TripleO Containers Architecture

TripleO Containers Architecture


The TripleO containers support is still under heavy development. Things documented here may change during the Pike cycle.

This document explains the details around TripleO’s containers architecture. The document goes into the details of how the containers are created from TripleO, how the configuration files are generated and how the containers are eventually run.

Like other areas of TripleO, the containers based deployment requires a couple of different projects to play together. The next section will cover each of the parts that allow for deploying OpenStack on containers using TripleO.

Kolla Build

Kolla is an OpenStack team that aims to create tools to allow for deploying OpenStack on container technologies. Kolla (or Kolla Build) is one of the tools produced by this team and it allows for building and customizing container images for OpenStack services and their dependencies.

TripleO consumes these images and takes advantage of the customization capabilities provided by the Kolla build tool to install some packages that are required by other parts of TripleO.

The following template is an example of the template used for building the base images that are consumed by TripleO. Further customization is required for some of the services, like mariadb:

{% extends parent_template %}
{% set base_centos_binary_packages_append = ['puppet'] %}
{% set nova_scheduler_packages_append = ['openstack-tripleo-common'] %}


parent_template is the literal string to include. No need to replace it.

Use the following command to build an image using kolla-build and the template above (template-overrides.j2):

$ kolla-build --base centos \
  --template-override /usr/share/tripleo-common/container-images/tripleo_kolla_template_overrides.j2 \
  --template-override template-overrides.j2

TripleO maintains its complete list of kolla customization in the tripleo-common project.


The heat-config-docker-cmd hook is used to manage containers. This hook takes json as input and uses it to create and run containers on demand. The docker-cmd accepts different keys that allow for configuring the container process. Some of the keys are:

  • net: To specify what network to use. This is commonly set to host.
  • privileged: Whether to give full access to the host’s devices to the container, similar to what happens when the service runs directly on the host.
  • volumes: List of host path volumes, named volumes, or dynamic volumes to bind on the container.
  • environment: List of environment variables to set on the container.


The list above is not exhaustive and you should refer to the heat-config-docker-cmd docs for the complete list.

The json file passed to this hook is built out of the docker_config attribute defined in the service’s yaml file. Refer to the Docker specific settings section for more info on this.


The heat-json-config-file takes a json config as input and dumps it onto disk in the specified directory. This is used to write on disk the json required to run the kolla images and the docker-puppet-tasks.

TripleO Heat Templates

The TripleO Heat Templates repo is where most of the logic resides in the form of heat templates. These templates define each service, the containers’ configuration and the initialization or post-execution operations.

Anatomy of a containerized service template

Containerized services templates inherit almost everything from the puppet based templates, with some exceptions for some services. New properties have been added to define container specific configurations, which will be covered in this section.

Docker specific settings

Each service may define an output variable which returns a puppet manifest snippet that will run at each of the following steps. Earlier manifests are re-asserted when applying latter ones.

  • service_name: This setting is inherited from the puppet service templates and it shouldn’t be modified as some of the hiera tasks depend on this.

  • config_settings: This setting is generally inherited from the puppet/services templates and only need to be appended to on occasion if docker specific config settings are required. For example:

        - get_attr: [MongodbPuppetBase, role_data, config_settings]
        - mongodb::server::fork: false
  • step_config: This setting controls the manifest that is used to create docker config files via puppet. The puppet tags below are used along with this manifest to generate a config directory for this container.

  • kolla_config: Contains YAML that represents how to map config files into the kolla container. This config file is typically mapped into the container itself at the /var/lib/kolla/config_files/config.json location and drives how kolla’s external config mechanisms work. Example:

        command: /usr/bin/mongod --unixSocketPrefix=/var/run/mongodb --config /etc/mongod.conf run
        - dest: /etc/mongod.conf
          source: /var/lib/kolla/config_files/src/etc/mongod.conf
          owner: mongodb
          perm: '0600'
        - dest: /etc/mongos.conf
          source: /var/lib/kolla/config_files/src/etc/mongos.conf
          owner: mongodb
          perm: '0600'
  • docker_config: Data that is passed to the heat-config-docker-cmd hook to configure a container, or step of containers at each step. See the available steps below and the related docker-cmd hook documentation in the heat-agents project.

  • puppet_config:

    • step_config: Usually a reference to the one defined outside this section.

