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Using Flux with Helm
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Using Flux with Helm

You can release charts to your cluster via "GitOps", by combining Flux and the Flux Helm Operator (also in weaveworks/flux).

The essential mechanism is this: the declaration of a Helm release is represented by a custom resource, specifying the chart and its values. If you put such a resource in your git repo as a file, Flux will apply it to the cluster, and once it's in the cluster, the Helm Operator will make sure the release exists by installing or upgrading it.

The HelmRelease custom resource

Each release of a chart is declared by a HelmRelease resource. The schema for these resources is given in the custom resource definition. They look like this:

---
apiVersion: flux.weave.works/v1beta1
kind: HelmRelease
metadata:
  name: rabbit
  namespace: default
spec:
  releaseName: rabbitmq
  chart:
    repository: https://kubernetes-charts.storage.googleapis.com/
    name: rabbitmq
    version: 3.3.6
  values:
    replicas: 1

The releaseName will be given to Helm as the release name. If not supplied, it will be generated by affixing the namespace to the resource name. In the above example, if releaseName were not given, it would be generated as default-rabbitmq. Because of the way Helm works, release names must be unique in the cluster.

The chart section gives a pointer to the chart; in this case, to a chart in a Helm repo. Since the helm operator is running in your cluster, and doesn't have access to local configuration, the repository is given as a URL rather than an alias (the URL in the example is what's usually aliased as stable). The name and version specify the chart to release.

The values section is where you provide the value overrides for the chart. This is as you would put in a values.yaml file, but inlined into the structure of the resource. See below for examples.

Using a chart from a Git repo instead of a Helm repo

You can refer to a chart from a git repo, rather than a chart repo, with a chart: section like this:

spec:
  chart:
    git: [email protected]:weaveworks/flux-get-started
    ref: master
    path: charts/ghost

In this case, the git repo will be cloned, and the chart will be released from the ref given (which defaults to master, if not supplied). Commits to the git repo may result in releases, if they update the chart at the path given.

Note that you will usually need to provide an SSH key to grant access to the git repository. The example deployment shows how to mount a secret at the expected location of the key (/etc/fluxd/ssh/). If you need more than one SSH key, you'll need to also mount an adapted ssh_config; this is also demonstrated in the example deployment.

What the Helm Operator does

When the Helm Operator sees a HelmRelease resource in the cluster, it either installs or upgrades the named Helm release so that the chart is released as specified.

It will also notice when a HelmRelease resource is updated, and take action accordingly.

Supplying values to the chart

You can supply values to be used with the chart when installing it, in two ways.

.spec.values

This is a YAML map as you'd put in a file and supply to Helm with -f values.yaml, but inlined into the HelmRelease manifest. For example,

apiVersion: flux.weave.works/v1beta1
kind: HelmRelease
# metadata: ...
spec:
  # chart: ...
  values:
    foo: value1
    bar:
    baz: value2
    oof:
    - item1
    - item2

.spec.valueFileSecrets

This is a list of secrets (in the same namespace as the HelmRelease) from which to take values. The secrets must each contain an entry for values.yaml.

The values are merged in the order given, with later values overwriting earlier. These values always have a lower priority that those passed via the .spec.values parameter.

This is useful if you want to have defaults such as the region, clustername, environment, a local docker registry URL, etc., or if you simply want to have values not checked into git as plaintext.

Example of spec.valueFileSecrets

Say you have a values.yaml that looks like this:

# values.yaml
clusterName: "my-cluster"
dockerRegistry: "registry.local"
mySecretValue: "foo"

You would create a secret in the cluster by doing this:

kubectl -n dev create secret generic default-values --from-file=values.yaml

If you did kubectl get -n dev secret default-values you would get:

apiVersion: v1
kind: Secret
type: Opaque
metadata:
  name: default-values
  namespace: dev
data:
  values.yaml: <base64 encoded values.yaml>

Then, you could refer to the secret in a HelmRelease, and the values would be used when the chart was installed:

apiVersion: flux.weave.works/v1beta1
kind: HelmRelease
metadata:
  name: uses-secret
  namespace: dev
spec:
  # chart: ...
  valueFileSecrets:
  - name: default-values

Upgrading images in a HelmRelease using Flux

If the chart you're using in a HelmRelease lets you specify the particular images to run, you will usually be able to update them with Flux, the same way you can with Deployments and so on.

