Capacity Reservations

aws/billing aws/compute aws/concept

💡 Definition

Capacity Reservations allow you to reserve compute capacity for your EC2 instances in a specific Availability Zone for any duration. This ensures that you have access to EC2 capacity whenever you need it, even during times of high demand.

🔑 Key Concepts

Capacity Assurance: Guarantees that you will have the specified EC2 capacity available whenever you need to launch instances.
Availability Zone Specific: Capacity is reserved in a particular AZ.
Billing: You are billed for the reserved capacity whether you use it or not. Any On-Demand Instances that match the attributes of the Capacity Reservation will automatically consume the reservation and be billed at the On-Demand rate (or Reserved Instance rate if you have RIs that apply).
Duration: Can be created for any duration (hourly, daily, monthly, etc.) and cancelled at any time.

⚙️ How it Works

You specify the instance type, platform, and quantity of instances you want to reserve in a particular AZ. AWS then allocates that capacity for your exclusive use. When you launch EC2 instances matching these attributes into that AZ, they will use your reserved capacity.

🎯 Use Cases

Critical Workloads: For applications that require uninterrupted access to compute capacity, even during peak periods or when demand for EC2 is high.
Specific Events: Ensuring capacity for planned events, testing, or production deployments where instance availability is paramount.
Disaster Recovery: Guaranteeing resources for recovery in a specific region/AZ.

💰 Pricing Model

Hourly: You pay the full On-Demand rate for the reserved capacity, whether you use it or not.
Reserved Instance Benefits: If you have Reserved Instances or Savings Plans that match the attributes of your Capacity Reservation, they can provide a discount on the Capacity Reservation charges.

📝 Exam Tips (CLF-C02)

Keywords: "Reserve capacity", "Guaranteed availability", "Specific Availability Zone".
The key benefit is guaranteed capacity, not cost savings (though RIs can cover the cost).
Contrast with Reserved Instances, which provide cost savings but do not guarantee capacity unless combined with a Capacity Reservation (zonal RIs implicitly provide capacity reservation).

See Also: * EC2 * On-Demand Instances * Reserved Instances * Savings Plans * AZ