    • puppet_tags: Puppet resource tag names that are used to generate config files with puppet. Only the named config resources are used to generate a config file. Any service that specifies tags will have the default tags of ‘file,concat,file_line’ appended to the setting. For example:

      puppet_tags: keystone_config

      Some puppet modules do a bit more than just generating config files. Some have custom resources with providers that execute commands. It’s possible to overwrite these providers by changing the step_config property. For example:

      puppet_tags: keystone_config
          - "\n"
          - - "['Keystone_user', 'Keystone_endpoint', 'Keystone_domain', 'Keystone_tenant', 'Keystone_user_role', 'Keystone_role', 'Keystone_service'].each |String $val| { noop_resource($val) }"
            - {get_attr: [KeystoneBase, role_data, step_config]}

      The example above will overwrite the provider for all the Keystone_* puppet tags (except keystone_config) using the noop_resource function that comes with puppet-tripleo. This function dynamically configures the default provider for each of the puppet_tags in the array.

    • config_volume: The name of the docker volume where config files will be generated for this service. Use this as the location to bind mount into the running Kolla container for configuration.

    • config_image: The name of the docker image that will be used for generating configuration files. This is often the same container that the runtime service uses. Some services share a common set of config files which are generated in a common base container.

  • docker_puppet_tasks: This section provides data to drive the tool directly. The task is executed only once within the cluster (not on each node) and is useful for several puppet snippets we require for initialization of things like keystone endpoints, database users, etc. See for formatting. Here’s an example of Keystone’s docker_puppet_tasks:

      # Keystone endpoint creation occurs only on single node
        - 'keystone_init_tasks'
        - 'keystone_config,keystone_domain_config,keystone_endpoint,keystone_identity_provider,keystone_paste_ini,keystone_role,keystone_service,keystone_tenant,keystone_user,keystone_user_role,keystone_domain'
        - 'include ::tripleo::profile::base::keystone'
        - list_join:
          - '/'
          - [ {get_param: DockerNamespace}, {get_param: DockerKeystoneImage} ]
  • host_prep_tasks: Ansible tasks to execute on the host before any containers are started. Useful e.g. for ensuring existence of directories that we want bind mounted into the containers.

  • upgrade_tasks: Ansible tasks to execute during upgrade. First these tasks are run on all nodes, and then the normal puppet/docker operations happen the same way as during a fresh deployment.

Docker steps

Similar to baremetal, docker containers are brought up in a stepwise manner. The current architecture supports bringing up baremetal services alongside of containers. Therefore, baremetal steps may be required depending on the service and they are always executed before the corresponding container step.

The list below represents the correlation between the baremetal and the containers steps. These steps are executed sequentially:

  1. Containers config files generated per hiera settings.
  2. Load Balancer configuration baremetal
    1. Step 1 baremetal
    2. Step 1 containers
  3. Core Services (Database/Rabbit/NTP/etc.)
    1. Step 2 baremetal
    2. Step 2 containers
  4. Early Openstack Service setup (Ringbuilder, etc.)
    1. Step 3 baremetal
    2. Step 3 containers
  5. General OpenStack Services
    1. Step 4 baremetal
    2. Step 4 containers
    3. Keystone containers post initialization (tenant, service, endpoint creation)
  6. Service activation (Pacemaker)
    1. Step 5 baremetal
    2. Step 5 containers

Service Bootstrap

Bootstrapping services is a one-shot operation for most services and it’s done by defining a separate container that shares the same structure as the main service container commonly defined under the docker_step number 3 (see Docker steps section above).

Unlike normal service containers, the bootstrap container should be run in the foreground - detach: false - so there can be more control on when the execution is done and whether it succeeded or not.

Example taken from Glance’s service file:

      image: *glance_image
      net: host
      privileged: false
      detach: false
      volumes: &glance_volumes
        - /var/lib/kolla/config_files/glance-api.json:/var/lib/kolla/config_files/config.json
        - /etc/localtime:/etc/localtime:ro
        - /lib/modules:/lib/modules:ro
        - /var/lib/config-data/glance_api/:/var/lib/kolla/config_files/src:ro
        - /run:/run
        - /dev:/dev
        - /etc/hosts:/etc/hosts:ro
        - KOLLA_BOOTSTRAP=True
      image: *glance_image
      net: host
      privileged: false
      restart: always
      volumes: *glance_volumes
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