Flux interprets certain commonly used structures in the values section of a HelmRelease as referring to images. The following are understood (showing just the values section):

values:
  image: repo/image:version
values:
  image: repo/image
  tag: version
values:
  image:
    repository: repo/image
    tag: version

These can appear at the top level (immediately under values:), or in a subsection (under a key, itself under values:). Other values may be mixed in arbitrarily. Here's an example of a values section that specifies two images, along with some other configuration:

values:
  persistent: true

  # image that will be labeled "chart-image"
  image: repo/image1:version

  subsystem:
    # image that will be labeled "subsystem"
    image:
      repository: repo/image2
      tag: version
      imagePullPolicy: IfNotPresent
    port: 4040

Using annotations to control updates to HelmRelease resources

You can use the same annotations in the HelmRelease as you would for a Deployment or other workload, to control updates and automation. For the purpose of specifying filters, the container name is either chart-image (if at the top level), or the key under which the image is given (e.g., "subsystem" from the example above).

Why use URLs to refer to repositories, rather than names? ^

A HelmRelease must be able to stand on its own. If we used names in the spec, which were resolved to URLs elsewhere (e.g., in a repositories.yaml supplied to the operator), it would be possible to change the meaning of a HelmRelease without altering it. This is undesirable because it makes it hard to specify exactly what you want, in the one place; or to read exactly what is being specified, in the one place. In other words, it's better to be explicit.

Authentication

At present, per-resource authentication is not implemented. The HelmRelease definition includes a field chartPullSecret for attaching a repositories.yaml file, but this is ignored for now.

Instead, you need to provide the operator with credentials and keys (see the following Authentication for Heml repos section for how to do this).

Authentication for Helm repos

As a workaround, you can mount a repositories.yaml file with authentication already configured, into the operator container.

Note: When using a custom repositories.yaml the default that ships with the operator is overwritten. This means that for any repository you want to make use of you should manually add an entry to your repositories.yaml file.

To prepare a file, add the repo locally as you would normally:

helm repo add <URL> --username <username> --password <password>

You need to doctor this file a little, since it will likely contain absolute paths that will be wrong when mounted inside the container. Copy the file and replace all the cache entries with just the filename.

cp ~/.helm/repository/repositories.yaml .
sed -i -e 's/^\( *cache: \).*\/\(.*\.yaml\)/\1\2/g'

Now you can create a secret in the same namespace as you're running the Helm operator, from the repositories file:

kubectl create secret generic flux-helm-repositories --from-file=./repositories.yaml

Lastly, mount that secret into the container. This can be done by setting helmOperator.configureRepositories.enable to true for the flux Helm release, or as shown in the commented-out sections of the example deployment.

Azure ACR repositories

For Azure ACR repositories, the entry in repositories.yaml created by running az acr helm repo add is unsufficient for the Helm operator. Instead you will need to create a service principal and use the plain text id and password this gives you. For example:

- caFile: ""
  cache: <repository>-index.yaml
  certFile: ""
  keyFile: ""
  name: <repository>
  url: https://<repository>.azurecr.io/helm/v1/repo
  username: <service principal id>
  password: <service principal password>

Authentication for Git repos

In general, it's necessary to have an SSH key to clone a git repo. This is sometimes (e.g., on GitHub) called a "deploy key". To use a chart from git, the Helm Operator needs a key with read-only access.

To provide an SSH key, put the key in a secret under the entry identity, and mount it into the operator container as shown in the example deployment. The default ssh_config expects an identity file at /etc/fluxd/ssh/identity, which is where it'll be if you just uncomment the blocks from the example.

If you're using more than one repository, you may need to provide more than one SSH key. In that case, you can create a secret with an entry for each key, and mount that as well as an ssh_config file mentioning each key as an IdentityFile.