Adding a Load Balancer to an Existing App

Bit the Chipmunk, AWS Expert published on October 26, 2024

6 min, 1100 words

Hey there, cloud builders! Bit the Chipmunk here, ready to scamper into one of the trickier topics on the AWS Advanced Networking exam: how to integrate load balancers into existing environments. 🐾

You’ve already got running workloads—maybe EC2 fleets, containerized services, or even hybrid systems—and now it’s time to weave in a load balancer for higher availability, smarter scaling, or stronger security. The challenge? Doing it without breaking what’s already working.

Let’s explore how to retrofit AWS load balancers into live deployments, from the simple to the super-sophisticated—complete with key exam clues along the way.

🧭 1. Why Integration Matters

Real-world environments rarely start from scratch. Often, you need to insert a load balancer into an existing app—for example:

To scale EC2 instances across AZs.
To enable HTTPS/TLS offload.
To inspect or filter traffic via a security appliance.
To migrate apps gradually (blue/green, canary).

For the exam, watch for keywords like:

“Existing application needs to add load balancing for scalability and security with minimal downtime.”

🧩 2. Common Integration Patterns

a) Retrofitting an ALB for a Web Tier

Use Case: Existing web servers or containers (HTTP/HTTPS).

Integration steps:

Create an Application Load Balancer (ALB) in the same or a connected VPC.
Configure listeners (ports 80/443) and routing rules (host or path-based).
Create target groups using either instance targets (for EC2) or IP targets (for containers, hybrid, or cross-VPC targets).
Gradually redirect traffic to the ALB via DNS (Route 53 alias).
Integrate AWS WAF for added security or ACM for HTTPS offload.

Exam clue: “Need host/path routing, SSL termination, and WAF for existing EC2 app.”

💡 Advanced pattern: If your web servers are in another VPC (peered or via TGW), use IP targets to register them with the ALB.

b) Adding an NLB in Front of Legacy or Non-HTTP Services

Use Case: TCP/UDP workloads needing low latency or client IP visibility.

Integration steps:

Deploy a Network Load Balancer (NLB) (Internet-facing or internal).
Define a listener for TCP, TLS, UDP, or TCP_UDP protocols.
Create a target group with instance or IP targets (for cross-VPC, hybrid, or on-prem via Direct Connect).
Assign Elastic IPs (EIPs) for static endpoints.
Update clients or DNS to use the NLB DNS name or EIPs.

Exam clue: “Must preserve source IP for a legacy TCP app.”

💡 Gotcha alert: Cross-zone load balancing is disabled by default on NLBs. If each AZ doesn’t have healthy targets, traffic may fail. Enabling it improves resilience but can add inter-AZ data charges.

c) Inserting a Gateway Load Balancer (GWLB) for Centralized Inspection

Use Case: You need to insert a security appliance layer (firewall, IDS, proxy) between the internet and app VPCs.

Integration steps:

Deploy a GWLB in a security/inspection VPC.
Create GWLB endpoints (GWLBe) in the ingress/egress VPCs to forward traffic transparently through the appliance fleet.
Register the appliances as targets in the GWLB’s target group (protocol GENEVE).
Route traffic via the GWLBe, ensuring symmetric paths.

Exam clue: “Centralized inspection layer using GWLB and endpoints.”

💡 GWLB supports cross-account and cross-VPC integration through PrivateLink, a powerful design for shared ingress/egress architectures.

d) Gradual Migration with Weighted Target Groups

Use Case: Migrate an existing version to a new one with minimal downtime.

Integration steps:

Configure an ALB with two target groups — one for the old app, one for the new version.
Use weighted routing to shift traffic incrementally (e.g., 10 → 50 → 100%).
Monitor metrics and roll back if needed.

Exam clue: “Migrate an application version with minimal downtime using load balancing.”

e) Cross-VPC or Cross-Account Integration

Use Case: Centralized load balancing in a “hub” or “DMZ” VPC that fronts apps in “spoke” VPCs or other accounts.

Integration patterns:

ALB/NLB with IP targets: Register private IPs of targets in other VPCs via VPC peering or Transit Gateway.
PrivateLink: For NLB, you can expose its front end as a PrivateLink endpoint service to other VPCs (advanced pattern).
Shared VPCs: Alternatively, deploy the load balancer in a shared network account via AWS RAM.

Exam clue: “Central ingress VPC must front applications in multiple spoke VPCs.”

f) EKS and Service Mesh Integrations

Use Case: Existing Kubernetes services running on Amazon EKS.

Integration options:

AWS Load Balancer Controller: Automatically provisions ALBs for Ingress resources or NLBs for Service objects of type LoadBalancer.
Use NLBs for internal TCP, or UDP services; ALBs for external HTTP(S) or gRPC ingress with path or host routing.
Combine with App Mesh or GWLB for advanced east-west traffic control and security inspection.

Exam clue: “Expose an EKS service to the internet securely using AWS Load Balancer Controller.”

⚙️ 3. Key Considerations & Exam Traps

Topic	Key Insight
Health Checks	Match the correct endpoint (path and port). Wrong checks = target deregistration.
AZ Coverage	Ensure the load balancer spans the same AZs as the backend targets for resilience.
Cross-Zone LB	ALB: always enabled (no cost). NLB: disabled by default, may cause uneven load.
Target Types	ALB/NLB: Instance, IP, or Lambda (ALB only). IP targets are key for hybrid and cross-VPC designs.
Client IP Visibility	Use NLB (native) or ALB with X-Forwarded-For / Proxy Protocol v2.
Security Groups	Remember: LBs have their own ENIs — allow inbound from LBs and outbound to targets.
Session Management	Use sticky sessions (ALB) or token-based auth if migrating from sessionful backends.
DNS and Migration	Use Route 53 weighted or alias records for safe traffic shifting during cutover.

📚 Further Reading

Blue/Green Deployments with Application Load Balancer — AWS DevOps Blog
Best Practices for Deploying Gateway Load Balancer — AWS Networking Blog
Using IP Targets for Cross-VPC Load Balancing — AWS Networking Blog
AWS Load Balancer Controller on Amazon EKS
Elastic Load Balancing Documentation — official AWS reference

🐿️ Bit’s Final Nut

Integrating a load balancer into an existing deployment isn’t just “plug and play” — it’s about smart placement, target flexibility, and cross-VPC awareness.

For the exam, remember:

Choose ALB for HTTP brains, NLB for TCP brawn, GWLB for inspection power.
Watch for cross-zone load balancing defaults, target type flexibility, and cross-account connectivity.
And always — always — match your health checks to the app’s heartbeat. ❤️‍🔥

Now go forth and balance wisely, little cloud adventurers! 🌩️🐿️