Heat is an OpenStack component responsible for Orchestration. Its purpose is to deliver automation engine and optimize processes.
Heat uses templates in most-popular for devops culture syntax → yaml. Those templates describe the whole infrastructure that you are able to deploy. The deployed environment is being called stack. It can consists of many different resources.

You can leverage Heat templates using python-heatclient in CLI or using Horizon dashboard in the web browser.

CLI

For installing python-heatclient run pip command (strongly recommended virtual environment activated):

pip install python-heatclient

To run a prepared template in order to deploy a stack, you have to use this command:

openstack stack create -t template.yml <stackname>
 
-t : assign template for deployment
 
<stackname>: defines a name for the stack

GUI

Log in to the Horizon dashboard and choose Orchestration tab.

Click on the Stacks.

Navigate to the right part of the screen and choose “Launch Stack”.

Enroll Template Source selector and choose a particular file, Direct Input or URL to your template.

Basic template

Using this snippet, you can create one virtual machine, booted from ephemeral disk.

heat_template_version: 2015-04-30       
                                          
resources:                              
  instance:                             
    type: OS::Nova::Server              
    properties:                         
      flavor: eo1.xsmall                
      image: Ubuntu 18.04 LTS (19.18)
      networks:                         
        - network: private_network_0XXXX
        - network: eodata               
      key_name: your_key
      security_groups:
        - allow_ping_ssh_rdp
        - default

Explanation

heat_template_version → strictly a version of heat template. Each of them varies in many ways (including different modules support, more parameters customization etc.)

resources → entry to commence providing particular components for deployment

instance → name of resource (you can type in anything on your own)

type → definition of an OpenStack component (a comprehensive list: https://docs.openstack.org/heat/latest/template_guide/openstack.html)

properties → under this tab provide required parameters for deploying an component (in the case of virtual machine such as flavor, image networks etc.)

More advanced template

In this example, we will attach parameters, ResourceGroup with counter, VM booted from Cinder Volume and predefined outputs.

heat_template_version: 2015-04-30

parameters:
    key_name:
        type: string
        label: Key Name
        description: SSH key to be used for all instances
        default: your_key
    image_id:
        type: string
        label: Image ID
        description: Image to be used. Check all available options in Horizon dashboard or by using openstack image list command.
        default: Ubuntu 18.04 LTS (19.18)
    private_net_id:
        type: string
        description: ID/Name of private network
        default: private_network_0XXXX
    eodata_net_id:
        type: string
        description: ID/Name of eodata network
        default: eodata



resources:
        Group_of_VMs:
                type: OS::Heat::ResourceGroup
                properties:
                        count: 2
                        resource_def:
                                type: OS::Nova::Server
                                properties:
                                        name: my_vm%index%
                                        flavor: eo1.xsmall
                                        image: { get_param: image_id }
                                        networks:
                                                - network: { get_param: private_net_id }
                                                - network: { get_param: eodata_net_id }
                                        key_name: { get_param: key_name }
                                        security_groups:
                                                - allow_ping_ssh_rdp
                                                - default

        VOL_FAQ:
                type: OS::Cinder::Volume
                properties:
                        name: vol
                        size: 20
                        image : { get_param: image_id }


        With_volume:
                type: OS::Nova::Server
                properties:
                        flavor: eo1.xsmall
                        block_device_mapping: [{"volume_size": 20, "volume_id": { get_resource: VOL_FAQ }, "delete_on_termination": False, "device_name": "/dev/vda" }]
                        networks:
                                 - network: { get_param: private_net_id }
                                 - network: { get_param: eodata_net_id }
                        key_name: { get_param: key_name }
                        security_groups:
                                 - allow_ping_ssh_rdp
                                 - default
                        image : { get_param: image_id }

outputs:
        SERVER_DETAILS:
                description: Shows details of all virtual servers.
                value: { get_attr: [ Group_of_VMs, show ] }     

Explanation

Parameters →  a solution to provide default values, which you can later inject into resource defintions using {get param: param_name }

ResourceGroup → component being used for repeating deployment, e.g two identical VM’s

Count → defines a variable for iterative operations

resource_def → starting statement for defining group resources

%index% → in this way we can easily add iterative number to the VM name, increasing values starting from 0.

block_device_mapping → property to define a bootable Cinder volume for instance

outputs → Additional information concerning deployed elements of the stack. In this case it returns a “show” attribute output. You can examine this kind of information by using openstack stack output list. Available attributes for every component can be found here: https://docs.openstack.org/heat/latest/template_guide/openstack.html

Documentation

https://docs.openstack.org/heat/latest/getting_started/create_a_stack.html

https://docs.openstack.org/heat/rocky/template_guide/hot_guide.html

https://docs.openstack.org/heat/latest/template_guide/hot_spec.html