AWS Cost Management: On-Demand Instances
In the AWS ecosystem, On-Demand Instances represent the most flexible way to consume compute capacity. You pay for compute capacity by the second or hour (depending on the OS) with no long-term commitments or upfront payments. It is the “default” pricing model for Amazon EC2.
Core Concepts & Billing
On-Demand Instances are ideal for users who want the low cost and flexibility of EC2 without making an upfront payment or long-term commitment. Billing starts the moment an instance launches and ends when the instance is terminated or stopped.
- Billing Increments: Linux, Windows, and Ubuntu are billed per second (60-second minimum). Other OS types are billed per hour.
- No Termination Risk: Unlike Spot instances, AWS will not reclaim On-Demand instances for internal capacity needs.
- Tenancy Options: Can run on shared hardware (default) or Dedicated Hosts/Instances.
Comparison of EC2 Purchasing Models
| Feature | On-Demand | Reserved Instances | Spot Instances |
|---|---|---|---|
| Commitment | None (Pay-as-you-go) | 1 or 3 years | None |
| Cost Savings | Baseline (Highest) | Up to 72% discount | Up to 90% discount |
| Reliability | High (No interruptions) | High (Capacity reservation) | Low (Can be interrupted) |
| Best Use Case | Unpredictable workloads | Steady-state workloads | Fault-tolerant tasks |
Architecture Patterns & Scenario-Based Learning
When to Choose On-Demand
- New Applications: When you haven’t benchmarked the load yet and don’t know your “steady state.”
- Short-term Workloads: Projects lasting only a few weeks or days.
- Spiky Traffic: Handling sudden bursts in traffic that exceed your Reserved Instance coverage.
- Dev/Test Environments: Instances that are turned off after work hours.
Decision Matrix / If–Then Guide
- IF the workload is unpredictable and cannot be interrupted THEN use On-Demand.
- IF you are running a production database 24/7 for a year THEN use Reserved Instances.
- IF you are running a batch job that can restart if interrupted THEN use Spot Instances.
- IF you need to meet regulatory requirements for physical isolation THEN use Dedicated Hosts (On-Demand pricing applies).
Exam Tips and Gotchas
- The “Cost Optimization” Trap: If an exam question asks for the “most cost-effective” solution for a steady-state application, On-Demand is usually the wrong answer. Look for Reserved Instances or Savings Plans.
- Default Choice: On-Demand is the best choice for “unpredictable” or “short-term” requirements where the application cannot be interrupted.
- Capacity Limits: While highly available, On-Demand instances are subject to account-level service quotas (vCPU limits).
- Transitioning: You can convert an active On-Demand instance into a Reserved Instance benefit simply by purchasing a matching RI; no reboot is required.
Topics covered:
Summary of key subtopics covered in this guide:
- On-Demand billing mechanics (per-second vs. per-hour).
- Comparison with Spot and Reserved purchasing models.
- Workload suitability (unpredictable vs. steady-state).
- Tenancy and isolation options.
- Strategic placement within a cost-optimized architecture.
Integration: Seamlessly works with Auto Scaling Groups (ASG) to handle traffic spikes. Integrated with IAM for permissions and CloudWatch for monitoring billing alarms.
Consistent: Provides consistent CPU and memory performance. Unlike “Burstable” T-instances that use credits, standard On-Demand instances provide fixed performance indefinitely.
Strategy: Use On-Demand for the “variable” portion of your architecture. Map the “base load” to Savings Plans and use On-Demand for anything that exceeds that base.
Production Use Case: The “Viral Launch”
A startup launches a new marketing campaign. They don’t know if they will have 100 or 100,000 users. They use On-Demand Instances combined with Auto Scaling. Once the traffic stabilizes after 2 months, they purchase Savings Plans for the remaining consistent load to reduce costs by 60%.