Scalability is the capability of a system to handle a growing amount of work by adding resources to the system. There are two types of scaling:

1. **Vertical Scaling (Scale Up):**
   - Adding more power to existing resources
   - Example: Upgrading CPU/RAM

2. **Horizontal Scaling (Scale Out):**
   - Adding more resources
   - Example: Adding more servers

High Availability (HA) is a characteristic of a system that aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period.

Key components:
1. **Redundancy:**
   - Multiple instances
   - No single point of failure

2. **Monitoring:**
   - Health checks
   - Automated failover

3. **Load Balancing:**
   - Traffic distribution
   - Resource optimization

Load Balancing is the process of distributing network traffic across multiple servers to ensure no single server bears too much demand.

Common Load Balancing algorithms:
1. **Round Robin**
2. **Least Connections**
3. **IP Hash**
4. **Weighted Round Robin**
5. **Resource-Based**

Example of Nginx Load Balancer configuration:
```nginx
http {
    upstream backend {
        server backend1.example.com;
        server backend2.example.com;
        server backend3.example.com;
    }

    server {
        listen 80;
        location / {
            proxy_pass http://backend;
        }
    }
}
```

Auto Scaling is a feature that automatically adjusts the number of compute resources based on the current demand.

Key concepts:
1. **Scaling Policies:**
   - Target tracking
   - Step scaling
   - Simple scaling

2. **Metrics:**
   - CPU utilization
   - Memory usage
   - Request count
   - Custom metrics

Example of AWS Auto Scaling configuration:
```yaml
AutoScalingGroup:
  MinSize: 1
  MaxSize: 10
  DesiredCapacity: 2
  HealthCheckType: ELB
  HealthCheckGracePeriod: 300
  LaunchTemplate:
    LaunchTemplateId: !Ref LaunchTemplate
    Version: !GetAtt LaunchTemplate.LatestVersionNumber
```