Splitting the Overcloud stack into multiple independent Heat stacks

split-stack is a feature in TripleO that splits the overcloud stack into multiple independent stacks in Heat.

The overcloud stack is split into an overcloud-baremetal and overcloud-services stack. This allows for independent and isolated management of the baremetal and services part of the Overcloud deployment. It is a more modular design than deploying a single overcloud stack in that it allows either the baremetal or services stack to be replaced by tooling that is external to TripleO if desired.

The overcloud-services stack makes extensive use of the deployed-server feature, documented at Using Already Deployed Servers in order to orchestrate the deployment and configuration of the services separate from the baremetal deployment.

split-stack allows for mixing baremetal systems deployed by TripleO and those deployed by external tooling when creating the services stack. Since the baremetal resources are completely abstracted behind the deployed-server interface when deploying the services stack, it does not matter whether the servers were actually created with TripleO or not.

split-stack Requirements

A default split-stack deployment (detailed in the later steps) can be deployed without any special requirements.

More advanced deployments where baremetal servers provisioned by TripleO will be mixed with those not provisioned by TripleO will want to pay attention to the requirements around using already deployed servers from Using Already Deployed Servers. The requirements for using deployed servers will apply when not using servers provisioned by TripleO.

Default split-stack deployment

split-stack will be deployed by running 2 separate openstack overcloud deploy commands to deploy the separate stacks.

If applicable, prepare the custom roles files and any custom environments initially. The custom roles file and an environment setting the role counts should be passed to both deployment commands so that enough baremetal nodes are deployed per what the overcloud-services stack expects.

Baremetal Deployment Command

Run the deployment command to deploy the overcloud-baremetal stack. An additional environment file, overcloud-baremetal.yaml, is passed to the deployment to enable deploying just the baremetal stack.

Enough baremetal nodes should be deployed to match how many nodes per role will be needed when the services stack is deployed later. Be sure that the environment file being used to set the role counts is passed to the baremetal deployment command:

openstack overcloud deploy \
  <other cli arguments> \
  --stack overcloud-baremetal \
  -r roles-data.yaml \
  -e /usr/share/openstack-tripleo-heat-templates/environments/overcloud-baremetal.yaml \
  -e /usr/share/openstack-tripleo-heat-templates/environments/split-stack-consistent-hostname-format.yaml

The --stack argument sets the name of the Heat stack to overcloud-baremetal. This will also be the name of the Swift container that stores the stack’s plan (templates) and of the Mistral environment.

The roles-data.yaml roles file illustrates passing a custom roles file to the deployment command. It is not necessary to use custom roles when using split stack, however if custom roles are used, the same roles file should be used for both stacks.

The overcloud-baremetal.yaml environment will set the parameters for the deployment such that no services will be deployed.

The split-stack-consistent-hostname-format.yaml environment will set the respective <role-name>HostnameFormat parameters for each role defined in the role files used. The server hostnames for the 2 stacks must be the same, otherwise the servers will not be able to pull their deployment metadata from Heat.


Do not pass any network isolation templates or NIC config templates to the overcloud-baremetal stack deployment command. These will only be passed to the overcloud-services stack deployment command.

An output on the overcloud-baremetal stack produces the contents of an environment file that needs to be passed to the overcloud-services command. Use the following command to save the output value:

openstack stack output show overcloud-baremetal DeployedServerEnvironment -f value -c output_value > deployed-server-environment-output.json

Services Deployment Command

The services stack, overcloud-services will now be deployed with a separate deployment command:

openstack overcloud deploy \
  <other cli arguments> \
  --stack overcloud-services \
  --disable-validations \
  -r roles-data.yaml \
  -e /usr/share/openstack-tripleo-heat-templates/environments/deployed-server-environment.yaml \
  -e /usr/share/openstack-tripleo-heat-templates/environments/deployed-server-deployed-neutron-ports.yaml \
  -e /usr/share/openstack-tripleo-heat-templates/environments/deployed-server-bootstrap-environment-centos.yaml \
  -e /usr/share/openstack-tripleo-heat-templates/environments/overcloud-services.yaml \
  -e /usr/share/openstack-tripleo-heat-templates/environments/split-stack-consistent-hostname-format.yaml
  -e deployed-server-environment-output.json

The overcloud-services stack makes use of the “deployed-server” feature. The additional environments needed are shown in the above command. See Using Already Deployed Servers for more information on how to fully configure the feature.

The roles file, roles-data.yaml is again passed to the services stack as the same roles file should be used for both stacks.

Also, instead of passing the overcloud-baremetal.yaml environment, overcloud-services.yaml is now passed.

The split-stack-consistent-hostname-format.yaml environment is again passed, so that the hostnames used for the server resources created by Heat are the same as were created in the previous baremetal stack.

Pass the deployed-server-environment-output.json environment file that was generated from the value of the DeployedServerEnvironment output of the overcloud-baremetal stack.

During this deployment, any network isolation environments and/or NIC config templates should be passed for the desired network configuration.

The stack should complete and the generated overcloudrc can be used to interact with the Overcloud.