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Flux sharding and horizontal scaling
When Flux is managing tens of thousands of applications, it is advised to adopt a sharding strategy to spread the load between multiple instances of Flux controllers. To enable horizontal scaling, each controller can be deployed multiple times with a unique label selector which is used as the sharding key.
What follows is a guide on how to bootstrap multiple controller instances and how to
shard the reconciliation of Flux resources using the sharding.fluxcd.io/key label.
Bootstrap with sharding
At bootstrap time, you can define the number of shards and spin up a Flux controller instance for each shard.
First you’ll need to create a Git repository and clone it locally, then create the file structure required by bootstrap with:
mkdir -p clusters/my-cluster/flux-system
touch clusters/my-cluster/flux-system/gotk-components.yaml \
clusters/my-cluster/flux-system/gotk-sync.yaml \
clusters/my-cluster/flux-system/kustomization.yaml
Then you’ll create a dedicated directory inside flux-system for each shard:
mkdir -p clusters/my-cluster/flux-system/shard1
touch clusters/my-cluster/flux-system/shard1/kustomization.yaml
Configure controller sharding
In the shard1 directory generate a set of controller deployments that
will reconcile the Flux resources labels with sharding.fluxcd.io/key: shard1.
To spin up a dedicated source-controller, kustomize-controller and helm-controller instance,
use the following patches in clusters/my-cluster/flux-system/shard1/kustomization.yaml:
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
namespace: flux-system
resources:
- ../gotk-components.yaml
nameSuffix: "-shard1"
commonAnnotations:
sharding.fluxcd.io/role: "shard"
patches:
- target:
kind: (Namespace|CustomResourceDefinition|ClusterRole|ClusterRoleBinding|ServiceAccount|NetworkPolicy)
labelSelector: "app.kubernetes.io/part-of=flux"
patch: |
apiVersion: v1
kind: all
metadata:
name: all
$patch: delete
- target:
labelSelector: "app.kubernetes.io/component=notification-controller"
patch: |
apiVersion: v1
kind: all
metadata:
name: all
$patch: delete
- target:
kind: Deployment
name: (image-reflector-controller|image-automation-controller)
patch: |
apiVersion: v1
kind: Deployment
metadata:
name: all
$patch: delete
- target:
kind: Service
name: source-controller
patch: |
- op: replace
path: /spec/selector/app
value: source-controller-shard1
- target:
kind: Deployment
name: source-controller
patch: |
- op: replace
path: /spec/selector/matchLabels/app
value: source-controller-shard1
- op: replace
path: /spec/template/metadata/labels/app
value: source-controller-shard1
- op: replace
path: /spec/template/spec/containers/0/args/6
value: --storage-adv-addr=source-controller-shard1.$(RUNTIME_NAMESPACE).svc.cluster.local.
- target:
kind: Deployment
name: kustomize-controller
patch: |
- op: replace
path: /spec/selector/matchLabels/app
value: kustomize-controller-shard1
- op: replace
path: /spec/template/metadata/labels/app
value: kustomize-controller-shard1
- target:
kind: Deployment
name: helm-controller
patch: |
- op: replace
path: /spec/selector/matchLabels/app
value: helm-controller-shard1
- op: replace
path: /spec/template/metadata/labels/app
value: helm-controller-shard1
- target:
kind: Deployment
name: (source-controller|kustomize-controller|helm-controller)
patch: |
- op: add
path: /spec/template/spec/containers/0/args/-
value: --watch-label-selector=sharding.fluxcd.io/key=shard1
The above configuration will generate three deployments source-controller-shard1,
kustomize-controller-shard1 and helm-controller-shard1 all configured
with --watch-label-selector=sharding.fluxcd.io/key=shard1.
To enable these deployments at bootstrap, add the shard1 directory to
the clusters/my-cluster/flux-system/kustomization.yaml resources:
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- gotk-components.yaml
- gotk-sync.yaml
- shard1
patches:
- target:
kind: Deployment
name: "(source-controller|kustomize-controller|helm-controller)"
annotationSelector: "!sharding.fluxcd.io/role"
patch: |
- op: add
path: /spec/template/spec/containers/0/args/0
value: --watch-label-selector=!sharding.fluxcd.io/key
Note how this configuration excludes the sharding keys from the main controllers watch with
--watch-label-selector=!sharding.fluxcd.io/key. This ensures
that the main controllers will not reconcile any Flux resources labels with the sharding keys.
