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AWS Elastic Load Balancing

Elastic Load Balancing (ELB) automatically distributes incoming application traffic across multiple targets and virtual appliances in one or more Availability Zones (AZs).

  • Instances monitored by ELB are reported as; InService, or OutofService

  • Load Balancer Types ALB, NLB, CLB

    • ALB | Application Load Balancer

      ALB

      • has Listeners, Rules, and Target Groups to route traffic.
      • for HTTP and HTTPS traffic (OS Layer 7)
      • can create advanced request routing, sending specified requests to specific web servers.
      • (O) can attach a Web Application Firewall(WAF)
        • (X) no WAF for NLB and CLB.

    • NLB | Network Load Balancer

      NLB

      • uses Listeners and Target Groups to route traffic.
      • for TCP traffic (OS Layer 4)
      • can handling millions of requests per second, while maintaining ultra-low latencies.
      • for extreme performance.
      • handle sudden and volatile traffic patterns while using a single static IP address per Availability Zone.

    • CLB | Classic Load Balancer (legacy)

      • uses Listeners and EC2 instances are directly registered as targets to CLB
      • HTTP/HTTPS (OS Layer 7), TCP (OS Layer 4)
      • recommended to use ALB or NLB

  • Load Balancer Sate

    • provisioning:
      The load balancer is being set up.

    • active:
      The load balancer is fully set up and ready to route traffic.

    • active_imparied:
      The load balancer is routing traffic but does not have the resources it needs to scale.

    • failed:
      The load balancer could not be set up.

  • 504 Error:

    • Gateway has timed out ⇒ the application not responding within the idle timeout period.
      • Issue could be either at the Web Server layer or at the Database Layer.

  • X-Forwarded-For Header:
    If you need the IPv4 address of your end user

    • get the original IP of incoming traffic passing through ELB.
    • helps you identify the IP address of a client when you use an HTTP or HTTPS load balancer.

  • Health Check

    Your Application Load Balancer periodically sends requests to its registered targets to test their status. These tests are called health checks.

    ELB health check

    • health check don't support WebSockets.

  • Sticky Sessions

    • enable your users to stick to the same EC2 instance. ⇒ Can be useful if you are storing information locally to that instance.
    • can be enabled for CLB or ALB and sessions are remembered via Cookie.

  • Cross Zone Load Balancing

    • enables you to load balance across multiple AZs.

  • Path Pattern allow you to direct traffic to different EC2 instances based on the URL contained in the request.

    • Route based on path patterns in the request URLs.
      • e.g. /img/*/pics