AWS Compute Services: EC2, Lambda, ECS & EKS

Compute service selection is one of the most common interview topics. Understanding when to use each service demonstrates architectural maturity.

EC2: The Foundation

Instance Types & Selection

Family	Use Case	Example
General Purpose (M/T)	Web servers, small DBs	m7i.xlarge, t3.medium
Compute Optimized (C)	Batch processing, gaming	c7i.2xlarge
Memory Optimized (R/X)	In-memory databases	r7i.4xlarge, x2idn.large
Storage Optimized (I/D)	Data warehousing, HDFS	i4i.large, d3.xlarge
Accelerated (P/G)	ML training, graphics	p5.48xlarge, g5.xlarge

Interview Question: Right-sizing

Q: "Your web application uses m5.2xlarge instances but CPU averages 15%. What do you recommend?"

A: Analyze CloudWatch metrics for CPU, memory, and network. Consider:

1. Downsize to m5.large (4x smaller, 4x cheaper)
2. Use Compute Savings Plans for predictable workloads (up to 72% savings)
3. Consider Graviton (m7g) for additional 20-40% cost reduction
4. Implement auto-scaling to handle peaks

Lambda: Serverless Compute

Key Characteristics

• Max duration: 15 minutes
• Memory: 128 MB to 10,240 MB
• Concurrency: 1,000 default (can increase)
• Cold start: 100ms-1s (varies by runtime)

When to Use Lambda

Good Fit:

• Event-driven processing (S3 triggers, API Gateway)
• Short-running tasks (< 15 min)
• Variable/unpredictable traffic
• Microservice APIs with low latency requirements

Poor Fit:

• Long-running processes (use Step Functions or ECS)
• High-frequency, consistent load (EC2 cheaper at scale)
• GPU-intensive workloads

Interview Question: Lambda vs EC2 Cost

Q: "Your API receives 10M requests/month, each taking 200ms and using 512MB. Should you use Lambda or EC2?"

A: Calculate Lambda cost:

• Requests: 10M × $0.20/1M = $2.00
• Compute: 10M × 0.2s × 0.5GB × $0.0000166667 = $16.67
• Total: ~$19/month

EC2 t3.medium could handle this but requires management overhead. Lambda wins for simplicity at this scale. Beyond ~50M requests/month, EC2 becomes more cost-effective.

ECS: Container Orchestration

Launch Types

Type	Use Case	Pricing
EC2	Control over infrastructure	Pay for EC2 instances
Fargate	Serverless containers	Pay per vCPU/memory-hour

ECS Architecture Components

• Cluster: Logical grouping of services
• Service: Maintains desired task count
• Task Definition: Container configuration (like Dockerfile + docker-compose)
• Task: Running container instance

Interview Question: ECS vs EKS

Q: "When would you choose ECS over EKS?"

A: Choose ECS when:

• AWS-native integration is priority
• Team lacks Kubernetes expertise
• Simpler deployment model is acceptable
• Vendor lock-in is not a concern

Choose EKS when:

• Kubernetes expertise exists on team
• Multi-cloud portability needed
• Rich Kubernetes ecosystem required
• Complex networking (service mesh) needed

EKS: Managed Kubernetes

EKS Pricing

• Control plane: $0.10/hour (~$73/month)
• Worker nodes: EC2/Fargate pricing
• No additional charge for managed add-ons

Interview Question: EKS Node Types

Q: "What are the trade-offs between managed node groups, self-managed nodes, and Fargate for EKS?"

Option	Pros	Cons
Managed Node Groups	AWS handles updates, easy scaling	Less customization
Self-Managed Nodes	Full control, custom AMIs	You manage updates
Fargate	Serverless, no nodes to manage	Higher cost per vCPU, limited features

Compute Decision Framework

Need GPU? → P/G instance types
Need serverless?
  └── Short-lived (< 15min)? → Lambda
  └── Longer? → Fargate or Step Functions
Need containers?
  └── Kubernetes required? → EKS
  └── Simpler model? → ECS
Default → EC2 with Auto Scaling

Interview Tip: Always discuss cost implications, scaling behavior, and operational overhead when comparing compute options.

Next, we'll explore AWS storage services and selection criteria. :::