How it works

Flagger can be configured to automate the release process for Kubernetes workloads with a custom resource named canary.

Canary resource

The canary custom resource defines the release process of an application running on Kubernetes and is portable across clusters, service meshes and ingress providers.

For a deployment named podinfo, a canary release with progressive traffic shifting can be defined as:

apiVersion: flagger.app/v1beta1
kind: Canary
metadata:
name: podinfo
spec:
targetRef:
apiVersion: apps/v1
kind: Deployment
name: podinfo
service:
port: 9898
analysis:
interval: 1m
threshold: 10
maxWeight: 50
stepWeight: 5
metrics:
- name: request-success-rate
thresholdRange:
min: 99
interval: 1m
- name: request-duration
thresholdRange:
max: 500
interval: 1m
webhooks:
- name: load-test
url: http://flagger-loadtester.test/
metadata:
cmd: "hey -z 1m -q 10 -c 2 http://podinfo-canary.test:9898/"

When you deploy a new version of an app, Flagger gradually shifts traffic to the canary, and at the same time, measures the requests success rate as well as the average response duration. You can extend the canary analysis with custom metrics, acceptance and load testing to harden the validation process of your app release process.

If you are running multiple service meshes or ingress controllers in the same cluster, you can override the global provider for a specific canary with spec.provider.

Canary target

A canary resource can target a Kubernetes Deployment or DaemonSet.

Kubernetes Deployment example:

spec:
progressDeadlineSeconds: 60
targetRef:
apiVersion: apps/v1
kind: Deployment
name: podinfo
autoscalerRef:
apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
name: podinfo

Based on the above configuration, Flagger generates the following Kubernetes objects:

  • deployment/<targetRef.name>-primary

  • hpa/<autoscalerRef.name>-primary

The primary deployment is considered the stable release of your app, by default all traffic is routed to this version and the target deployment is scaled to zero. Flagger will detect changes to the target deployment (including secrets and configmaps) and will perform a canary analysis before promoting the new version as primary.

If the target deployment uses secrets and/or configmaps, Flagger will create a copy of each object using the -primary prefix and will reference these objects in the primary deployment. You can disable the secrets/configmaps tracking with the -enable-config-tracking=false command flag in the Flagger deployment manifest under containers args or by setting --set configTracking.enabled=false when installing Flagger with Helm.

Note that the target deployment must have a single label selector in the format app: <DEPLOYMENT-NAME>:

apiVersion: apps/v1
kind: Deployment
metadata:
name: podinfo
spec:
selector:
matchLabels:
app: podinfo
template:
metadata:
labels:
app: podinfo

Besides app Flagger supports name and app.kubernetes.io/name selectors. If you use a different convention you can specify your label with the -selector-labels=my-app-label command flag in the Flagger deployment manifest under containers args or by setting --set selectorLabels=my-app-label when installing Flagger with Helm.

The autoscaler reference is optional, when specified, Flagger will pause the traffic increase while the target and primary deployments are scaled up or down. HPA can help reduce the resource usage during the canary analysis.

The progress deadline represents the maximum time in seconds for the canary deployment to make progress before it is rolled back, defaults to ten minutes.

Canary service

A canary resource dictates how the target workload is exposed inside the cluster. The canary target should expose a TCP port that will be used by Flagger to create the ClusterIP Services.

spec:
service:
name: podinfo
port: 9898
portName: http
targetPort: 9898
portDiscovery: true

The container port from the target workload should match the service.port or service.targetPort. The service.name is optional, defaults to spec.targetRef.name. The service.targetPort can be a container port number or name. The service.portName is optional (defaults to http), if your workload uses gRPC then set the port name to grpc.

If port discovery is enabled, Flagger scans the target workload and extracts the containers ports excluding the port specified in the canary service and service mesh sidecar ports. These ports will be used when generating the ClusterIP services.

Based on the canary spec service, Flagger creates the following Kubernetes ClusterIP service:

  • <service.name>.<namespace>.svc.cluster.local

    selector app=<name>-primary

  • <service.name>-primary.<namespace>.svc.cluster.local

    selector app=<name>-primary

  • <service.name>-canary.<namespace>.svc.cluster.local

    selector app=<name>

This ensures that traffic to podinfo.test:9898 will be routed to the latest stable release of your app. The podinfo-canary.test:9898 address is available only during the canary analysis and can be used for conformance testing or load testing.

