Chapter 10. Templates

Labels are used to manage and organize generated objects, such as pods. The labels specified in the template are applied to every object that is generated from the template.

There is also the ability to add labels in the template from the command line.

$ oc process -f <filename> -l name=otherLabel

The list of parameters that you can override are listed in the parameters section of the template. You can list them with the CLI by using the following command and specifying the file to be used:

$ oc process --parameters -f <filename>

Alternatively, if the template is already uploaded:

$ oc process --parameters -n <project> <template_name>

For example, the following shows the output when listing the parameters for one of the Quickstart templates in the default openshift project:

$ oc process --parameters -n openshift rails-postgresql-example
NAME                         DESCRIPTION                                                                                              GENERATOR           VALUE
SOURCE_REPOSITORY_URL        The URL of the repository with your application source code                                                                  https://github.com/sclorg/rails-ex.git
SOURCE_REPOSITORY_REF        Set this to a branch name, tag or other ref of your repository if you are not using the default branch
CONTEXT_DIR                  Set this to the relative path to your project if it is not in the root of your repository
APPLICATION_DOMAIN           The exposed hostname that will route to the Rails service                                                                    rails-postgresql-example.openshiftapps.com
GITHUB_WEBHOOK_SECRET        A secret string used to configure the GitHub webhook                                                     expression          [a-zA-Z0-9]{40}
SECRET_KEY_BASE              Your secret key for verifying the integrity of signed cookies                                            expression          [a-z0-9]{127}
APPLICATION_USER             The application user that is used within the sample application to authorize access on pages                                 openshift
APPLICATION_PASSWORD         The application password that is used within the sample application to authorize access on pages                             secret
DATABASE_SERVICE_NAME        Database service name                                                                                                        postgresql
POSTGRESQL_USER              database username                                                                                        expression          user[A-Z0-9]{3}
POSTGRESQL_PASSWORD          database password                                                                                        expression          [a-zA-Z0-9]{8}
POSTGRESQL_DATABASE          database name                                                                                                                root
POSTGRESQL_MAX_CONNECTIONS   database max connections                                                                                                     10
POSTGRESQL_SHARED_BUFFERS    database shared buffers                                                                                                      12MB

The output identifies several parameters that are generated with a regular expression-like generator when the template is processed.

Using the CLI, you can process a file defining a template to return the list of objects to standard output:

$ oc process -f <filename>

Alternatively, if the template has already been uploaded to the current project:

$ oc process <template_name>

You can create objects from a template by processing the template and piping the output to oc create:

$ oc process -f <filename> | oc create -f -

Alternatively, if the template has already been uploaded to the current project:

$ oc process <template> | oc create -f -

You can override any parameter values defined in the file by adding the -p option for each <name>=<value> pair you want to override. A parameter reference may appear in any text field inside the template items.

For example, in the following the POSTGRESQL_USER and POSTGRESQL_DATABASE parameters of a template are overridden to output a configuration with customized environment variables:

$ oc process -f my-rails-postgresql \
    -p POSTGRESQL_USER=bob \
    -p POSTGRESQL_DATABASE=mydatabase

The JSON file can either be redirected to a file or applied directly without uploading the template by piping the processed output to the oc create command:

$ oc process -f my-rails-postgresql \
    -p POSTGRESQL_USER=bob \
    -p POSTGRESQL_DATABASE=mydatabase \
    | oc create -f -

If you have large number of parameters, you can store them in a file and then pass this file to oc process:

$ cat postgres.env
POSTGRESQL_USER=bob
POSTGRESQL_DATABASE=mydatabase
$ oc process -f my-rails-postgresql --param-file=postgres.env

You can also read the environment from standard input by using "-" as the argument to --param-file:

$ sed s/bob/alice/ postgres.env | oc process -f my-rails-postgresql --param-file=-

The template description covers information that informs users what your template does and helps them find it when searching in the web console. In addition to general descriptive information, it includes a set of tags. Useful tags include the name of the language your template is related to (e.g., java, php, ruby, etc.).

