EC2 vs Lambda: When Should You Choose Which?
EC2 is ideal for long-running server workloads, while Lambda excels at event-driven short tasks. Learn the selection criteria from an SAA-C03 exam perspective.
Related Exam Domains
- Domain 3: Design High-Performing Architectures
- Domain 4: Design Cost-Optimized Architectures
Key Takeaway
EC2 is best for continuously running server workloads, while Lambda excels at event-driven short tasks. Choose EC2 when traffic is predictable and 24/7 operation is needed. Choose Lambda for intermittent, short-running tasks.
Exam Tip
Exam Essential: "Serverless", "event-driven", "short execution" → Lambda / "Continuous operation", "stateful", "custom runtime" → EC2
When Should You Use EC2 vs Lambda?
This is the most common question when selecting AWS compute services. These two services take fundamentally different approaches.
EC2 is Best For
- Web servers requiring 24/7 operation
- Applications that need to maintain state
- Batch jobs running longer than 15 minutes
- Workloads requiring GPU or specialized hardware
- Custom OS or runtime environments
Lambda is Best For
- Event-driven processing (S3 uploads, API requests, etc.)
- Intermittent short-running tasks
- API backends with high traffic variability
- Reducing server management overhead
- Tasks completing within 15 minutes
EC2 vs Lambda: Which Should You Choose?
| Comparison | EC2 | Lambda |
|---|---|---|
| Billing | Per hour/second (while running) | Requests + execution time |
| Max Execution Time | Unlimited | 15 minutes |
| Cold Start | None (always running) | Yes (ms to seconds) |
| Scalability | Requires Auto Scaling setup | Automatic scaling |
| State Management | Can store on instance | Stateless (external storage needed) |
| OS Access | Full access | Not available |
| Memory | Up to 24TB | Up to 10GB |
Selection Criteria
- Choose EC2: 24/7 operation, tasks >15 minutes, state persistence needed, special hardware/OS required
- Choose Lambda: Event-driven, intermittent execution, reduce server management, need automatic scaling
How to Reduce Costs
EC2 Cost Optimization
- Reserved Instances: Up to 72% discount with 1-3 year commitment
- Spot Instances: Up to 90% discount (for interruptible workloads)
- Savings Plans: Flexible usage commitment
Lambda Cost Optimization
- Memory Optimization: Balance cost/performance with appropriate memory settings
- Provisioned Concurrency: Reduce cold starts, predictable costs
- ARM Architecture: 20% cheaper with Graviton2
SAA-C03 Exam Focus Points
- ✅ Lambda Limitations: Max 15 minutes execution, 10GB memory, Stateless
- ✅ Cost Optimization: Intermittent workload → Lambda, Continuous workload → EC2 + Reserved
- ✅ Architecture Patterns: API Gateway + Lambda (serverless API), ELB + EC2 (traditional web)
Exam Tip
Sample Exam Question: "A web application receives requests only a few times per day, with each request taking 10 seconds to process. What is the most cost-effective solution?" → Answer: Lambda (intermittent + short execution time)
Frequently Asked Questions
Q: How long is Lambda's cold start?
It varies by language and package size. Python/Node.js typically takes 100-200ms, while Java/.NET can take 1-5 seconds. Provisioned Concurrency can solve this issue.
Q: Can I use EC2 and Lambda together?
Yes, many architectures use both. For example, the main application runs on EC2, while asynchronous tasks like image processing are handled by Lambda.
Q: How do I connect to databases from Lambda?
Using RDS Proxy is recommended. It prevents database connection explosion caused by Lambda's concurrent executions.
Q: What if execution time exceeds 15 minutes?
Use Step Functions to chain multiple Lambda functions, or switch to EC2/Fargate. AWS Batch is also a good alternative for batch workloads.
Q: Is Lambda always cheaper than EC2?
No. For consistently high traffic, EC2 + Reserved Instances may be cheaper. You need to calculate the breakeven point.