Besides the port mapping, the service specification can contain URI match and rewrite rules, timeout and retry polices:

spec:
service:
port: 9898
match:
- uri:
prefix: /
rewrite:
uri: /
retries:
attempts: 3
perTryTimeout: 1s
timeout: 5s

When using Istio as the mesh provider, you can also specify HTTP header operations, CORS and traffic policies, Istio gateways and hosts. The Istio routing configuration can be found here.

Canary status

You can use kubectl to get the current status of canary deployments cluster wide:

kubectl get canaries --all-namespaces
NAMESPACE NAME STATUS WEIGHT LASTTRANSITIONTIME
test podinfo Progressing 15 2019-06-30T14:05:07Z
prod frontend Succeeded 0 2019-06-30T16:15:07Z
prod backend Failed 0 2019-06-30T17:05:07Z

The status condition reflects the last known state of the canary analysis:

kubectl -n test get canary/podinfo -oyaml | awk '/status/,0'

A successful rollout status:

status:
canaryWeight: 0
failedChecks: 0
iterations: 0
lastAppliedSpec: "14788816656920327485"
lastPromotedSpec: "14788816656920327485"
conditions:
- lastTransitionTime: "2019-07-10T08:23:18Z"
lastUpdateTime: "2019-07-10T08:23:18Z"
message: Canary analysis completed successfully, promotion finished.
reason: Succeeded
status: "True"
type: Promoted

The Promoted status condition can have one of the following reasons: Initialized, Waiting, Progressing, Promoting, Finalising, Succeeded or Failed. A failed canary will have the promoted status set to false, the reason to failed and the last applied spec will be different to the last promoted one.

Wait for a successful rollout:

kubectl wait canary/podinfo --for=condition=promoted

CI example:

# update the container image
kubectl set image deployment/podinfo podinfod=stefanprodan/podinfo:3.0.1
# wait for Flagger to detect the change
ok=false
until ${ok}; do
kubectl get canary/podinfo | grep 'Progressing' && ok=true || ok=false
sleep 5
done
# wait for the canary analysis to finish
kubectl wait canary/podinfo --for=condition=promoted --timeout=5m
# check if the deployment was successful
kubectl get canary/podinfo | grep Succeeded

Canary finalizers

The default behavior of Flagger on canary deletion is to leave resources that aren't owned by the controller in their current state. This simplifies the deletion action and avoids possible deadlocks during resource finalization. In the event the canary was introduced with existing resource(s) (i.e. service, virtual service, etc.), they would be mutated during the initialization phase and no longer reflect their initial state. If the desired functionality upon deletion is to revert the resources to their initial state, the revertOnDeletion attribute can be enabled.

spec:
revertOnDeletion: true

When a deletion action is submitted to the cluster, Flagger will attempt to revert the following resources:

  • Canary target replicas will be updated to the primary replica count

  • Canary service selector will be reverted

  • Mesh/Ingress traffic routed to the target

The recommended approach to disable canary analysis would be utilization of the skipAnalysis attribute, which limits the need for resource reconciliation. Utilizing the revertOnDeletion attribute should be enabled when you no longer plan to rely on Flagger for deployment management.

Note When this feature is enabled expect a delay in the delete action due to the reconciliation.

Canary analysis

The canary analysis defines:

Spec:

analysis:
# schedule interval (default 60s)
interval:
# max number of failed metric checks before rollback
threshold:
# max traffic percentage routed to canary
# percentage (0-100)
maxWeight:
# canary increment step
# percentage (0-100)
stepWeight:
# total number of iterations
# used for A/B Testing and Blue/Green
iterations:
# canary match conditions
# used for A/B Testing
match:
- # HTTP header
# key performance indicators
metrics:
- # metric check
# alerting
alerts:
- # alert provider
# external checks
webhooks:
- # hook

The canary analysis runs periodically until it reaches the maximum traffic weight or the number of iterations. On each run, Flagger calls the webhooks, checks the metrics and if the failed checks threshold is reached, stops the analysis and rolls back the canary. If alerting is configured, Flagger will post the analysis result using the alert providers.