kind: "Template"
apiVersion: "v1"
metadata:
  name: "cakephp-mysql-example" 1
  annotations:
  openshift.io/display-name: "CakePHP MySQL Example" 2
  description: "An example CakePHP application with a MySQL database.\n\nFor more information see https://github.com/sclorg/cakephp-ex" 3
  tags: "instant-app,php,cakephp,mysql" 4
  iconClass: "icon-php" 5
message: "Your admin credentials are ${ADMIN_USERNAME}:${ADMIN_PASSWORD}" 6

Templates can include a set of labels. These labels will be added to each object created when the template is instantiated. Defining a label in this way makes it easy for users to find and manage all the objects created from a particular template.

kind: "Template"
apiVersion: "v1"
...
labels:
  template: "cakephp-mysql-example" 1

Parameters allow a value to be supplied by the user or generated when the template is instantiated. Then, that value is substituted wherever the parameter is referenced. References can be defined in any field in the objects list field. This is useful for generating random passwords or allowing the user to supply a host name or other user-specific value that is required to customize the template. Parameters can be referenced in two ways:

When using the ${PARAMETER_NAME} syntax, multiple parameter references can be combined in a single field and the reference can be embedded within fixed data, such as "http://${PARAMETER_1}${PARAMETER_2}". Both parameter values will be substituted and the resulting value will be a quoted string.

When using the ${{PARAMETER_NAME}} syntax only a single parameter reference is allowed and leading/trailing characters are not permitted. The resulting value will be unquoted unless, after substitution is performed, the result is not a valid json object. If the result is not a valid json value, the resulting value will be quoted and treated as a standard string.

A single parameter can be referenced multiple times within a template and it can be referenced using both substitution syntaxes within a single template.

A default value can be provided, which is used if the user does not supply a different value:

parameters:
  - name: USERNAME
    description: "The user name for Joe"
    value: joe

Parameter values can also be generated based on rules specified in the parameter definition:

parameters:
  - name: PASSWORD
    description: "The random user password"
    generate: expression
    from: "[a-zA-Z0-9]{12}"

In the example above, processing will generate a random password 12 characters long consisting of all upper and lowercase alphabet letters and numbers.

The syntax available is not a full regular expression syntax. However, you can use \w, \d, and \a modifiers:

Here is an example of a full template with parameter definitions and references:

kind: Template
apiVersion: v1
objects:
  - kind: BuildConfig
    apiVersion: v1
    metadata:
      name: cakephp-mysql-example
      annotations:
        description: Defines how to build the application
    spec:
      source:
        type: Git
        git:
          uri: "${SOURCE_REPOSITORY_URL}" 1
          ref: "${SOURCE_REPOSITORY_REF}"
        contextDir: "${CONTEXT_DIR}"
  - kind: DeploymentConfig
    apiVersion: v1
    metadata:
      name: frontend
    spec:
      replicas: "${{REPLICA_COUNT}}" 2
parameters:
  - name: SOURCE_REPOSITORY_URL 3
    displayName: Source Repository URL 4
    description: The URL of the repository with your application source code 5
    value: https://github.com/sclorg/cakephp-ex.git 6
    required: true 7
  - name: GITHUB_WEBHOOK_SECRET
    description: A secret string used to configure the GitHub webhook
    generate: expression 8
    from: "[a-zA-Z0-9]{40}" 9
  - name: REPLICA_COUNT
    description: Number of replicas to run
    value: "2"
    required: true
message: "... The GitHub webhook secret is ${GITHUB_WEBHOOK_SECRET} ..." 10

The main portion of the template is the list of objects which will be created when the template is instantiated. This can be any valid API object, such as a BuildConfig, DeploymentConfig, Service, etc. The object will be created exactly as defined here, with any parameter values substituted in prior to creation. The definition of these objects can reference parameters defined earlier.

kind: "Template"
apiVersion: "v1"
objects:
  - kind: "Service" 1
    apiVersion: "v1"
    metadata:
      name: "cakephp-mysql-example"
      annotations:
        description: "Exposes and load balances the application pods"
    spec:
      ports:
        - name: "web"
          port: 8080
          targetPort: 8080
      selector:
        name: "cakephp-mysql-example"

Rather than writing an entire template from scratch, you can export existing objects from your project in template form, and then modify the template from there by adding parameters and other customizations. To export objects in a project in template form, run:

$ oc export all --as-template=<template_name> > <template_filename>

You can also substitute a particular resource type or multiple resources instead of all. Run oc export -h for more examples.

The object types included in oc export all are: