A Red Hat OpenShift template is a custom resource, usually defined as a YAML or a JSON file, that contains a set of OpenShift and Kubernetes resources. Templates accept parameters, so you can declare configurable values when you define these resources. OpenShift processes templates by replacing parameter references with actual values and creating a customized set of resources.

A template is useful when you want to deploy a set of resources as a single bundle, rather than deploying them one by one. The following examples are common use cases of templates:

apiVersion: template.openshift.io/v1
kind: Template 
metadata:
  name: mytemplate
  annotations:
    description: "Description" 
objects: 
- apiVersion: v1
  kind: Pod
  metadata:
    name: myapp
  spec:
    containers:
    - env:
      - name: MYAPP_CONFIGURATION
        value: ${MYPARAMETER} 
      image: myorganization/myapplication
      name: myapp
      ports:
      - containerPort: 80
        protocol: TCP
parameters: 
- description: Myapp configuration data
  name: MYPARAMETER
  required: true
labels: 
  mylabel: myapp

The preceding example template demonstrates the definition of a required parameter, by setting the required property to true. You can define optional parameters by omitting this property, or by setting its value to false.

Both optional and required parameters can provide default values by using the value property. For example:

parameters:
- description: Myapp configuration data
  name: MYPARAMETER
  value: /etc/myapp/config.ini

Additionally, OpenShift can generate random default values for parameters if you specify a pattern with the from property, as the following example shows:

parameters:
- description: ACME cloud provider API key
  name: APIKEY
  generate: expression
  from:"[a-zA-Z0-9]{12}"

This feature is useful for secrets and passwords. The syntax of the from property is a subset of the Perl regular expression syntax. See the references at the end of this section for the full template parameter expression syntax.

To add a template to OpenShift, use either the oc create CLI command or the Import YAML page of the web console. Adding a template creates a new Template resource in the cluster as specified in the template definition file, the same way you would for any other kind of resource.

If the template is meant to be used by a single deployment, then you can keep the template definition in a file. If the template is meant to be reused by multiple developers, then you might want to create the Template resource in a shared project. Creating projects that act as template collections is a common practice. These projects usually include other potentially shared resources, such as image streams.

By default, OpenShift provides several templates in the openshift project. Among these templates, you can find quick start templates, which are useful to bootstrap new applications in different languages. All cluster users have read access to the openshift project, but only cluster administrators may create or delete templates in this project.

You can deploy an application directly from a template definition file. The oc new-app and oc process commands can use a template as an input and process this file to create resources.

The oc new-app command can process a template file to create resources in OpenShift, as follows:

[user@host ~]$ oc new-app --file mytemplate.yaml -p PARAM1=value1 \
-p PARAM2=value2

You can also use a template stored in the cluster. This example processes a template called mysql-persistent:

[user@host ~]$ oc new-app --template mysql-persistent \
-p MYSQL_USER=student -p MYSQL_PASSWORD=mypass

The oc process command processes a template and produces a resource list. You can save the resource list to a local file as follows:

[user@host ~]$ oc process -f mytemplate.yaml -p PARAM1=value1 \
-p PARAM2=value2 myresourcelist.json

The oc process command does not create the resources in OpenShift. If you want to create the resources in the cluster, then you can pass the file that contains the resource list to the oc create command, as follows:

[user@host ~]$ oc create -f myresourcelist.json

You can combine the previous two commands to process the template and create the resources in the cluster.

[user@host ~]$ oc process -f mytemplate.yaml -p PARAM1=value1 \
-p PARAM2=value2 | oc create -f -

You can print the available parameters in a template, by using the --parameters option:

[user@host ~]$ oc process -f mytemplate.yaml --parameters

For both the oc new-app and oc process commands, define parameters with the --param (-p) option. Use as many -p options as needed to declare all your parameters.

Both commands also accept the -l option, which adds a label to all resources created from the template.

By default, you cannot create applications from custom templates via the web console. Cluster administrators can install additional templates into the Developer Catalog. You can use the templates in the Developer Catalog to create new applications.