Ephemeral Heat


Ephemeral Heat is a means to install the overcloud by using an ephemeral Heat process instead of a system installed Heat process. This change is possible beginning in the Wallaby release.

In a typical undercloud, Heat is installed on the undercloud and processes are run in podman containers for heat-api and heat-engine. When using ephemeral Heat, there is no longer a requirement that Heat is installed on the undercloud, instead these processes are started on demand by the deployment, update, and upgrade commands.

This model has been in use within TripleO already for both the undercloud and standalone installation methods, which start an on demand all in one heat-all process in order to perform only the installation. Using ephemeral Heat in this way allows for re-use of the Heat templates from tripleo-heat-templates without having to require an already fully installed undercloud.


Ephemeral Heat is enabled by passing the --heat-type argument to openstack overcloud deploy. The ephemeral process can also be launched outside of a deployment with the openstack tripleo launch heat command. The latter command also takes a --heat-type argument to enable selecting the type of Heat process to use.

Heat types

The --heat-type argument allows for the following options described below.


Use the system Heat installation. This is the historical TripleO usage of Heat with Heat fully installed on the undercloud. This is the default value, and requires a fully installed undercloud.


Use an ephemeral heat-all process. The process will be started natively on the system executing tripleoclient commands by way of an OS (operating system) fork.


A podman container will be started on the executing system that runs a single heat-all process.


A podman pod will be started on the executing system that runs containers for heat-api and heat-engine.

In all cases, the process(es) are terminated at the end of the deployment.


The native and container methods are limited in scale due to being a single Heat process. Deploying more than 3 nodes or 2 roles will significantly impact the deployment time with these methods as Heat has only a single worker thread.

Using the installed or pod methods enable scaling node and role counts as is typically required.


The following example shows using --heat-type to enable ephemeral Heat:

openstack overcloud deploy \
  --stack overcloud \
  --work-dir ~/overcloud-deploy/overcloud \
  --heat-type <pod|container|native> \
  <other cli arguments>

With ephemeral Heat enabled, several additional deployment artifacts are generated related to the management of the Heat process(es). These artifacts are generated under the working directory of the deployment in a heat-launcher subdirectory. The working directory can be overridden with the --work-dir argument.

Using the above example, the Heat artifact directory would be located at ~/overcloud-deploy/overcloud/heat-launcher. An example of the directory contents is shown below:

[centos@ephemeral-heat ~]$ ls -l ~/overcloud-deploy/overcloud/heat-launcher/
total 41864
-rw-rw-r--. 1 centos centos      650 Mar 24 18:39 api-paste.ini
-rw-rw-r--. 1 centos centos     1054 Mar 24 18:39 heat.conf
-rw-rw-r--. 1 centos centos 42852118 Mar 24 18:31 heat-db-dump.sql
-rw-rw-r--. 1 centos centos     2704 Mar 24 18:39 heat-pod.yaml
drwxrwxr-x. 2 centos centos       49 Mar 24 16:02 log
-rw-rw-r--. 1 centos centos     1589 Mar 24 18:39 token_file.json

The directory contains the necessary files to inspect and debug the Heat process(es), and if necessary reproduce the deployment.


The consolidated log file for the Heat process is the log file in the heat-launcher directory.

Launching Ephemeral Heat

Outside of a deployment, the ephemeral Heat process can also be started with the openstack tripleo launch heat command. This can be used to interactively use the ephemeral Heat process or to debug a previous deployment.

When combined with --heat-dir and --restore-db, the command can be used to restore the Heat process and database from a previous deployment:

openstack tripleo launch heat \
  --heat-type pod \
  --heat-dir ~/overcloud-deploy/overcloud/heat-launcher \

The command will exit after launching the Heat process, and the Heat process will continue to run in the background.

Interacting with ephemeral Heat

With the ephemeral Heat process launched and running, openstackclient can be used to interact with the Heat API. The following shell environment configuration must be set in order to disable authentication:

IP=`cat /etc/hosts | grep "undercloud.localdomain undercloud$" | awk '{print $1}'`

Once the environment is configured, openstackclient work as expected against the Heat API:

[centos@ephemeral-heat ~]$ openstack stack list
| ID                                   | Stack Name | Project | Stack Status    | Creation Time        | Updated Time |
| 761e2a54-c6f9-4e0f-abe6-c8e0ad51a76c | overcloud  | admin   | CREATE_COMPLETE | 2021-03-22T20:48:37Z | None         |

Killing ephemeral Heat

To stop the ephemeral Heat process previously started with openstack tripleo launch heat, use the --kill argument:

openstack tripleo launch heat \
  --heat-type pod \
  --heat-dir ~/overcloud-deploy/overcloud/heat-launcher \


Ephemeral Heat currently only supports new deployments. Update and Upgrade support for deployments that previously used the system installed Heat will be coming.