Resolves#371
---
This adds the support for `corev1.Service` as the `targetRef.kind`, so that we can use Flagger just for canary analysis and traffic-shifting on existing and pre-created services. Flagger doesn't touch deployments and HPAs in this mode.
This is useful for keeping your full-control on the resources backing the service to be canary-released, including pods(behind a ClusterIP service) and external services(behind an ExternalName service).
Major use-case in my mind are:
- Canary-release a K8s cluster. You create two clusters and a master cluster. In the master cluster, you create two `ExternalName` services pointing to (the hostname of the loadbalancer of the targeted app instance in) each cluster. Flagger runs on the master cluster and helps safely rolling-out a new K8s cluster by doing a canary release on the `ExternalName` service.
- You want annotations and labels added to the service for integrating with things like external lbs(without extending Flagger to support customizing any aspect of the K8s service it manages
**Design**:
A canary release on a K8s service is almost the same as one on a K8s deployment. The only fundamental difference is that it operates only on a set of K8s services.
For example, one may start by creating two Helm releases for `podinfo-blue` and `podinfo-green`, and a K8s service `podinfo`. The `podinfo` service should initially have the same `Spec` as that of `podinfo-blue`.
On a new release, you update `podinfo-green`, then trigger Flagger by updating the K8s service `podinfo` so that it points to pods or `externalName` as declared in `podinfo-green`. Flagger does the rest. The end result is the traffic to `podinfo` is gradually and safely shifted from `podinfo-blue` to `podinfo-green`.
**How it works**:
Under the hood, Flagger maintains two K8s services, `podinfo-primary` and `podinfo-canary`. Compared to canaries on K8s deployments, it doesn't create the service named `podinfo`, as it is already provided by YOU.
Once Flagger detects the change in the `podinfo` service, it updates the `podinfo-canary` service and the routes, then analyzes the canary. On successful analysis, it promotes the canary service to the `podinfo-primary` service. You expose the `podinfo` service via any L7 ingress solution or a service mesh so that the traffic is managed by Flagger for safe deployments.
**Giving it a try**:
To give it a try, create a `Canary` as usual, but its `targetRef` pointed to a K8s service:
```
apiVersion: flagger.app/v1alpha3
kind: Canary
metadata:
name: podinfo
spec:
provider: kubernetes
targetRef:
apiVersion: core/v1
kind: Service
name: podinfo
service:
port: 9898
canaryAnalysis:
# schedule interval (default 60s)
interval: 10s
# max number of failed checks before rollback
threshold: 2
# number of checks to run before rollback
iterations: 2
# Prometheus checks based on
# http_request_duration_seconds histogram
metrics: []
```
Create a K8s service named `podinfo`, and update it. Now watch for the services `podinfo`, `podinfo-primary`, `podinfo-canary`.
Flagger tracks `podinfo` service for changes. Upon any change, it reconciles `podinfo-primary` and `podinfo-canary` services. `podinfo-canary` always replicate the latest `podinfo`. In contract, `podinfo-primary` replicates the latest successful `podinfo-canary`.
**Notes**:
- For the canary cluster use-case, we would need to write a K8s operator to, e.g. for App Mesh, sync `ExternalName` services to AppMesh `VirtualNode`s. But that's another story!
