How to use Session Affinity on requests to Kubernetes service?

Deployment: Stateless workload

I could not find a documentation that specifies how Kubernetes service behaves when the affiliated deployment is scaled with multi replicas.

Pods deployed with Deployment is supposed to be stateless.

Ingress to Service routing

When using Ingress, L7-proxy, the routing can be based on http request content, but this depends on what implementation of an IngressController you are using. E.g. Ingress-nginx has some support for sticky sessions and other implementations may have what you are looking for. E.g. Istio has support similar settings.

Ambassador

Ambassador that you write about does also have some support for session affinity / sticky sessions.

Configuring sticky sessions makes Ambassador route requests to the same backend service in a given session. In other words, requests in a session are served by the same Kubernetes pod

Pod to Service routing

When a pod in your cluster does an http request to a Service within the cluster, the kube-proxy does routing in a round robin way by default.

By default, kube-proxy in userspace mode chooses a backend via a round-robin algorithm.

If you want session affinity on pod-to-service routing, you can set the SessionAffinity: ClientIP field on a Service object.

If you want to make sure that connections from a particular client are passed to the same Pod each time, you can select the session affinity based on client’s IP addresses by setting service.spec.sessionAffinity to “ClientIP” (the default is “None”).