# Charts
Helm uses a packaging format called _charts_ . A chart is a collection of files
that describe a related set of Kubernetes resources. A single chart
might be used to deploy something simple, like a memcached pod, or
something complex, like a full web app stack with HTTP servers,
databases, caches, and so on.
Charts are created as files laid out in a particular directory tree,
then they can be packaged into versioned archives to be deployed.
This document explains the chart format, and provides basic guidance for
building charts with Helm.
## The Chart File Structure
A chart is organized as a collection of files inside of a directory. The
directory name is the name of the chart (without versioning information). Thus,
a chart describing Wordpress would be stored in the `wordpress/` directory.
Inside of this directory, Helm will expect a structure that matches this:
```
wordpress/
Chart.yaml # A YAML file containing information about the chart
LICENSE # OPTIONAL: A plain text file containing the license for the chart
README.md # OPTIONAL: A human-readable README file
values.yaml # The default configuration values for this chart
charts/ # OPTIONAL: A directory containing any charts upon which this chart depends.
templates/ # OPTIONAL: A directory of templates that, when combined with values,
# will generate valid Kubernetes manifest files.
```
Helm will silently strip out any other files.
## The Chart.yaml File
The `Chart.yaml` file is required for a chart. It contains the following fields:
```yaml
name: The name of the chart (required)
version: A SemVer 2 version (required)
description: A single-sentence description of this project (optional)
keywords:
- A list of keywords about this project (optional)
home: The URL of this project's home page (optional)
sources:
- A list of URLs to source code for this project (optional)
maintainers: # (optional)
- name: The maintainer's name (required for each maintainer)
email: The maintainer's email (optional for each maintainer)
engine: gotpl # The name of the template engine (optional, defaults to gotpl)
```
If you are familiar with the `Chart.yaml` file format for Helm Classic, you will
notice that fields specifying dependencies have been removed. That is because
the new Chart format expresses dependencies using the `charts/` directory.
Other fields will be silently ignored.
### Charts and Versioning
Every chart must have a version number. A version must follow the
[SemVer 2 ](http://semver.org/ ) standard. Unlike Helm Classic, Kubernetes
Helm uses version numbers as release markers. Packages in repositories
are identified by name plus version.
For example, an `nginx` chart whose version field is set to `version:
1.2.3` will be named:
```
nginx-1.2.3.tgz
```
More complex SemVer 2 names are also supported, such as
`version: 1.2.3-alpha.1+ef365` . But non-SemVer names are explicitly
disallowed by the system.
**NOTE:** Whereas Helm Classic and Deployment Manager were both
very GitHub oriented when it came to charts, Kubernetes Helm does not
rely upon or require GitHub or even Git. Consequently, it does not use
Git SHAs for versioning at all.
The `version` field inside of the `Chart.yaml` is used by many of the
Helm tools, including the CLI and the Tiller server. When generating a
package, the `helm package` command will use the version that it finds
in the `Chart.yaml` as a token in the package name. The system assumes
that the version number in the chart package name matches the version number in
the `Chart.yaml` . Failure to meet this assumption will cause an error.
## Chart Dependencies
In Helm, one chart may depend on any number of other charts. These
dependencies are expressed explicitly by copying the dependency charts
into the `charts/` directory.
**Note:** The `dependencies:` section of the `Chart.yaml` from Helm
Classic has been completely removed.
For example, if the Wordpress chart depends on the Apache chart, the
Apache chart (of the correct version) is supplied in the Wordpress
chart's `charts/` directory:
```
wordpress:
Chart.yaml
# ...
charts/
apache/
Chart.yaml
# ...
mysql/
Chart.yaml
# ...
```
The example above shows how the Wordpress chart expresses its dependency
on Apache and MySQL by including those charts inside of its `charts/`
directory.
**TIP:** _To drop a dependency into your `charts/` directory, use the
`helm fetch` command._
## Templates and Values
By default, Helm Chart templates are written in the Go template language, with the
addition of 50 or so [add-on template
functions](https://github.com/Masterminds/sprig). (In the future, Helm
may support other template languages, like Python Jinja.)
