Why EC2 pricing models (on-demand, reserved, spot) in AWS? - Purpose & Use Cases

The Big Idea

What if you could save big on servers without risking your website's speed?

The Scenario

Imagine you run a small online store and need computer servers to handle your website traffic. You try to guess how many servers you need and pay for each one every hour manually. Sometimes you pay too much when traffic is low, and other times your site slows down because you don't have enough servers.

The Problem

Manually choosing and paying for servers hour by hour is slow and confusing. You might waste money paying for unused servers or lose customers when your site is too slow. It's hard to predict exactly how many servers you need, and changing plans takes time and effort.

The Solution

EC2 pricing models let you pick the best way to pay for servers based on your needs. On-demand lets you pay only when you use servers, reserved saves money if you commit long-term, and spot offers big discounts for flexible use. This makes managing costs and capacity easier and smarter.

Before vs After

✗ Before

Pay $0.10 per hour for each server, no discounts, no flexibility

✓ After

Use on-demand for flexibility, reserved for savings, spot for cheap spare capacity

What It Enables

You can run your applications reliably while saving money by choosing the right pricing model for your server needs.

Real Life Example

A startup uses on-demand servers during launch, switches to reserved servers for steady traffic, and uses spot servers for batch jobs to save costs.

Key Takeaways

Manual server payment is costly and inflexible.

EC2 pricing models offer flexible and cost-effective options.

Choosing the right model helps balance cost and performance.

Practice

1. Which EC2 pricing model allows you to pay only for the compute time you use without any long-term commitment?

easy

A. Dedicated hosts

B. Reserved instances

C. Spot instances

D. On-demand instances

Solution

1. Step 1: Understand On-demand pricing

   On-demand instances let you pay per hour or second without any upfront commitment.

2. Step 2: Compare with other models

   Reserved requires commitment; Spot can be interrupted; Dedicated hosts are physical servers.

3. Final Answer:

   On-demand instances -> Option D

4. Quick Check:

   Pay-as-you-go = On-demand [OK]

Hint: No commitment means On-demand pricing [OK]

Common Mistakes:

• Confusing Reserved with On-demand
• Thinking Spot is always available
• Mixing Dedicated hosts with pricing models

2. Which of the following is the correct way to describe Spot instances in AWS EC2?

easy

A. Instances with a fixed monthly fee

B. Instances that use spare capacity and can be interrupted

C. Instances reserved for 3 years with upfront payment

D. Instances that run only on dedicated hardware

Solution

1. Step 1: Define Spot instances

   Spot instances use spare AWS capacity and are cheaper but can be stopped anytime.

2. Step 2: Eliminate other options

   Fixed monthly fee applies to Reserved; dedicated hardware is Dedicated hosts.

3. Final Answer:

   Instances that use spare capacity and can be interrupted -> Option B

4. Quick Check:

   Spot = spare capacity + interruption [OK]

Hint: Spot = cheap but can stop anytime [OK]

Common Mistakes:

• Thinking Spot has fixed fees
• Confusing Reserved with Spot
• Mixing Dedicated hosts with Spot

3. You launch an EC2 instance using Spot pricing. What happens if AWS needs the capacity back?

medium

A. Your instance is stopped or terminated with a 2-minute warning

B. You are charged the full On-demand price instead

C. Your instance continues running without interruption

D. Your instance is automatically converted to Reserved pricing

Solution

1. Step 1: Understand Spot instance interruptions

   AWS can reclaim Spot instances anytime, giving a 2-minute warning before stopping or terminating.

2. Step 2: Check other options

   Instances do not continue uninterrupted; no automatic price change or conversion to Reserved.

3. Final Answer:

   Your instance is stopped or terminated with a 2-minute warning -> Option A

4. Quick Check:

   Spot interruption = 2-minute warning stop [OK]

Hint: Spot instances get 2-minute stop warning [OK]

Common Mistakes:

• Assuming Spot instances never stop
• Thinking Spot switches to On-demand automatically
• Believing Spot instances run like Reserved

4. A user wants to save costs by committing to a 1-year usage but accidentally selects On-demand pricing. What is the main issue with this choice?

medium

A. On-demand pricing does not offer cost savings for long-term commitment

B. On-demand pricing requires upfront payment for 1 year

C. On-demand pricing instances cannot be launched immediately

D. On-demand pricing instances are always interrupted

Solution

1. Step 1: Understand On-demand pricing characteristics

   On-demand pricing charges per use with no upfront or commitment discounts.

2. Step 2: Identify the cost-saving option

   Reserved instances offer savings with 1 or 3 year commitments, unlike On-demand.

3. Final Answer:

   On-demand pricing does not offer cost savings for long-term commitment -> Option A

4. Quick Check:

   Commitment savings = Reserved, not On-demand [OK]

Hint: Commitment savings only with Reserved pricing [OK]

Common Mistakes:

• Thinking On-demand requires upfront payment
• Believing On-demand instances are delayed
• Confusing interruption with Spot pricing

5. A company runs a critical web application that must be available 24/7 with predictable costs. They also want to reduce expenses by committing to usage. Which EC2 pricing model should they choose and why?

hard

A. Spot instances, because they are cheapest and can be interrupted

B. On-demand instances, because they have no commitment and flexible pricing

C. Reserved instances, because they offer cost savings with a commitment and stable availability

D. Dedicated hosts, because they provide physical isolation

Solution

1. Step 1: Analyze availability and cost needs

   Critical 24/7 apps need stable availability and predictable costs.

2. Step 2: Match pricing models to needs

   Reserved instances provide cost savings with 1 or 3 year commitment and stable availability; Spot can be interrupted; On-demand is flexible but more costly.

3. Final Answer:

   Reserved instances, because they offer cost savings with a commitment and stable availability -> Option C

4. Quick Check:

   Critical + savings + commitment = Reserved [OK]

Hint: Stable + savings + commitment = Reserved instances [OK]

Common Mistakes:

• Choosing Spot for critical apps despite interruptions
• Picking On-demand for cost savings
• Confusing Dedicated hosts with pricing benefits