Search code examples
kubernetesload-balancing

How does Kubernetes balance requests among cluster's nodes?

I've been studying Kubernetes' main features for days, I understood many things, really I did. But I found nowhere the answer to this question: how does Kubernetes balance requests among cluster's nodes?

Well, I mean, suppose an on premise private Kubernetes cluster: LoadBalancer type actually makes a service publish his ports to the network with an IP; Ingress service is a service which sets the rules for some third-part IngressController, which handles requests and forward them to the correct service.

What I do not understand:

  1. Does any or all of these components, or others perhaps, actually monitors nodes' (or pods', I don't know) available resources and chooses to which node (or pod) forward the requests?
  2. If any real load balancer is present natively in Kubernates, what criteria does it adopt? Maybe the aforementioned resources, or the network latency, or just adopts a round robin.
  3. If there is a default policy for balancing, is it possible to customize it and implement your own rules?

Please, tell me if I misunderstood anything and I'll try to focus better on that one. Thank you all.

Solution

  • If you don't have something in place that does load balancing externally (f.e. istio) all your mentioned options boil down to getting tcp connections into the cluster.

    Inside the cluster a ClusterIP is the real concept for load balancing: All Pods that are assigned to a Service with a ClusterIP will be used (roughly) in a round robin fashion. This is handled by iptables DNAT rules configured by kubeproxy on each node.

    The external LoadBalancer or Ingress usually do not do load balancing, even if the name might suggest it.