Task Statement 4.2: Design cost-optimized compute solutions.
📘AWS Certified Solutions Architect – (SAA-C03)
1. Overview: Why Choosing the Right Compute Service Matters
In AWS, compute services run your applications.
Choosing the wrong service can:
- Increase costs
- Reduce performance
- Add unnecessary complexity
Choosing the right one helps you:
- Pay only for what you use
- Scale efficiently
- Reduce operational work
2. Main AWS Compute Services for the Exam
You must clearly understand these three:
1. AWS Lambda (Serverless)
2. Amazon EC2 (Virtual Servers)
3. AWS Fargate (Serverless Containers)
3. AWS Lambda (Serverless Compute)
What is AWS Lambda?
AWS Lambda runs your code without managing servers.
You only upload code, and AWS handles:
- Servers
- Scaling
- Maintenance
Cost Model
You pay only for:
- Number of requests
- Execution time (milliseconds)
No cost when idle.
Key Characteristics
- Fully serverless
- Automatic scaling
- Event-driven
- Short execution time (max 15 minutes)
When to Use (Exam Focus)
Use Lambda when:
- Workloads are intermittent or unpredictable
- Execution time is short
- You want zero server management
IT-Based Use Cases
- Processing files uploaded to S3
- Running backend APIs using API Gateway
- Data transformation tasks
- Real-time log processing
Cost Optimization Benefits
- No idle cost
- Auto-scaling (no over-provisioning)
- Fine-grained billing (milliseconds)
Limitations (Important for Exam)
- Not suitable for long-running tasks
- Limited memory and CPU
- Cold start latency (for infrequent use)
4. Amazon EC2 (Elastic Compute Cloud)
What is EC2?
Amazon EC2 provides virtual servers where you control everything:
- OS
- Software
- Scaling (manual or auto)
Cost Model
You pay for:
- Running instances (per second/hour)
- Storage, networking, etc.
Key Characteristics
- Full control over environment
- Flexible instance types
- Supports all workloads
When to Use (Exam Focus)
Use EC2 when:
- You need full control
- Workloads are long-running
- Applications require custom configuration
IT-Based Use Cases
- Hosting web servers (e.g., Apache, Nginx)
- Running databases (self-managed)
- Enterprise applications
- Legacy systems
Cost Optimization Options (VERY IMPORTANT)
To reduce cost in EC2, use:
1. Spot Instances
- Up to 90% cheaper
- Can be interrupted
2. Reserved Instances / Savings Plans
- Commit for 1–3 years
- Lower cost
3. Auto Scaling
- Add/remove instances based on demand
Cost Optimization Benefits
- Flexible pricing options
- Can optimize based on workload predictability
Limitations
- You manage servers
- Can have idle cost
- Requires scaling configuration
5. AWS Fargate (Serverless Containers)
What is Fargate?
AWS Fargate runs containers without managing servers.
Works with:
- Amazon ECS
- Amazon EKS
Cost Model
You pay for:
- CPU and memory used per second
No cost when containers are not running.
Key Characteristics
- Serverless container platform
- No infrastructure management
- Scales automatically
When to Use (Exam Focus)
Use Fargate when:
- You are using containers
- You want serverless compute
- You don’t want to manage EC2 instances
IT-Based Use Cases
- Microservices architecture
- Containerized APIs
- Batch processing jobs in containers
Cost Optimization Benefits
- No need to pay for idle EC2 instances
- Pay per task usage
- Simplified scaling
Limitations
- More expensive than EC2 for constant workloads
- Less control than EC2
6. Key Comparison (VERY IMPORTANT FOR EXAM)
| Feature | AWS Lambda | Amazon EC2 | AWS Fargate |
|---|---|---|---|
| Server Management | None | Full control | None |
| Pricing | Per request + duration | Per instance time | Per CPU & memory |
| Best For | Short tasks, event-driven | Long-running apps | Containerized apps |
| Scaling | Automatic | Manual/Auto Scaling | Automatic |
| Idle Cost | None | Yes | None |
| Control Level | Low | High | Medium |
7. How to Choose the Most Cost-Effective Service
Step 1: Check Workload Type
Short, event-driven tasks
→ Use Lambda
Long-running applications
→ Use EC2
Container-based workloads
→ Use Fargate
Step 2: Check Usage Pattern
Spiky or unpredictable
→ Lambda or Fargate
Steady usage
→ EC2 with Reserved Instances or Savings Plans
Step 3: Check Management Requirement
Want no infrastructure management
→ Lambda or Fargate
Need full control
→ EC2
8. Exam Tips (VERY IMPORTANT)
1. Always look for keywords in questions
| Keyword | Best Choice |
|---|---|
| Event-driven | Lambda |
| Short execution | Lambda |
| Full control | EC2 |
| Containers | Fargate |
| No server management | Lambda / Fargate |
2. Cost Optimization Clues
- “Avoid paying for idle resources” → Lambda / Fargate
- “Predictable workload” → EC2 Reserved Instances
- “Flexible workload, interruptible” → Spot Instances
3. Common Exam Traps
- Choosing EC2 when Lambda is cheaper for short tasks
- Choosing Fargate when EC2 is cheaper for steady workloads
- Ignoring Spot Instances for cost savings
9. Final Summary
- AWS Lambda → Best for event-driven, short tasks, lowest cost for intermittent workloads
- Amazon EC2 → Best for full control and long-running applications
- AWS Fargate → Best for containers without managing servers
If you understand:
- When to use each service
- Their pricing models
- Their scaling behavior
👉 You are fully prepared for this part of the SAA-C03 exam.
