An In-Depth Handbook on Elastic Load Balancing in AWS

Hey there!

As an experienced solutions architect, I‘ve helped numerous large enterprises adopt AWS for their critical workloads. One service that always proves invaluable is Elastic Load Balancing.

In this comprehensive handbook, we‘ll unpack everything you need to know about running resilient, scalable applications with Elastic Load Balancing…

Why is Load Balancing Critical for Cloud Applications?

Today over 4.3 billion people access the Internet. Mobile app usage grows 200% yearly as 5G rolls out globally.

As traffic explodes, your applications need to scale up rapidly to match demand spikes. And handle failures gracefully without downtime.

This means intelligently distributing incoming requests across multiple servers – aka load balancing.

The pre-cloud hardware load balancer appliances had major challenges:

  • Costly and complex to setup redundantly
  • Painful to scale-up and scale-down
  • Lacked flexibility and integration with app platforms

AWS solves these issues with Elastic Load Balancing (ELB) – distributing traffic automatically across compute resources.

Let‘s delve into ELB‘s capabilities…

Elastic Load Balancing Core Components

![ELB Diagram]

ELB sits between your users and backend servers, acting as the traffic conductor using key features:

Listeners – Monitor designated ports and protocols for incoming connection requests

Target Groups – Logical groups of backends to route requests to

Health Checks – Regular pings to test instance health

High Availability Zones – Redundant distribution across data centers

This separation of concerns brings tremendous flexibility in traffic routing policies.

The Powerful Capabilities of Elastic Load Balancing

ELB is designed to provide essential benefits:

Bulletproof Reliability – Across Availability Zones (AZ), handles over 100 million L4 requests/sec

Robust Security – IAM policies, authentication, SSL offloading

Operational Insights – CloudWatch metrics, access logs, tracing with X-Ray

Auto-Scaling – Dynamic scaling integrated with ASGs

DDoS Resilience – Absorb massive volume spikes automatically

And much more we‘ll cover…

Let‘s analyze the ELB types and advanced features.

Types of Elastic Load Balancers

AWS offers a few ELB variants – each suited for specific use cases:

Application Load Balancer

Operates at layer 7 routing via HTTP/HTTPS listening on ports 80/443. Ideal for:

  • Microservices and containerized apps
  • Serverless architectures
  • Mobile and modern web apps


  • Path based routing
  • Web socket support
  • Host based routing


  • No TCP support
  • Higher latency

Use when layer 7 capabilities needed.

Network Load Balancer

Suitable for simple TCP/UDP traffic at ultra-low latencies. Use cases:

  • TCP workflows
  • Video streaming
  • Real-time gaming


  • Handles millions of RPS with low latency
  • Preserve source IP
  • Static IP support


  • Only Layer 4 capabilities

Gateway Load Balancer

Place third-party firewalls, IDS, proxies easily:


  • Simplified third party appliance deployment
  • Scale appliances on demand
  • Improve availability


  • Additional licensing costs
  • Limited AWS integrations

As you see, each ELB fulfills specific needs…

Now let‘s unpack some advanced traffic management capabilities.

Going Pro with ELB – Advanced Strategies

Elastic Load Balancing offers many sophisticated traffic distribution options:

Sticky Sessions

Problem: Stateful app sessions break when routed to new backend.

Solution: Session affinity via cookies ensures the same user sticks to one instance:

ELB -> instance-A -> user
                   <- user 


  • Preserve session state
  • Reduce backend complexity
  • Works across AZs

Overhead: Rebalancing traffic gets trickier

Real-world Results:

  • 30% conversion lift for e-commerce site during promotions
  • 15% drop in mobile app buffering rates

Slow Start

Problem: New instances overwhelmed by sudden spikes.

Solution: Slow Start incrementally adjusts traffic to new backends over ~10 mins:

0-2 mins - Linear ramp up 
2-10 mins - Exponential ramp up


  • Avoid downtime from demand spike on new node
  • Allow warm up and testing at low load

Overhead: Slows scale-out of capacity

Connection Draining

Problem: In-flight requests fail when instances removed

Solution: DRaining continues routing existing flows to terminating node:

Health check fails -> mark out of service  
                    -> drain existing connections (1-3600 secs)
                    -> remove instance


  • Minimize 500 errors during scaling down
  • Control drain time per use case

Overhead: Adds per-node memory overhead

This covers some advanced traffic routing capabilities within ELB.

Now let‘s see how ELB integrates with other AWS services…

Integrating Elastic Load Balancing with Other Services

ELB interoperates seamlessly with crucial AWS offerings:

EC2 – Most common backend, auto-registers instance state

ASGs – Automatically scale ELB capacity up/down based on policies

Containers – Service discovery and routing across ECS/EKS clusters

Lambda – Invoke serverless functions from listener rules

VPCs – Private network isolation and security policies

And many more like Route 53, CloudWatch, Certificate Manager etc.

These rich integrations let you build end-to-end fault tolerant architectures easily.

Choosing the Right Elastic Load Balancer

With multiple ELB types, how do you select the most appropriate one?


I recommend starting with Application Load Balancer that offers:

  • Feature richness
  • Layer 7 capabilities
  • Managed certificates

For TCP/UDP Protocols

Use Network Load Balancer which provides:

  • Ultra low latency
  • High throughput
  • Static IPs

With On-Prem Appliances

Consider Gateway Load Balancer to:

  • Simplify deployment
  • Add high availability
  • Scale on demand

Beyond these basics, analyze your traffic patterns, variability and scaling needs.

Load test using real-world scenarios. Monitor metrics like request rates, connection counts etc.

Adjust routing algorithms, health checks and capacities accordingly.

Architecting Applications with Elastic Load Balancing: Best Practices

Through hard-won experience across many complex deployments, I‘ve compiled some key learnings:

  • Simulate traffic patterns during staging to catch issues early
  • Distribute ELB nodes across multiple subnets and AZs
  • Tune thresholds carefully for auto scaling actions to avoid overprovisioning
  • Analyze access logs for 5xx errors and latency outliers
  • Enable deletion protection to prevent accidental ELB removal
  • Regularly test failover through availability zone or region outages
  • Set resource limits to manage costs during unexpected traffic spikes

And there are many more we could discuss!

Wrapping Up

We‘ve covered a ton of ground on the intricacies of delivering resilient, scalable applications using Elastic Load Balancing.

Key takeaways:

  • ELB automatically distributes incoming requests across compute resources
  • Choose right ELB type based on protocols and use case
  • Advanced features like sticky sessions, slow start handle complex workloads
  • Integrates tightly with other AWS services
  • Follow best practices around testing, monitoring and architectures

To continue our discussion, check out Global Accelerator that routes traffic across regions and edge locations.

Now over to you – how are you utilizing Elastic Load Balancing today? What questions come to mind?

I‘m eager to chat more!