What algorithm does Amazon ELB use to balance load?

Solution 1:

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.

Solution 2:

It depends on the type of ELB used. AWS has over the time introduced Application ELB and Network ELB along with the Classic ELB.

Application Load Balancers applies listener rules and assigns the (HTTP/HTTPS) request to a target group. It selects a target from that target group using the round robin routing algorithm

Network Load Balancers node that receives the connection, selects a target from its target group using a flow hash routing algorithm

Classic Load Balancers uses round robin routing algorithm for TCP listeners and least outstanding requests routing algorithm for HTTP and HTTPS listeners

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TLDR;

The modern ELBs use round robin routing algorithm for HTTP/HTTPS requests and flow hash routing algorithm for TCP requests.

Classic ELB used round robin routing algorithm for TCP requests and least outstanding requests routing algorithm for HTTP and HTTPS requests

Source / Further Reading : https://docs.aws.amazon.com/elasticloadbalancing/latest/userguide/how-elastic-load-balancing-works.html#routing-algorithm