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What algorithm does Amazon ELB use to balance load?



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


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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