Chloe McAree (McAteer)
Published on

AWS Elastic Load Balancers (ELB)


This is part of a blog series giving a high level overview of the different services examined on the AWS Solution Architect Associate exam, to view the whole series click here.

ELB Summary

ELB Logo

  • Designed to help balance the load of incoming traffic by distributing it across multiple targets to help improve scalability.

  • Can balance load across one or more Availability Zones.

  • Internal Load Balancers are load balancers that are inside private subnets

  • Load Balancers have their own DNS name — you will NEVER be given an IP address

  • X-Forward-For header → use if you need to get the IPv4 address of your end user

  • Cross Zone load Balancing → can enable all EC2 instances to get equal share of traffic no matter what zone they are in!

  • Path Patterns → can direct traffic to different EC2 instances based on request URL.

Types of ELB’s

AWS offers four different types of ELBs, all of which are highly available and can be used to make your application fault tolerant.

1. Application Load Balancer

  • Best suited for HTTP & HTTPS traffic

  • Operates at Layer 7 (Application layer of the OSI model)

  • Routes traffic based on request content. It is Intelligent and can send specific requests to specific servers.

  • Secure as it uses SSL/TLS ciphers and protocols

2. Network Load Balancer

  • Best suited for TCP & UDP traffic

  • Used when extreme performance is required

  • Operates at Layer 4 (Transport layer of the OSI model)

  • Can handle millions of requests per second (ultra-low latency)

3. Classic Load Balancers

  • Used to balance the load across multiple EC2 instances.

  • Can operate at both Layer 7 (Application layer) and Layer 4 (Transport layer).

  • Not very intelligent — it can’t route traffic based on content like Application Load Balancers.

4. Gateway Load Balancer

  • For scaling virtual applications e.g. firewalls.

  • Operates at Layer 3 (Network layer of the OSI model)

  • Automatically scales virtual appliances based on demand.

Sticky Sessions

  • Also known as Session Affinity

  • Allows you to bind a users session to a specific instance, ensuring all requests in that specific session are sent to the same instance.

  • If you are writing to an EC2 instance or disk, you will want to enable sticky sessions.