High Availability and Scalability


  • Application / system can handle greater loads by adapting
  • Scalability is linked but different to high availability
  • Elasticity
    • System can adapt to workload changes by provisioning and de-provisioning resources automatically to match current demand as closely as possible.
  • Two kinds of scalability
    • Vertical Scalability: scale up & down
      • Increasing the size of the instance e.g. from t2.micro to t2.large
      • Common use case
        • Distributed systems, such as database
        • RDS, ElastiCache can scale vertically
      • Hardware limits how much you can scale
    • Horizontal Scalability (= elasticity): scale in & out
      • Increase number of instances / systems for application
      • Common for web applications / modern applications
      • Easy with e.g. Amazon EC2 through right-clicking
        • For EC2, you can use Auto Scaling Groups or Load Balancers for horizontal scaling

High Availability

  • Goes hand in hand with horizontal scaling
  • Running application in at least 2 data centers (= Availability Zones)
    • Goal is to survive a data center loss
  • Can be:
    • Passive e.g. RDS Multi AZ
    • Active e.g. for horizontal scaling
  • For EC2, you can use Auto Scaling Group with multi AZ enabled and Load Balancer with multi AZ enabled.
  • 💡 Design for failure as it can happen
    • 🤗 Netflix has simian army (monkeys) to inject failure into their production systems.
      • E.g.
        • Chaos Monkey => Terminate random EC2 instances
        • Chaos Gorilla => Delete entire AZ
        • Latency Monkey => introduces latency.
        • and more

Common architectures

  • Single region HA architecture
    • Single region AZ failure tolerant architecture
    • 3 tier application with 3 tiered security

      Tier Role Example service Security Group
      1 Client Application Load Balancer Allow HTTP + HTTPs
      2 Application Application servers (EC2) Allow HTTP from Client
      3 Database Amazon RDS Allow e.g. MySQL from application SG
    • In a single VPC
      • At least 2 AZ and and in each AZ:
        • A public subnet and in each public subnet:
          • Deploy NAT gateway
          • Load balance public subnet in AZs through application load balancer
        • A private subnet and in each private subnet:
          • Deploy application server in same auto-scaling group
          • Create route table that routes internet ( to the NAT gateway of a public subnet in same AZ
          • Configure auto-scaling group to put servers in public Load balancer
          • Deploy RDS instance in one, and RDS multi-AZ standby in other one
    • 📝 E.g. if you have minimum 6 instances running and can tolerate failure of 1 AZ failure:
      • Deploy 3 AZ with 3 instances in each AZ so you always have 6 instances running.
  • Multi region HA architecture
    • E.g. using S3
      • Multi region HA architecture using S3
    • E.g. using RDS
      • Multi region HA architecture using RDS

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