Load Balancing

Elastic Load Balancers

• They allows us to balance load between different servers.
• Types
  • Application Load Balancer : Layer 7. support advanced request routing based on HTTP request characteristics like path, headers, etc.
  • Network Load Balancer : Very High Performance, Layer 4, Most expensive. Support millions of request per second.
  • Classic Load Balancer : Dumber Layer 7, Legacy. Also supports Layer 4. The only thing supported at Layer 7 is X-Forwarded-For and sticky sessions.
• ELB responds with HTTP 504 Gateway Timeout when the application does not respond.
• The DNS names for the load balancers are {LB-name}.{region}-elb.amazonaws.com
• The healthcheck statuses for instances behind LB can be InService or OutOfService.
• When a healtcheck for an instance fails, the load balancer stops sending traffic to that instance.
• When configuring ELB health checks, bear in mind that you may want to create a file like healthcheck.html or point the ping path of the health check to the main index file in application
• Remember the health check interval is how often a health check will occur
• Healthy/Unhealthy thresholds are how many times either will check before marking the origin either healthy or unhealthy
  • Health Check Interval: 10 seconds
  • Unhealthy Threshold: 2
  • Healthy Threshold: 3
  • This means that if the health check interval occurs twice without success, then the source will be marked as unhealthy. This is 2 checks @ 10 seconds per check, so basically after 20 seconds the origin will be marked unhealthy
  • Likewise, if the healthy threshold is marked at 3, then it would be 3 x health check interval or 10 seconds being 30 seconds. After 30 seconds with 3 consecutive success checks, the origin will be marked as healthy.
• Enable Cross-Zone Load Balancing will distribute load across all back-end instances, even if they exist in different AZ’s
• ELBs are NEVER given public IP Addresses, only a public DNS name
• ELBs can be In Service or Out of Service depending on health check results
• Charged by the hour and on a per GB basis of usage
• Must be configured with at least one listener
• A listener must be configured with a protocol and a port for front end (client to ELB connection), as well as a protocol and port for backed end (ELB to instances connection)
• ELBs support HTTP, HTTPS, TCP, and SSL (Secure TCP)
• ELBs support all ports (1-65535)
• ELBs do not support multiple SSL certificates
• Classic ELBs support the following ports:
  • 25 (SMTP)
  • 80 (HTTP)
  • 443 (HTTPS)
  • 465 (SMTPS)
  • 587 (SMTPS)
  • 1024-65535

ELB Error Messages

HTTP Error Codes:

• 200 – The request has succeeded
• 3xx – Redirection
• 4xx – Client Error (404 not found)
• 5xx – Server Error