It is explained in detail in istio documentation under VirtualService.

There is also nice simple explanation and example here:

Canary Deployments

A canary deployment is a strategy for safely rolling out a new version of a service. With Istio, you can use percentage-based traffic splitting to direct a small amount of traffic to the new version. Then you can run a canary analysis on v2 (like check latency and error rate), and finally direct more traffic at the new version until it's serving all traffic.

apiVersion: networking.istio.io/v1alpha3kind: VirtualServicemetadata:  name: helloworldspec:  hosts:    - helloworld  http:  - route:    - destination:        host: helloworld        subset: v1      weight: 90    - destination:        host: helloworld        subset: v2      weight: 10

apiVersion: networking.istio.io/v1alpha3kind: DestinationRulemetadata:  name: helloworldspec:  host: helloworld  subsets:  - name: v1    labels:      version: v1  - name: v2    labels:      version: v2

UPDATE:

Session affinity aka sticky session can be added with DestinationRule using hashed based loadbalancing according to istio documentation.

Hope it helps.