All template files are stored in a chart's `templates/` folder. When
Helm renders the charts, it will pass every file in that directory
through the template engine.
Values for the templates are supplied two ways:
- Chart developers may supply a file called `values.yaml` inside of a
chart. This file can contain default values.
- Chart users may supply a YAML file that contains values. This can be
provided on the command line with `helm install` .
When a user supplies custom values, these values will override the
values in the chart's `values.yaml` file.
### Template Files
Template files follow the standard conventions for writing Go templates
(see [the text/template Go package documentation ](https://golang.org/pkg/text/template/ )
for details).
An example template file might look something like this:
```yaml
apiVersion: v1
kind: ReplicationController
metadata:
name: deis-database
namespace: deis
labels:
heritage: deis
spec:
replicas: 1
selector:
app: deis-database
template:
metadata:
labels:
app: deis-database
spec:
serviceAccount: deis-database
containers:
- name: deis-database
image: {{.imageRegistry}}/postgres:{{.dockerTag}}
imagePullPolicy: {{.pullPolicy}}
ports:
- containerPort: 5432
env:
- name: DATABASE_STORAGE
value: {{default "minio" .storage}}
```
The above example, based loosely on [https://github.com/deis/charts ](https://github.com/deis/charts ), is a template for a Kubernetes replication controller.
It can use the following four template values:
- `imageRegistry` : The source registry for the Docker image.
- `dockerTag` : The tag for the docker image.
- `pullPolicy` : The Kubernetes pull policy.
- `storage` : The storage backend, whose default is set to `"minio"`
All of these values are defined by the template author. Helm does not
require or dictate parameters.
### Predefined Values
The following values are pre-defined, are available to every template, and
cannot be overridden. As with all values, the names are _case
sensitive_.
- `Release.Name` : The name of the release (not the chart)
- `Release.Time` : The time the chart release was last updated. This will
match the `Last Released` time on a Release object.
- `Release.Namespace` : The namespace the chart was released to.
- `Release.Service` : The service that conducted the release. Usually
this is `Tiller` .
- `Chart` : The contents of the `Chart.yaml` . Thus, the chart version is
obtainable as `Chart.Version` and the maintainers are in
`Chart.Maintainers` .
**NOTE:** Any unknown Chart.yaml fields will be dropped. They will not
be accessible inside of the `Chart` object. Thus, Chart.yaml cannot be
used to pass arbitrarily structured data into the template.
### Values files
Considering the template in the previous section, a `values.yaml` file
that supplies the necessary values would look like this:
```yaml
imageRegistry: "quay.io/deis"
dockerTag: "latest"
pullPolicy: "alwaysPull"
storage: "s3"
```
A values file is formatted in YAML. A chart may include a default
`values.yaml` file. The Helm install command allows a user to override
values by supplying additional YAML values:
```console
$ helm install --values=myvals.yaml wordpress
```
When values are passed in this way, they will be merged into the default
values file. For example, consider a `myvals.yaml` file that looks like
this:
```yaml
storage: "gcs"
```
When this is merged with the `values.yaml` in the chart, the resulting
generated content will be:
```yaml
imageRegistry: "quay.io/deis"
dockerTag: "latest"
pullPolicy: "alwaysPull"
storage: "gcs"
```
Note that only the last field was overridden.
**NOTE:** The default values file included inside of a chart _must_ be named
`values.yaml` . But files specified on the command line can be named
anything.
### Scope, Dependencies, and Values
Values files can declare values for the top-level chart, as well as for
any of the charts that are included in that chart's `charts/` directory.
Or, to phrase it differently, a values file can supply values to the
chart as well as to any of its dependencies. For example, the
demonstration Wordpress chart above has both `mysql` and `apache` as
dependencies. The values file could supply values to all of these
components:
```yaml
title: "My Wordpress Site" # Sent to the Wordpress template
mysql:
max_connections: 100 # Sent to MySQL
password: "secret"
apache:
port: 8080 # Passed to Apache
```
Charts at a higher level have access to all of the variables defined
beneath. So the wordpress chart can access `.mysql.password` . But lower
level charts cannot access things in parent charts, so MySQL will not be
able to access the `title` property. Nor, for that matter, can it access
`.apache.port` .
