What algorithm does Amazon ELB use to balance load

amazon-elbamazon-web-servicesload balancing

I found this in the official ELB documentation

By default, a load balancer routes each request independently to the
application instance with the smallest load.

but an article on Newvem says that ELB supports only Round Robin algorithm

Algorithms supported by Amazon ELB – Currently Amazon ELB only supports Round Robin (RR) and Session Sticky Algorithms.

So which one is it?

[1] http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/US_StickySessions.html

[2] http://www.newvem.com/dissecting-amazon-elastic-load-balancer-elb-18-facts-you-should-know/?lead_source=popup_ebook&oid=00DD0000000lsYR&email=muneeb%40olacabs.com

Best Answer

It's request count based for HTTP(S), round robin for other.

http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/TerminologyandKeyConcepts.html#request-routing

Before a client sends a request to your load balancer, it first resolves the load balancer's domain name with the Domain Name System (DNS) servers. The DNS server uses DNS round robin to determine which load balancer node in a specific Availability Zone will receive the request.

The selected load balancer node then sends the request to healthy instances within the same Availability Zone. To determine the healthy instances, the load balancer node uses either the round robin (for TCP connections) or the least outstanding request (for HTTP/HTTPS connections) routing algorithm. The least outstanding request routing algorithm favors back-end instances with the fewest connections or outstanding requests.

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