AWS Study Guide: Reserved Instances (RI)

Reserved Instances (RIs) are a powerful cost-optimization mechanism in AWS that provide a significant discount (up to 75%) compared to On-Demand pricing. Unlike On-Demand instances, which you pay for by the second or hour with no commitment, RIs require a commitment to a specific instance configuration for a 1-year or 3-year term.

The Real-World Analogy

Think of a Gym Membership.

• On-Demand: You pay a “drop-in” fee every time you visit. It’s flexible but expensive if you go every day.
• Reserved Instance: You sign a 1-year contract. Because you committed to the gym, they give you a much lower monthly rate. Even if you don’t show up one day, you still pay for that day because you “reserved” your spot.

Core Concepts & Configuration

1. Commitment Terms & Payment Options

The discount depth depends on two main factors: how long you commit and how you pay.

• Term: 1 Year or 3 Years (3 years offers higher discounts).
• Payment Options:
  • All Upfront: Entire cost paid at start. Highest discount.
  • Partial Upfront: Portion paid at start; remaining balance paid monthly.
  • No Upfront: Nothing paid at start; entire discounted cost paid monthly.

2. RI Types: Standard vs. Convertible

For the SAA-C03 exam, you must distinguish between the flexibility of these two types:

• Standard RI: Highest discount. Cannot change instance families. Best for steady-state usage where the application architecture is mature.
• Convertible RI: Slightly lower discount but allows you to exchange for another Convertible RI of equal or greater value. You can change instance families, OS, and tenancies.

3. Scope: Regional vs. Zonal

This is a frequent exam topic regarding Capacity Reservations:

• Regional RI: The discount applies to any instance in any AZ within that Region. It provides no capacity reservation.
• Zonal RI: You specify a specific Availability Zone (e.g., us-east-1a). This reserves capacity in that specific AZ, ensuring you can launch the instance when needed.

Comparison: RI vs. Other Pricing Models

Decision Matrix / If–Then Guide

• If the workload is steady-state and the instance family won’t change: Choose Standard RI (3-year All Upfront for max savings).
• If the workload is steady-state but you might move from C5 to M5 instances later: Choose Convertible RI.
• If you need to ensure capacity is available in a specific AZ for a critical DR failover: Choose Zonal RI.
• If you want the RI discount to apply to any size (e.g., m5.large and m5.xlarge): Choose Regional RI with Linux/Unix (Instance Size Flexibility).

Exam Tips and Gotchas

Topics covered :

Summary of key subtopics covered in this guide:

• Standard vs. Convertible RI flexibility
• Payment options (All, Partial, No Upfront)
• Regional vs. Zonal scope and capacity implications
• Instance Size Flexibility (Normalization factors)
• RI Marketplace for selling unused capacity
• Comparison with On-Demand and Spot instances