ELB & ASG

 Elastic Load Balancer and Auto Scaling Groups

Scalability & High Availability

  • Scalability means that an application / system can handle greater loads by adapting.
  • There are two kinds of scalability:
    • Vertical Scalability
    • Horizontal Scalability ( = elasticity)
  • Scalability is linked but different to High Availability

Vertical Scalability

  • Vertical Scalability means increasing the size of the instance
  • For example, your application runs on a t2.micro
  • Scaling that application vertically means running it on a t2.large
  • Vertical scalability is very common for non distributed systems, such as a database.
  • There's usually a limit to how much you can vertically scale (hardware limit)

Horizontal Scalability

  • Horizontal Scalability means increasing the number of instances / systems for your application
  • Horizontal scaling implies distributed systems.
  • This is very common for web applications / modern applications
  • It's easy to horizontally scale thanks the cloud offerings such as Amazon EC2

High Availability

  • High Availability usually goes hand in hand with horizontal scaling
  • High availability means running your application / system in at least 2 Availability Zones
  • The goal of high availability is to survive a data center loss(disaster)

High Availability & Scalability for EC2

  • Vertical Scaling: Increase instance size (= scale up / down)
    • From: t2.nano - 0.5G of RAM, 1 vCPU
    • To: u-12tbl.metal - 12.3 TB of RAM, 448 vCPUs
  • Horizontal Scaling: Increase number of instances ( = scale out / in)
    • Auto Scaling Group
    • Load Balancer
  • High Availability: Run instances for the same application across multi AZ
    • Auto Scaling Group multi AZ
    • Load Balancer multi AZ

Scalability vs Elasticity ( vs Agility)

  • Scalability: ability to accommodate a larger load by making the hardware stronger (scale up), or by adding nodes (scale out)
  • Elasticity: once a system is scalable, elasticity means that there will be some "auto-scaling" so that the system can scale based on the load. This is "cloud-friendly": pay-per-use, match demand, optimize costs
  • Agility: (not related to scalability - distractor) new IT resources are only a click away, which means that you reduce the time to make those resources available to your developers from weeks to just minutes.

What is load balancing?

  • Load balancer are servers that forward internet traffic to multiple servers (EC2 Instances) downstream.

Load Balancer importance

  • Spread load across multiple downstream instances
  • Expose a single point of access (DNS) to your application
  • Seamlessly handle failures of downstream instances
  • Do regular health checks to your instances
  • Provide SSL termination (HTTPS) for your websites
  • High availability across zones

Elastic Load Balancer

  • An ELB (Elastic Load Balancer) is a managed load balancer
    • AWS guarantees that it will be working
    • AWS takes care of upgrades, maintenance, high availability
    • AWS provides only a few configuration knobs
  • It costs less to setup your own load balancer but it will be a lot more effort on your end (maintenance, integrations)
  • 4 kinds of load balancers offered by AWS:
    • Application Load Balancer (HTTPS / HTTP only) - Layer 7
    • Network Load Balancer (ultra-high performance, allows for TCP) - Layer 4
    • Gateway Load Balancer - Layer 3
    • Classic Load Balancer (retired in 2023) - Layer 4 & 7

Auto Scaling Group

  • In real-life, the load on your websites and application can change
  • In the cloud, you can create and get rid of servers very quickly
  • The goal of an Auto Scaling Group (ASG) is to:
    • Scale out (add EC2 instances) to match an increased load
    • Scale in (remove EC2 instances) to match a decreased load
    • Ensure we have a minimum and a maximum number of machines running
    • Automatically register new instances to a load balancer
    • Replace unhealthy instances
  • Cost Savings: only run at an optimal capacity (principle of the cloud)

Auto Scaling Groups - Scaling Strategies

  • Manual Scaling: Update the size of an ASG manually
  • Dynamic Scaling: Response to changing demand
    • Simple/Step Scaling
      • When CloudWatch alarm is triggered (example CPU > 70%), then add 2 units
      • When CloudWatch alarm is triggered (example CPU < 30%), then remove 1 units
    • Target Tracking Scaling
      • Example: I want the average ASG CPU to stay at around 10%
    • Schedule Scaling
      • Anticipate a scaling based on known usage pattern
      • Example: increase the min. capacity to 10 at 5 pm on Fridays
    • Predictive Scaling - Useful when your load has predictable time based pattern
      • Uses Machine Learning to predict future traffic ahead of time
      • Automatically provisions the right number of EC2 instances in advance

ELB & ASG - Summary

  • High Availability vs Scalability (Vertical and horizontal) vs Elasticity vs Agility in the Cloud
  •  Elastic Load Balancers (ELB)
    • Distribute traffic across backend EC2 instances, can be Multi-AZ
    • Supports health checks
    • 4 types: Classic (old), Application (HTTP - L7), Network (TCP - L4), Gateway (L3)
  • Auto Scaling Groups (ASG)
    • Implement Elasticity for your application, across multiple AZ
    • Scaling EC2 instances based on the demand on your system, replace unhealthy
    • Integrated with the ELB

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