Install and Upgrade shards
Push the changes to main branch:
git add -A && git commit -m "init flux" && git push
And run the bootstrap for clusters/my-cluster:
flux bootstrap git \
--url=ssh://git@<host>/<org>/<repository> \
--branch=main \
--path=clusters/my-cluster
Verify that the main controllers and the ones assigned to shard1 are running:
$ kubectl -n flux-system get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
helm-controller 1/1 1 1 1m
helm-controller-shard1 1/1 1 1 1m
kustomize-controller 1/1 1 1 1m
kustomize-controller-shard1 1/1 1 1 1m
notification-controller 1/1 1 1 1m
source-controller 1/1 1 1 1m
source-controller-shard1 1/1 1 1 1m
When upgrading Flux, either by rerunning bootstrap with a newer version or by using the Flux GitHub Actions, the sharded controllers will be automatically upgraded along with the main ones.
Assign resources to shards
To assign a group of Flux resources to a particular shard, label
them with sharding.fluxcd.io/key.
For example, assuming you want to assign the reconciliation of an application
to the shard1 controllers, label both the Flux source and its Kustomization
with sharding.fluxcd.io/key: shard1:
---
apiVersion: source.toolkit.fluxcd.io/v1
kind: GitRepository
metadata:
name: podinfo
namespace: default
labels:
sharding.fluxcd.io/key: shard1
spec:
interval: 10m
url: https://github.com/stefanprodan/podinfo
ref:
semver: 6.x
---
apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
name: podinfo
namespace: default
labels:
sharding.fluxcd.io/key: shard1
spec:
interval: 10m
targetNamespace: default
sourceRef:
kind: GitRepository
name: podinfo
path: ./kustomize
prune: true
Note that Source object kinds which have a dependency on another kind
(i.e. HelmChart on a HelmRepository) need to have the same labels
applied to work as expected.
For example, assuming you want to assign the reconciliation of a Helm release
to the shard1 controllers, label the HelmRelease, its chart and its repository
with sharding.fluxcd.io/key: shard1:
apiVersion: source.toolkit.fluxcd.io/v1beta2
kind: HelmRepository
metadata:
name: podinfo
namespace: default
labels:
sharding.fluxcd.io/shard: shard1
spec:
interval: 10m
type: oci
url: oci://ghcr.io/stefanprodan/charts
---
apiVersion: helm.toolkit.fluxcd.io/v2beta1
kind: HelmRelease
metadata:
name: podinfo
namespace: default
labels:
sharding.fluxcd.io/key: shard1
spec:
interval: 10m
chart:
metadata:
labels:
sharding.fluxcd.io/key: shard1
spec:
chart: podinfo
sourceRef:
kind: HelmRepository
name: podinfo
Note that with .spec.chart.metadata.labels we set the sharding key on the
generated Flux HelmChart object, so that both the Helm repository and charts
are managed by source-controller-shard1 instance.
Bulk assign shards
Instead of manually labeling each Flux resource with a shard key, use a top-level Flux Kustomization and automatically label all resources.
For example, assuming you want to assign a tenant to a particular shard, in the root Flux Kustomization that reconcile the tenant’s Flux sources, kustomizations and Helm releases label these resources as follows:
apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
name: tenant1
namespace: tenant1
spec:
interval: 10m0s
path: ./apps
prune: true
sourceRef:
kind: GitRepository
name: tenant1
targetNamespace: tenant1
commonMetadata:
labels:
sharding.fluxcd.io/key: shard1
patches:
- target:
kind: HelmRelease
patch: |
apiVersion: helm.toolkit.fluxcd.io/v2beta1
kind: HelmRelease
metadata:
name: all
spec:
chart:
metadata:
labels:
sharding.fluxcd.io/key: shard1
With .spec.commonMetadata.labels we set the shading key on all
the Flux resources and with the .spec.patches we set the same shading key
for all generated HelmCharts.