Chapter 6. Composable services

The core heat template collection contains two sets of composable service templates:

Each template contains a description that identifies its purpose. For example, the ntp.yaml service template contains the following description:

description: >
  NTP service deployment using puppet, this YAML file
  creates the interface between the HOT template
  and the puppet manifest that actually installs
  and configure NTP.

These service templates are registered as resources specific to a RHOSP deployment. This means you can call each resource using a unique heat resource namespace defined in the overcloud-resource-registry-puppet.j2.yaml file. All services use the OS::TripleO::Services namespace for their resource type.

Some resources use the base composable service templates directly:

resource_registry:
  ...
  OS::TripleO::Services::Ntp: puppet/services/time/ntp.yaml
  ...

However, core services require containers and as such use the containerized service templates. For example, the keystone containerized service uses the following:

resource_registry:
  ...
  OS::TripleO::Services::Keystone: docker/services/keystone.yaml
  ...

These containerized templates usually reference back to the base templates in order to include Puppet configuration. For example, the docker/services/keystone.yaml template stores the output of the base template in the KeystoneBase parameter:

KeystoneBase:
  type: ../../puppet/services/keystone.yaml

The containerized template can then incorporate functions and data from the base template.

The overcloud.j2.yaml heat template includes a section of Jinja2-based code to define a service list for each custom role in the roles_data.yaml file:

{{role.name}}Services:
  description: A list of service resources (configured in the Heat
               resource_registry) which represent nested stacks
               for each service that should get installed on the {{role.name}} role.
  type: comma_delimited_list
  default: {{role.ServicesDefault|default([])}}

For the default roles, this creates the following service list parameters: ControllerServices, ComputeServices, BlockStorageServices, ObjectStorageServices, and CephStorageServices.

You define the default services for each custom role in the roles_data.yaml file. For example, the default Controller role contains the following content:

- name: Controller
  CountDefault: 1
  ServicesDefault:
    - OS::TripleO::Services::CACerts
    - OS::TripleO::Services::CephMon
    - OS::TripleO::Services::CephExternal
    - OS::TripleO::Services::CephRgw
    - OS::TripleO::Services::CinderApi
    - OS::TripleO::Services::CinderBackup
    - OS::TripleO::Services::CinderScheduler
    - OS::TripleO::Services::CinderVolume
    - OS::TripleO::Services::Core
    - OS::TripleO::Services::Kernel
    - OS::TripleO::Services::Keystone
    - OS::TripleO::Services::GlanceApi
    - OS::TripleO::Services::GlanceRegistry
...

These services are then defined as the default list for the ControllerServices parameter.

You can also use an environment file to override the default list for the service parameters. For example, you can define ControllerServices as a parameter_default in an environment file to override the services list from the roles_data.yaml file.

This example examines how to create a user-defined composable service and focuses on implementing a message of the day (motd) service. In this example, the overcloud image contains a custom motd Puppet module loaded either through a configuration hook or through modifying the overcloud images. For more information, see Chapter 3, Working with overcloud images.

When you create your own service, you must define the following items in the heat template of your service:

The following is an example heat template (service.yaml) for the motd service:

heat_template_version: 2016-04-08

description: >
  Message of the day service configured with Puppet

parameters:
  ServiceNetMap:
    default: {}
    type: json
  DefaultPasswords:
    default: {}
    type: json
  EndpointMap:
    default: {}
    type: json
  MotdMessage: 1
    default: |
      Welcome to my Red Hat OpenStack Platform environment!

    type: string
    description: The message to include in the motd

outputs:
  role_data:
    description: Motd role using composable services.
    value:
      service_name: motd
      config_settings: 2
        motd::content: {get_param: MotdMessage}
      step_config: | 3
        if hiera('step') >= 2 {
          include ::motd
        }

You can configure the custom motd service only on the overcloud Controller nodes. This requires a custom environment file and custom roles data file included with your deployment. Replace example input in this procedure according to your requirements.

This includes our custom motd service in our deployment and configures the service on Controller nodes only.