABASE_URL,           // Primary (writes)
      replicaUrl: process.env.REPLICA_URL,     // Replica (reads)
    },
  },
});
```

---

### Connection Pooling

**PgBouncer** for connection pooling.

**docker-compose.yml**:
```yaml
pgbouncer:
  image: pgbouncer/pgbouncer:latest
  container_name: pgbouncer-changemaker
  environment:
    DATABASES_HOST: changemaker-v2-postgres
    DATABASES_PORT: 5432
    DATABASES_USER: changemaker
    DATABASES_PASSWORD: ${V2_POSTGRES_PASSWORD}
    DATABASES_DBNAME: changemaker_v2
    POOL_MODE: transaction
    MAX_CLIENT_CONN: 1000
    DEFAULT_POOL_SIZE: 20
  ports:
    - "6432:6432"
```

**Update DATABASE_URL**:
```bash
# Before (direct)
DATABASE_URL=postgresql://changemaker:pass@changemaker-v2-postgres:5432/changemaker_v2

# After (pooled)
DATABASE_URL=postgresql://changemaker:pass@pgbouncer:6432/changemaker_v2
```

**Benefits**:
- Handles 1000+ client connections with only 20 PostgreSQL connections
- Reduces connection overhead
- Prevents "too many connections" errors

---

## API Scaling

### Multiple API Containers

**docker-compose.yml**:
```yaml
api:
  # ... existing config
  deploy:
    replicas: 3  # Run 3 API containers
```

**Or manual scaling**:
```bash
docker compose up -d --scale api=3
```

**Load balancer** (Nginx upstream):
```nginx
upstream api_backend {
    least_conn;  # Load balancing algorithm
    server changemaker-v2-api-1:4000;
    server changemaker-v2-api-2:4000;
    server changemaker-v2-api-3:4000;
}

server {
    location /api/ {
        proxy_pass http://api_backend;
    }
}
```

**Session affinity** (sticky sessions):
```nginx
upstream api_backend {
    ip_hash;  # Route same IP to same backend
    server changemaker-v2-api-1:4000;
    server changemaker-v2-api-2:4000;
}
```

---

### Vertical Scaling (Resource Limits)

**Increase container resources**:
```yaml
api:
  deploy:
    resources:
      limits:
        cpus: '4'      # 4 CPU cores
        memory: 4G     # 4GB RAM
      reservations:
        cpus: '1'
        memory: 1G
```

**Node.js memory limit**:
```yaml
api:
  environment:
    - NODE_OPTIONS=--max-old-space-size=3072  # 3GB heap
```

---

## Redis Scaling

### Redis Cluster (Sharding)

**For >100GB datasets** or high throughput.

**docker-compose.yml** (6-node cluster):
```yaml
redis-cluster-1:
  image: redis:7-alpine
  command: redis-server --cluster-enabled yes --cluster-config-file nodes.conf

# ... repeat for redis-cluster-2 through redis-cluster-6
```

**Create cluster**:
```bash
docker compose exec redis-cluster-1 redis-cli --cluster create \
  redis-cluster-1:6379 \
  redis-cluster-2:6379 \
  redis-cluster-3:6379 \
  redis-cluster-4:6379 \
  redis-cluster-5:6379 \
  redis-cluster-6:6379 \
  --cluster-replicas 1
```

---

### Redis Sentinel (High Availability)

**Automatic failover** for Redis.

**docker-compose.yml**:
```yaml
redis-sentinel-1:
  image: redis:7-alpine
  command: redis-sentinel /etc/redis/sentinel.conf
  volumes:
    - ./configs/redis/sentinel.conf:/etc/redis/sentinel.conf

# ... repeat for sentinel-2, sentinel-3
```

**sentinel.conf**:
```ini
sentinel monitor mymaster redis-primary 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel parallel-syncs mymaster 1
sentinel failover-timeout mymaster 10000
```

---

## Media API Scaling

### Separate Media Containers

**docker-compose.yml**:
```yaml
media-api:
  deploy:
    replicas: 2  # Run 2 media API containers
```

**Nginx load balancer**:
```nginx
upstream media_backend {
    server changemaker-media-api-1:4100;
    server changemaker-media-api-2:4100;
}

location /api/media/ {
    proxy_pass http://media_backend;
}
```

**Shared volume** (read-only):
```yaml
media-api:
  volumes:
    - ${MEDIA_ROOT}:/media:ro  # All replicas read same library
```

---

### CDN for Static Media

**Cloudflare CDN** (or similar):

**Setup**:
1. Enable Cloudflare proxy (orange cloud)
2. Configure cache rules:
   - Cache `/media/library/*.mp4` for 30 days
   - Bypass cache for `/api/media/` (dynamic)

**Benefits**:
- Offload video bandwidth
- Global edge caching
- DDoS protection

---

## Frontend Scaling

### CDN for Static Assets

**Vite production build** → static files → CDN.

**Build**:
```bash
cd admin && npm run build
```

**Upload to CDN** (S3 + CloudFront):
```bash
aws s3 sync dist/ s3://changemaker-static/ --delete
aws cloudfront create-invalidation --distribution-id XYZ --paths "/*"
```

**Benefits**:
- Global edge caching
- Reduced origin load
- Faster page loads

---

### Nginx Caching

**Proxy cache for API responses**:
```nginx
proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=api_cache:10m max_size=1g inactive=60m;

