Auto Scaling Group (ASG)

💡 Definition

An Auto Scaling Group (ASG) is a collection of EC2 instances that are treated as a logical unit for the purposes of automatic scaling and management. It is the core component of Amazon Auto Scaling.

🔑 Key Concepts

• Capacity Settings:
  • Minimum: The fewest number of instances the group should ever have.
  • Maximum: The most instances the group is allowed to scale out to.
  • Desired: The target number of instances you want running right now.
• Self-Healing: If an instance becomes unhealthy (fails a health check), the ASG terminates it and launches a new one to maintain the Desired/Minimum capacity.
• Scaling Policies: Rules that tell the ASG when to launch (scale out) or terminate (scale in) instances based on metrics like CPU utilization.

⚙️ How it Works

1. Define: You create an ASG, specifying the Launch Template and the VPC subnets.
2. Monitor: The ASG monitors the health of instances.
3. Scale: Based on alarms or schedules, it changes the Desired Capacity.
4. Launch/Terminate: It calls the EC2 API to launch or terminate instances to match the Desired Capacity.

🎯 Use Cases

• Dynamic Workloads: Applications with traffic that varies throughout the day.
• High Availability: Ensuring at least X instances are always running across multiple AZs.
• Fleet Management: Automatically replacing failed hardware.

💰 Pricing Model

• Free: The ASG feature itself is free. You pay for the EC2 instances it runs.

📝 Exam Tips (CLF-C02)

• Defines WHERE (VPC/Subnets) and HOW MANY (Min/Max/Desired) instances to run.
• Uses a Launch Template to define WHAT to run.
• Can span multiple Availability Zones.

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See Also: * Launch Template * Load Balancer * Auto Scaling