Canary releases

Canary releasing is a deployment pattern where a small percentage of traffic is diverted to an early (“canary”) release of a particular service. This technique lets you test a release on a small subset of users, mitigating the impact of any given bug. Canary releasing also allows you to quickly roll back to a known good version in the event of an unexpected error. Detailed monitoring of core service metrics is an essential part of canary releasing, as monitoring enables the rapid detection of problems in the canary release.

Canary releases in Kubernetes

Kubernetes supports a basic canary release workflow using its core objects. In this workflow, a service owner can create a Kubernetes service. This service can then be pointed to multiple deployments. Each deployment can be a different version. By specifying the number of replicas in a given deployment, you can control how much traffic goes between different versions. For example, you could set replicas: 3 for v1, and replicas: 1 for v2, to ensure that 25% of traffic goes to v2. This approach works but is fairly coarse-grained unless you have lots of replicas. Moreover, auto-scaling doesn’t work well with this strategy.

Canary releases in Emissary

Emissary supports fine-grained canary releases. Emissary uses a weighted round-robin scheme to route traffic between multiple services. Full metrics are collected for all services, making it easy to compare the relative performance of the canary and production.

The weight attribute

The weight attribute specifies how much traffic for a given resource will be routed using a given mapping. Its value is an integer percentage between 0 and 100. Emissary will balance weights to make sure that, for every resource, the mappings for that resource will have weights adding to 100%. (In the simplest case, a single mapping is guaranteed to receive 100% of the traffic no matter whether it’s assigned a weight or not.)

Specifying a weight only makes sense if you have multiple mappings for the same resource, and typically you would not assign a weight to the “default” mapping (the mapping expected to handle most traffic): letting Emissary assign that mapping all the traffic not otherwise spoken for tends to make life easier when updating weights.

Here’s an example, which might appear during a canary deployment:

---
apiVersion: getambassador.io/v2
kind:  Mapping
metadata:
  name:  quote-backend
spec:
  prefix: /backend/
  service: quote
---
apiVersion: getambassador.io/v2
kind:  Mapping
metadata:
  name:  quote-backend2
spec:
  prefix: /backend/
  service: quotev2
  weight: 10

In this case, the quote-backend2 will receive 10% of the requests for /backend/, and Emissary will assign the remaining 90% to the quote-backend.