Values are namespaced, but namespaces are pruned. So for the Wordpress
chart, it can access the MySQL password field as `.mysql.password` . But
for the MySQL chart, the scope of the values has been reduced and the
namespace prefix removed, so it will see the password field simply as
`.password` .
#### Global Values
As of 2.0.0-Alpha.2, Helm supports special "global" value. Consider
this modified version of the previous example:
```yaml
title: "My Wordpress Site" # Sent to the Wordpress template
global:
app: MyWordpress
mysql:
max_connections: 100 # Sent to MySQL
password: "secret"
apache:
port: 8080 # Passed to Apache
```
The above adds a `global` section with the value `app: MyWordpress` .
This value is available to _all_ charts as `.Values.global.app` .
For example, the `mysql` templates may access `app` as `{{.Values.global.app}}` , and
so can the `apache` chart. Effectively, the values file above is
regenerated like this:
```yaml
title: "My Wordpress Site" # Sent to the Wordpress template
global:
app: MyWordpress
mysql:
global:
app: MyWordpress
max_connections: 100 # Sent to MySQL
password: "secret"
apache:
global:
app: MyWordpress
port: 8080 # Passed to Apache
```
This provides a way of sharing one top-level variable with all
subcharts, which is useful for things like setting `metadata` properties
like labels.
If a subchart declares a global variable, that global will be passed
_downward_ (to the subchart's subcharts), but not _upward_ to the parent
chart. There is no way for a subchart to influence the values of the
parent chart.
_Global sections are restricted to only simple key/value pairs. They do
not support nesting._
For example, the following is **illegal** and will not work:
```yaml
global:
foo: # It is illegal to nest an object inside of global.
bar: baz
```
### References
When it comes to writing templates and values files, there are several
standard references that will help you out.
- [Go templates ](https://godoc.org/text/template )
- [Extra template functions ](https://godoc.org/github.com/Masterminds/sprig )
- [The YAML format]()
## Hooks
Helm provides a _hook_ mechanism to allow chart developers to intervene
at certain points in a release's life cycle. For example, you can use
hooks to:
- Load a ConfigMap or Secret during install before any other charts are
loaded.
- Execute a Job to backup up a database before installing a new chart,
and then execute a second job after the upgrade in order to restore
data.
- Run a Job before deleting a release to gracefully take a service out
of rotation before removing it.
Hooks work like regular templates, but they have special annotations
that cause Helm to utilize them differently. In this section, we cover
the basic usage pattern for hooks.
### The Available Hooks
The following hooks are defined:
- pre-install: Executes after templates are rendered, but before any
resources are created in Kubernetes.
- post-install: Executes after all resources are loaded into Kubernetes
- pre-delete: Executes on a deletion request before any resources are
deleted from Kubernetes.
- post-delete: Executes on a deletion request after all of the release's
resources have been deleted.
- pre-upgrade: Executes on an upgrade request after templates are
rendered, but before any resources are loaded into Kubernetes (e.g.
before a kuberntes apply operation).
- post-upgrade: Executes on an upgrade after all resources have been
upgraded.
### Hooks and the Release Lifecycle
Hooks allow you, the chart developer, an opportunity to perform
operations at strategic points in a release lifecycle. For example,
consider the lifecycle for a `helm install` . By default, the lifecycle
looks like this:
1. User runs `helm install foo`
2. Chart is loaded into Tiller
3. After some verification, Tiller renders the `foo` templates
4. Tiller loads the resulting resources into Kubernetes
5. Tiller returns the release name (and other data) to the client
6. The client exits
Helm defines two hooks for the `install` lifecycle: `pre-install` and
`post-install` . If the developer of the `foo` chart implements both
hooks, the lifecycle is altered like this:
1. User runs `helm install foo`
2. Chart is loaded into Tiller
3. After some verification, Tiller renders the `foo` templates
4. Tiller executes the `pre-install` hook (loading hook resources into
Kubernetes)
5. Tiller waits until the hook is "Ready"
6. Tiller loads the resulting resources into Kubernetes
7. Tiller executes the `post-install` hook (loading hook resources)
8. Tiller waits until the hook is "Ready"
9. Tiller returns the release name (and other data) to the client
10. The client exits
What does it mean to wait until a hook is ready? This depends on the
resource declared in the hook. If the resources is a `Job` kind, Tiller
will wait until the job successfully runs to completion. And if the job
fails, the release will fail. This is a _blocking operation_ , so the
Helm client will pause while the Job is run.
