HA Architecture

Created : April 02, 2020

This note was taken while learning AWS on Udemy.

Load Balancer

  • physical or virtual device that balance the network load across multiple web servers
  • if you need IPv4 address of your end user,

    • look for X-Forwarded-For header
  • instances monitored by ELB are reported as - InService or OutofService
  • have their own DNS name

    • you're never given an IP address

Types of Load Balancer

  • Application Load Balancer

    • best suited for HTTP,HTTPS traffic
  • Network Load Balancer

    • best suited for TCP traffic
    • use for extreme performance
  • Classic Load Balancer

    • legacy ELB(Elastic Load Balancer)
    • basic, cheaper one
    • if your application stops responding, ELB responds with 504 error

      • 504 error = gateway has timed out

Sticky Sessions

  • enable users to stick to same EC2 instance
  • useful if you're storing information locally to that instance
  • all requests from user during session are sent to same instance
  • when one of your EC2 instance not receiving any traffic,

    • disable sticky sessions!

Cross-Zone Load Balancing

Without Cross-Zone Load Balancing

haarchitecture 1

ELB cannot send traffic to another AZ

With Cross-Zone Load Balancing

haarchitecture 2

ELB send traffic across 5 instances evenly (across multiple AZ)

< Exam Scenario >

  • when instance in AZ-1 getting all traffic, and instance in AZ-2 is not getting any traffic?

    • enable cross-zone load balancing
    • ELB in AZ-1 will distribute traffic to AZ-2 as well,
    • so all instances have even traffic

Path Patterns

  • direct traffic to different EC2 instances based on URL contained in the request
  • path-based routing

    • create a listener with rules to forward requests based on URL path

haarchitecture 3

  • if enable path patterns,

    • Application Load Balancer (because it's reading URL path) in US-EAST-1A send traffic across two different AZ,
    • traffic for 'www.myurl.com/images' - direct to media EC2 instance in another AZ

Auto Scaling

3 Components

  • Groups

    • logical component where you put EC2 instances in
    • webserver/ application/ database group
  • Configuration Templates

    • each group always use launch template or launch configuration
  • Scaling Options

    • maintain current instance levels at all times

      • perform a periodic health check on running instances
      • when find an unhealthy instance, it terminates that instance and launch new one
    • scale manually

      • specify only the change in maximum, minimum, desired capacity of group
    • scale based on a schedule

      • useful when you know when to increase/decrease
    • scale based on demand

      • useful in response to changing conditions, when you don't know when conditions will change
    • use predictive scaling

      • predict based on your previous performance

HA (High Availability) Architecture

  • highly available = protect against failures

    • always design for failure
  • scaling out : use auto scaling groups and add additional EC2 instances
  • scaling up : increase the resources inside EC2 instances

< exam scenario >

You have a website that requires a minimum of 6 instances and it must be highly available. You must also be able to tolerate the failure of 1 AZ. What is the ideal architecture for this while also being the most cost effective?

  1. 2 AZs with 2 instances in each AZ. (X) - total instances 4 is less than 6
  2. 3 AZs with 3 instances in each AZ. (O) - even if 1 AZ fails, there are 6 instances still.
  3. 1 AZ with 6 instances in each AZ. (X) - 1 AZ is not highly available
  4. 3 AZs with 2 instances in each AZ. (X) - if 1 AZ fails, there are only 4 instances left which is less than 6.


  • a way of completely scripting your cloud environment
  • Quick Start : a bunch of CloudFormation templates already built by AWS Solutions Architects allowing you to create complex environments very quickly

Elastic Beanstalk

  • easy-to-use service for deploying and scaling web applications
  • just upload your code and Elastic Beanstalk automatically handles the deployment and scales application