# Cluster Templating

The command `kops replace` can replace a cluster desired configuration from the config in a yaml file (see [cli/kops_replace.md](../cli/kops_replace.md)).

It is possible to generate that yaml file from a template, using the command `kops toolbox template` (see [cli/kops_toolbox_template.md](../cli/kops_toolbox_template.md)).

This document details the template language used.

The file passed as `--template` must be a [go template](https://golang.org/pkg/text/template/). Example:
```yaml
# File cluster.tmpl.yaml
apiVersion: kops.k8s.io/v1alpha2
kind: InstanceGroup
metadata:
  labels:
  kops.k8s.io/cluster: {{ '{{.clusterName}}.{{.dnsZone}}' }}
  name: nodes
spec:
  image: coreos.com/CoreOS-stable-1409.6.0-hvm
  kubernetesVersion: {{ '{{.kubernetesVersion}}' }}
  machineType: m4.large
  maxPrice: "0.5"
  maxSize: 20
  minSize: 15
  role: Node
  rootVolumeSize: 100
  subnets:
  - {{ '{{.awsRegion}}' }}a
  - {{ '{{.awsRegion}}' }}b
  - {{ '{{.awsRegion}}' }}c
```

You can pass configuration such as an environment file by using the `--values PATH` command line option. Note `--values` is a slice so can be defined multiple times; the configuration is overridden by each configuration file *(so order is important assuming duplicating values)*; a use-case for this would be a default configuration which upstream clusters can override.

The file passed as `--values` must contain the variables referenced in the template. Example:
```yaml
# File values.yaml
clusterName: eu1
kubernetesVersion: 1.7.1
dnsZone: k8s.example.com
awsRegion: eu-west-1
```

When multiple environment files are passed using `--values` kOps performs a deep merge, for example given the following two files:
```yaml
# File values-a.yaml
instanceGroups:
  foo:
    ami: ami-1234567
    type: m4.large

# File values-b.yaml
instanceGroups:
  foo:
    type: t2.large
```

Would result in the `instanceGroups.foo` object having two properties: `{"ami": "ami-1234567", "type": "t2.large"}`.

Besides specifying values through an environment file it is also possible to pass variables directly on the command line using the `--set` and `--set-string` command line options. The difference between the two options is that `--set-string` will always yield a string value while `--set` will cause the value to be parsed as a YAML value, for example the value `true` would turn into a boolean with `--set` while with `--set-string` it will be the literal string `"true"`. The format for specifying a variable is as follows:

```shell
kops toolbox template --template mytemplate.tpl --set 'version=1.0,foo.bar=baz' --set-string 'foo.myArray={1,2,3}' --set 'foo.myArray[1]=false,foo.myArray[3]=4'
```

which would yield the same values as using the `--values` option with the following file

```yaml
version: 1.0
foo:
  bar: baz
  myArray:
  - "1"
  - false
  - "3"
  - 4
```


Running `kops toolbox template` replaces the placeholders in the template by values and generates the file output.yaml, which can then be used to replace the desired cluster configuration with `kops replace -f cluster.yaml`.

Note: when creating a cluster desired configuration template, you can

- use `kops get k8s-cluster.example.com -o yaml > cluster-desired-config.yaml` to create the cluster desired configuration file (see [cli/kops_get.md](../cli/kops_get.md)). The values in this file are defined in [cluster_spec.md](../cluster_spec.md).
- replace values by placeholders in that file to create the template.

### Templates

The `--template` command line option can point to either a specific file or a directory with a collection of templates. An example usage would be;

```shell
$ kops toolbox template --values dev.yaml --template cluster.yaml --template instance_group_directory
```

The cluster.yaml *(your main cluster spec for example)* would be written first followed by any templates found in the instance_group_directory directory. Note the toolbox will automatically add YAML separators between the documents for you.

### Snippets

The toolbox template also supports the reuse or break up of code blocks into snippets directories. By passing a `--snippets PATH` to a directory holding templates;

```shell
$ kops toolbox template --values dev.yaml --template cluster.yaml --template instancegroups --snippets snippets
```

The example below assumes you have placed the appropriate files i.e. *(nodes.json, master.json etc)* in to the snippets directory. Note, the namespace of the snippets are flat and always the basename() of the file path; so `snippets/components/docker.options` is still referred to as 'docker.options'.

```YAML
apiVersion: kops.k8s.io/v1alpha2
kind: Cluster
metadata:
  name: {{ '{{ .environment }}.{{ .dns_zone }}' }}
spec:
  docker:
    {{ '{{ include "docker" . | indent 4 }}' }}
  additionalPolicies:
    master: |
      {{ '{{ include "masters.json" . | indent 6 }}' }}
    node: |
      {{ '{{ include "nodes.json" . | indent 6 }}' }}
```

### Template Functions

The entire set of https://github.com/Masterminds/sprig functions are available within the templates for you. Note if you want to use the 'defaults' functions switch off the verification check on the command line by `--fail-on-missing=false`;

```YAML
image: {{ '{{ default $image $node.image }}' }}
machineType: {{ '{{ default $instance $node.machine_type }}' }}
maxSize: {{ '{{ default "10" $node.max_size }}' }}
minSize: {{ '{{ default "1" $node.min_size }}' }}
```

### Formatting

Formatting in golang templates is a pain! At the start or at the end of a statement can be infuriating to get right, so a `--format-yaml=true` *(defaults to false)* command line option has been added. This will first unmarshal the generated content *(performing a syntax verification)* and then marshal back the content removing all those nasty formatting issues, newlines etc.