For all other kinds, as soon as Kubernetes marks the resource as loaded
(added or updated), the resource is considered "Ready". When many
resources are declared in a hook, the resources are executed serially,
but the order of their execution is not guaranteed.
#### Hook resources are unmanaged
The resources that a hook creates are not tracked or managed as part of the
release. Once Tiller verifies that the hook has reached its ready state, it
will leave the hook resource alone.
Practically speaking, this means that if you create resources in a hook, you
cannot rely upon `helm delete` to remove the resources. To destroy such
resources, you need to write code to perform this operation in a `pre-delete`
or `post-delete` hook.
### Writing a Hook
Hooks are just Kubernetes manfiest files with special annotations in the
`metadata` section. Because they are template files, you can use all of
the normal template features, including reading `.Values` , `.Release` ,
and `.Template` .
For example, this template, stored in `templates/post-install-job.yaml` ,
declares a job to be run on `post-install` :
```yaml
apiVersion: batch/v1
kind: Job
metadata:
name: "{{.Release.Name}}"
labels:
heritage: {{.Release.Service | quote }}
release: {{.Release.Name | quote }}
chart: "{{.Chart.Name}}-{{.Chart.Version}}"
annotations:
# This is what defines this resource as a hook. Without this line, the
# job is considered part of the release.
"helm.sh/hook": post-install
spec:
template:
metadata:
name: "{{.Release.Name}}"
labels:
heritage: {{.Release.Service | quote }}
release: {{.Release.Name | quote }}
chart: "{{.Chart.Name}}-{{.Chart.Version}}"
spec:
restartPolicy: Never
containers:
- name: {{template "fullname" .}}-job
image: "alpine:3.3"
command: ["/bin/sleep","{{default "10" .Values.sleepyTime}}"]
```
What makes this template a hook is the annotation:
```
annotations:
"helm.sh/hook": post-install
```
One resource can implement multiple hooks:
```
annotations:
"helm.sh/hook": post-install,post-upgrade
```
Similarly, there is no limit to the number of different resources that
may implement a given hook. For example, one could declare both a secret
as a config map as a pre-install hook. It is important to keep in mind,
though, that there are no ordering guarantees about hooks.
When subcharts declare hooks, those are also evaluated. There is no way
for a top-level chart to disable the hooks declared by subcharts. And
again, there is no guaranteed ordering.
## Using Helm to Manage Charts
The `helm` tool has several commands for working with charts.
It can create a new chart for you:
```console
$ helm create mychart
Created mychart/
```
Once you have edited a chart, `helm` can package it into a chart archive
for you:
```console
$ helm package mychart
Archived mychart-0.1.-.tgz
```
You can also use `helm` to help you find issues with your chart's
formatting or information:
```console
$ helm lint mychart
No issues found
```
## Chart Repositories
A _chart repository_ is an HTTP server that houses one or more packaged
charts. While `helm` can be used to manage local chart directories, when
it comes to sharing charts, the preferred mechanism is a chart
repository.
Any HTTP server that can serve YAML files and tar files and can answer
GET requests can be used as a repository server.
Helm comes with built-in package server for developer testing (`helm
serve`). The Helm team has tested other servers, including Google Cloud
Storage with website mode enabled, and S3 with website mode enabled.
A repository is characterized primarily by the presence of a special
file called `index.yaml` that has a list of all of the packages supplied
by the repository, together with metadata that allows retrieving and
verifying those packages.
On the client side, repositories are managed with the `helm repo`
commands. However, Helm does not provide tools for uploading charts to
remote repository servers. This is because doing so would add
substantial requirements to an implementing server, and thus raise the
barrier for setting up a repository.