location /api/campaigns {
    proxy_cache api_cache;
    proxy_cache_valid 200 10m;
    proxy_cache_key "$scheme$request_method$host$request_uri";
    proxy_pass http://changemaker-v2-api:4000;
}
```

**Cacheable endpoints**:
- `/api/campaigns` (public listing, 10 minutes)
- `/api/representatives` (lookup cache, 1 hour)
- `/api/locations/public` (map data, 5 minutes)

**Never cache**:
- POST/PUT/DELETE requests
- Authenticated endpoints
- Real-time data (canvass sessions)

---

## Job Queue Scaling

### Multiple BullMQ Workers

**API container scaling** also scales workers (each container runs worker).

**Alternative**: Dedicated worker containers.

**docker-compose.yml**:
```yaml
email-worker:
  build:
    context: ./api
  container_name: email-worker
  command: node dist/workers/email-worker.js
  environment:
    - REDIS_URL=${REDIS_URL}
    - SMTP_HOST=${SMTP_HOST}
    # ... other env vars
  depends_on:
    - redis
```

**Worker script** (api/src/workers/email-worker.ts):
```typescript
import { emailQueue } from '../services/email-queue.service';

emailQueue.process(10, async (job) => {
  // Process email job
});

console.log('Email worker started');
```

**Scale workers**:
```bash
docker compose up -d --scale email-worker=5
```

---

## Monitoring Under Load

### Load Testing

**k6 script** (load-test.js):
```javascript
import http from 'k6/http';
import { check } from 'k6';

export let options = {
  stages: [
    { duration: '1m', target: 50 },   // Ramp to 50 users
    { duration: '3m', target: 50 },   // Stay at 50 users
    { duration: '1m', target: 100 },  // Ramp to 100 users
    { duration: '3m', target: 100 },  // Stay at 100 users
    { duration: '1m', target: 0 },    // Ramp down
  ],
};

export default function () {
  let res = http.get('http://api.cmlite.org/api/campaigns');
  check(res, {
    'status 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500,
  });
}
```

**Run test**:
```bash
k6 run load-test.js
```

---

### Prometheus Metrics

**Monitor scaling indicators**:
- `rate(http_requests_total[5m])` — Request rate
- `histogram_quantile(0.95, http_request_duration_seconds)` — P95 latency
- `container_cpu_usage_seconds_total` — CPU usage per container
- `container_memory_usage_bytes` — Memory usage per container

**Grafana alert**:
```yaml
- alert: HighAPILatency
  expr: histogram_quantile(0.95, rate(http_request_duration_seconds_bucket[5m])) > 0.5
  for: 5m
  labels:
    severity: warning
  annotations:
    summary: "P95 latency >500ms, consider scaling"
```

---

## Troubleshooting

### High CPU Usage

**Diagnosis**:
```bash
# Top processes
docker stats

# API CPU usage
docker stats changemaker-v2-api

# Profile Node.js
docker compose exec api node --prof dist/server.js
```

**Solutions**:
- Scale API containers (3-5 replicas)
- Increase CPU limit (2-4 cores)
- Optimize slow queries (add indexes)
- Enable caching (Nginx proxy cache)

---

### Memory Leaks

**Diagnosis**:
```bash
# Memory usage over time
docker stats --no-stream changemaker-v2-api

# Heap snapshot (Node.js)
docker compose exec api node --inspect dist/server.js
# Chrome DevTools → Memory → Take snapshot
```

**Solutions**:
- Restart containers daily (cron job)
- Increase memory limit (4-8GB)
- Fix code leaks (event listeners, circular refs)

---

### Database Connection Exhaustion

**Symptoms**: `Error: too many connections for role "changemaker"`

**Diagnosis**:
```bash
# Check connection count
docker compose exec v2-postgres psql -U changemaker -c \
  "SELECT COUNT(*) FROM pg_stat_activity WHERE usename='changemaker'"

# Check max connections
docker compose exec v2-postgres psql -U changemaker -c \
  "SHOW max_connections"
```

**Solutions**:
- Add PgBouncer (connection pooling)
- Increase `max_connections` (PostgreSQL config)
- Fix connection leaks (always close Prisma clients)

---

## Cost Optimization

### Resource Allocation

**Right-sizing** (don't over-provision):
- Start with 1 CPU, 1GB RAM per container
- Monitor actual usage (Prometheus)
- Scale based on metrics (not guesses)

**Example** (production workload):
- API: 2 CPUs, 2GB RAM (3 replicas)
- PostgreSQL: 2 CPUs, 4GB RAM
- Redis: 1 CPU, 512MB RAM
- Media API: 2 CPUs, 2GB RAM (2 replicas)

---

### Autoscaling (Docker Swarm)

**Docker Swarm mode** (alternative to Compose):
```bash
# Initialize swarm
docker swarm init

# Deploy stack
docker stack deploy -c docker-compose.yml changemaker

# Autoscale API
docker service scale changemaker_api=3

# Update with zero downtime
docker service update --image api:v2.1 changemaker_api
```

**Autoscaling**:
```yaml
api:
  deploy:
    replicas: 3
    update_config:
      parallelism: 1
      delay: 10s
    restart_policy:
      condition: on-failure
```

---

## Related Documentation

- **[Docker Compose](docker-compose.md)** — Container orchestration
- **[Monitoring Stack](monitoring-stack.md)** — Performance metrics
- **[Nginx Configuration](nginx.md)** — Load balancing
- **[Backup & Restore](backup-restore.md)** — Data protection at scale