ELB - Sticky Sessions

Session Affinity

• It is possible to implement stickiness so that the same client is always redirected to the same instance behind a load balancer

• This works for Classic Load Balancer, Application Load Balancer, Network Load Balancer

• The "cookie" used for stickiness has an expiration date you control

• Use case: make sure the user doesn't lose his session data

• Enabling stickiness may bring imbalance to the load over the backend EC2 instances

Cookie Names

• Application-based Cookies:

  • Custom cookie: generated by the target - don't use AWSALB, AWSALBAPP, AWSALBTG(reserved for use by ELB)
  • Applicaiton cookie: generated by the load balancer - cookie name is AWSALBAPP

• Duration-base Cookies: generated by the load balancer. Cookie name is AWSALB (for ALB), AWSEL (for CLB)

Cross-Zone Load Balancing

• Application Load Balancer

  • Enabled by default (can be disabled at the Target Group Level)
  • No charges for inter AZ data

• Network Load Balancer & Gateway Load Balancer

  • Disabled by default
  • You pay charges ($) for inter AZ data if enabled

• Classic Load Balancer

  • Disabled by default
  • No charges for inter AZ data if enabled