Load Balancers and Auto Scaling
Bit the Chipmunk, AWS Expert published on
Hey friends! Bit here, ready to zip through one of those essential cloud-nuts: how Auto Scaling Groups (ASGs) and Elastic Load Balancers (ELBs) work together in AWS. If your exam question mentions “scale based on demand,” “load balancer registers new instances,” or “healthy-target replacement,” you’re in the right place. Let’s dig in.
What’s an Auto Scaling Group, anyway? 🐿️
Think of an ASG as your trusty squirrel crew that keeps the nut pile perfectly stocked. Too much traffic? It adds more EC2 instances to handle the rush. Things calm down? It takes some away!
Even better — if one instance goes bad or an entire Availability Zone goes down, the ASG pops in healthy replacements automatically. That’s elasticity and resilience working hand in paw. 🐾
When you pair Auto Scaling with a Load Balancer, you get a dynamic duo that keeps your apps highly available, scalable, and self-healing.
🧱 1. Why combine Auto Scaling and Load Balancers?
When you link an Auto Scaling group with a load balancer you get three big benefits:
- High availability: Automatically adds new instances when demand spikes and replacing unhealthy ones if they fail.
- Scalability: As demand rises, the ASG adds instances; the load balancer auto-registers them (and deregisters when scale-in).
- Health-based traffic routing: Using ELB health checks + ASG health checks helps ensure only healthy instances serve traffic.
In short: ASG = “adjust number of servers”; LB = “send traffic to the right servers.” Together they form a self-adjusting, resilient foundation.
✨ 2. Key mechanics for integration
✅ Attach the target group or load balancer
When you create (or update) an Auto Scaling group you must attach the load balancer target group (ALB/NLB) so the ASG registers instances automatically.
Tip: Make sure the LB and ASG are in the same VPC & Region.
🚦 Health check integration
You can enable ELB health checks (in addition to the EC2 instance health checks). If an instance fails LB health, the ASG can terminate and replace it.
🔍 Metrics & scaling policies
Your ASG can scale based on standard metrics (CPU, network) and load-balancer metrics (e.g., requests per target for ALB) so you tie scaling to actual traffic.
🎯 3. Exam-centric implementation patterns
Here are some patterns you’ll see in the exam and how to choose/configure them:
| Scenario | What to do | Exam clue |
|---|---|---|
| Web app in multiple AZs needs to scale based on HTTP traffic | Create an ALB + target group, create ASG attached to that target group, metric = ALB requests per target. | “Scale based on request volume” |
| Backend service using TCP needs to scale out & preserve client IP | Use NLB + target group for ASG + ASG scale based on connections/bytes. | “Millions of TCP connections” or “preserve source IP” |
| Launch new instances but want boot-up time before sending traffic | Use ASG with Lifecycle hook (warm-up), attach to LB only after bootstrap. | “Initialize software before serving traffic” |
| Improve cost by scaling in when traffic drops | ASG configured with scale-in policy + LB deregisters targets before termination. | “Lower cost when idle” |
🧠 4. Best practices & traps to watch out
- AZ coverage: Make sure the LB spans the same AZs as the ASG for proper distribution.
- Target-deregistration delay: On scale-in, deregister targets first, let in-flight requests finish, then terminate instances. Don’t kill too early.
- Warm-up and cooldown: If you expect a big traffic spike, schedule scaling or use predictive scaling so you’re ready ahead of time.
- Metric selection: Don’t scale purely on CPU if your traffic is spiky based on number of requests; use LB metrics (requests/target) when possible.
- Security group/launch template alignment: The instances launched by ASG must allow LB health-check traffic and target traffic from LB.
📚 Further Reading
Here are some useful links if you want to dig deeper:
- Use Elastic Load Balancing with Auto Scaling groups (AWS Docs) https://docs.aws.amazon.com/autoscaling/ec2/userguide/autoscaling-load-balancer.html (Read more)
- Attach a load balancer (target group) to your Auto Scaling group https://docs.aws.amazon.com/autoscaling/ec2/userguide/attach-load-balancer-asg.html (Read more)
- Target groups for your Application Load Balancers (AWS Docs) https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-target-groups.html (Read more)
- Scaling strategies for Elastic Load Balancing (AWS Networking & Content Delivery Blog) https://aws.amazon.com/blogs/networking-and-content-delivery/scaling-strategies-for-elastic-load-balancing/ (Amazon Web Services, Inc.)
- 10 AWS Auto Scaling Best Practices 2024 (With Coherence blog) https://www.withcoherence.com/articles/10-aws-auto-scaling-best-practices-2024 (withcoherence.com)
🐿️ Bit’s final nut
To ace auto-scalling questions on the exam: whenever you see “load balancer + scaling” think of the ASG + target-group-to-LB integration pattern. Pick the right LB type, attach your ASG correctly, set health checks and scaling policies tied to real traffic — and you’re good.
Go crack some study — that acorn’s yours! 🎯