We devoted a great deal of content to the concept of upstreams. Just to remind you, upstreams are entities that generate HTTP responses that Nginx sends to the clients. Usually, an upstream contains several (or at least one) servers speaking one of the supported protocols, such as FastCGI or plain HTTP. Nginx uses sophisticated client code to very efficiently proxy the communication between the clients and the upstream in a transparent way by also optimizing the number of idle connections and wasted memory. There are a number of algorithms that Nginx uses to balance the client load on all the members of an upstream block, and one of those algorithms is known to bite the unsuspecting web administrators.
The configuration under discussion looks like this:
proxy_pass http://backend;
upstream backend {
server server1.example.com;
server server2.example.com;
server server3.example.com;
ip_hash;
}The first line sets up the handling of some...