Session Affinity

Session Affinity (or Sticky Session) is a technique that provides soft affinity for requests sharing the same hash key; it does not guarantee a permanent pod assignment.

Table of Contents

1. Session Affinity Overview
2. Consistent Hash Based
3. Cookie Based
4. HTTP Header Based
5. Source IP Based
6. Operational Considerations

Session Affinity Overview

Diagram showing consistent-hash session affinity: a client request carrying a user ID is hashed by the load balancer and always routed to the same pod, while the other pod replicas remain unused for that session.Open full screen ↗

Consistent Hash Based

HTTP Header Based

apiVersion: networking.istio.io/v1
kind: DestinationRule
metadata:
  name: reviews-session-affinity
spec:
  host: reviews
  trafficPolicy:
    loadBalancer:
      consistentHash:
        httpHeaderName: "x-user-id"

Cookie Based

apiVersion: networking.istio.io/v1
kind: DestinationRule
metadata:
  name: reviews-cookie-affinity
spec:
  host: reviews
  trafficPolicy:
    loadBalancer:
      consistentHash:
        httpCookie:
          name: "session-id"
          ttl: 0s  # Cookie expiration time (0s = session cookie)

Source IP Based

apiVersion: networking.istio.io/v1
kind: DestinationRule
metadata:
  name: reviews-ip-affinity
spec:
  host: reviews
  trafficPolicy:
    loadBalancer:
      consistentHash:
        useSourceIp: true

Operational Considerations

The diagram assumes an unchanged endpoint set and identical endpoint views at each proxy. Adding/removing pods or locality-based endpoint differences can remap requests. Store session state so that a remap or pod failure remains safe.

Choose one of these DestinationRules for the same host. HTTP header/cookie hashing requires HTTP processing; a missing header cannot identify a user. ttl: 0s creates a session cookie when it is absent, but the browser must return it. Cookie attributes and lifetime should match the application’s requirements.

Source-IP hashing uses the source address visible to the proxy. NAT and intervening load balancers can collapse multiple clients onto one address; check trusted proxy/client-IP handling before relying on it. These DestinationRule examples describe sidecar behavior; verify waypoint feature support separately.

References

• Istio Session Affinity