Traffic Splitting

The SMI Traffic Split API can be used to split outgoing traffic to multiple service backends. This can be used to orchestrate canary releases for multiple versions of the software.

What is supported

FSM implements the SMI traffic split v1alpha4 version.

It supports the following:

• Traffic splitting in both SMI and Permissive traffic policy modes
• HTTP and TCP traffic splitting
• Traffic splitting for canary or blue-green deployments

How it works

Outbound traffic destined to a Kubernetes service can be split to multiple service backends using the SMI Traffic Split API. Consider the following example where traffic to the bookstore.default.svc.cluster.local FQDN corresponding to the default/bookstore service is split to services default/bookstore-v1 and default/bookstore-v2, with a weight of 90 and 10 respectively.

apiVersion: split.smi-spec.io/v1alpha4
kind: TrafficSplit
metadata:
  name: bookstore-split
  namespace: default
spec:
  service: bookstore.default.svc.cluster.local
  backends:
  - service: bookstore-v1
    weight: 90
  - service: bookstore-v2
    weight: 10

For a TrafficSplit resource to be correctly configured, it is important to ensure the following conditions are met:

• metadata.namespace is a namespace added to the mesh
• metadata.namespace, spec.service, and spec.backends all belong to the same namespace
• spec.service specifies an FQDN of a Kubernetes service
• spec.service and spec.backends correspond to Kubernetes service objects
• The total weight of all backends must be greater than zero, and each backend must have a positive weight

When a TrafficSplit resource is created, FSM applies the configuration on client sidecars to split traffic directed to the root service (spec.service) to the backends (spec.backends) based the specified weights. For HTTP traffic, the Host/Authority header in the request must match the FQDNs of the root service specified in the TrafficSplit resource. In the above example, it implies that the Host/Authority header in the HTTP request originated by the client must match the Kubernetes service FQDNs of the default/bookstore service for traffic split to work.

Note: FSM does not configure Host/Authority header rewrites for the original HTTP requests, so it is necessary that the backend services referenced in a TrafficSplit resource accept requests with the original HTTP Host/Authority header.

It is important to note that a TrafficSplit resource only configures traffic splitting to a service, and does not give applications permission to communicate with each other. Thus, a valid TrafficTarget resource must be configured in conjunction with a TrafficSplit configuration to achieve traffic flow between applications as desired.

Refer to a demo on Canary rollouts using SMI Traffic Split to learn more.