Building a dynamic Ansible inventory from Proxmox

I am in the process of slowly migrating the VMs on my home servers from my old, custom configuration management scripts to a set of Ansible scripts. However, because I'm quite lazy, I don't want to have to update the playbook's inventories manually if I can avoid it. Ideally, I should only have to write a service's actual configuration, then run the playbook. The appropriate VMs should be selected automatically from the Proxmox cluster and used as the targets.

TL;DR version: trying to do as much of this as possible using only Ansible and YAML files results in a highly convoluted mess.

Target inventory structure

As far as inventory groups go, I want to have a somewhat nested hierarchy. First and foremost, all VMs that are managed by Ansible will be found under the managed group, which is organized with the following subgroups.

For example, a pair of clusters implementing LDAP services (one for testing purposes, one for actual use) might be organized as shown below :

 |- by_network
 |   |- net_dev -> [vm0, vm1, vm2, vm3, vm4]
 |   |- net_infra -> [vm5, vm6, vm7, vm8, vm9]
 |- by_environment
 |   |- env_dev -> [vm0, vm1, vm2, vm3, vm4]
 |   |- env_prod -> [vm5, vm6, vm7, vm8, vm9]
 |- by_failover_stack
 |   |- fostack_1 -> [vm0, vm2, vm5, vm7]
 |   |- fostack_2 -> [vm1, vm3, vm6, vm8]
 |   |- no_failover -> [vm4, vm9]
 |- svc_ldap
     |- svin_ldap_dev -> [vm0, vm1, vm2, vm3, vm4]
     |- svin_ldap_prod -> [vm5, vm6, vm7, vm8, vm9]
     |- svcm_ldap_front -> [vm0, vm1, vm5, vm6]
     |- svcm_ldap_ldap
        |- svcm_ldap_roldap -> [vm2, vm3, vm7, vm8]
        |- svcm_ldap_rwldap -> [vm5, vm9]


There are two main problems that need to be solved here.

First, Proxmox has very limited support for VM metadata. It supports "tags", which is a list of words with rather strong constraints on which characters are allowed, and as far as I know can only be set or read through the API or command line, but these would be insufficient for what I need to do. In addition, while I have no intention of managing my metadata manually, not having it in the GUI at all is a bit of a pain.

On the Ansible side, While there is a community plugin which can read inventory from Proxmox, and a core plugin that can construct groups from various variables, the latter cannot construct empty groups (e.g. the svc_ldap group in the example above), and there are limitations in the way facts can be generated by both plugins - chiefly, none of the facts being generated may refer to another such fact.


In the following sections, I will implement an inventory which generates the expected structure from the Proxmox inventory. I have created a repository on GitHub which contains the example, with each commit in the repository corresponding to the steps below.

Static structure

The first file of the inventory should generate the static parts of the structure. This is done using a simple inventory file ; that file must be read before the rest, so we will name it 00-static-structure.yml.


Testing at this point using ansible-inventory --playbook-dir . --graph shows the groups above plus the additional ungrouped group.

Fetching inventory from Proxmox

Now we need to fetch the list of VMs and their associated metadata from the Proxmox cluster using the community.general.proxmox plugin. We need Ansible to run it right after loading the static groups, so its name will start with 01-. In addition, the plugin requires the name of the file to end with .proxmox.yml.

We will configure the plugin to fetch all facts and write them to variables with the proxmox__ prefix. Similarly, groups generated by the plugin will use the same prefix.

plugin: community.general.proxmox

url: https://proxmox.example.org:8006
validate_certs: false
user: test@pve
password: ...
want_facts: true
facts_prefix: proxmox__
group_prefix: proxmox__
want_proxmox_nodes_ansible_host: false

If the Ansible configuration is used to restrict the list of plugins that can parse the inventory, it should be modified as well:

enable_plugins = community.general.proxmox, yaml

And it might be necessary to install the requests Python module (pip install requests in the same venv as Ansible should work).

Once this is done, and assuming the url, user and password are configured appropriately, ansible-inventory should display both the static structure from the section above and the VMs and groups that were fetched from the Proxmox cluster :

  |  |--@by_environment:
  |  |--@by_failover_stack:
  |  |  |--@no_failover:
  |  |--@by_network:
  |  |--vm1
  |  |--vm2
  |  |--vm3
  | ...

In addition, using ansible-inventory --host to display the facts for a VM should show a bunch of entries that correspond to the VM's settings :

    "proxmox__agent": "1",
    "proxmox__boot": {
        "order": "ide2;scsi0"
    "proxmox__cores": 4,
    "proxmox__cpu": "kvm64",
    "proxmox__description": "something",
    // ...
    "proxmox__net0": {
        "bridge": "vmbr0",
        "firewall": "1",
        "tag": "16",
        "virtio": "12:23:34:45:56:67"
    // ...

Storing metadata on the Proxmox cluster

As I mentionned in the introduction, VM tags are not sufficient for what we need to do. However, each VM can have arbitrary Markdown associated with it. This text can be seen in the "Notes" part of the Proxmox GUI.

One solution to the problem of storing arbitrary metadata would be to store it as JSON directly in the notes. It can then be read from the proxmox__description variable.

However, this approach is insufficient in two ways. First, the JSON itself is quite unreadable on the GUI, which renders the idea of having it visible there moot. Second, it makes adding human-readable notes impossible.

In order to solve that, we could surround the section of the notes that contain the JSON notes with the Markdown code block marker. This is still not enough, as it would prevent the notes from containing any other code block.

Instead, I chose to use the following structure:

(arbitrary Markdown here)
  "service": "ldap",
  "instance": "dev",
  "component": "ldap",
  "subcomponent": "roldap",
  "fostack": 1
(more Markdown here because why not)

Because of the ansible marker, it is possible to split the description right at the start of the block, then using the unmodified marker to remove the end of the description. The resulting string can then be parsed from JSON.

This can be achieved by adding a compose section to the Proxmox plugin configuration.

  inv__data: >-
    ( ( proxmox__description | split( '```ansible' ) )[1]
      | split( '```' ) )[0]
    | from_json

When the notes contain a block that follows the right format, the plugin will create a inv__data variable which will contain the parsed data. If the format is incorrect, or if there is no description, or if the block contains invalid JSON, the variable will simply not be defined (this is due to the Proxmox inventory plugin's strict option defaulting to false).

It is possible to use the ansible-inventory command to check for the variable after having added a test on one of the VMs :

    "inv__data": {
        "component": "ldap",
        "fostack": 1,
        "instance": "dev",
        "service": "ldap",
        "subcomponent": "roldap"
    "proxmox__agent": "1",
    // ...

Computing facts

We now need to deduce a few things from the various data we gathered.

Copying metadata to top-level variables

Because the inv__data variable might be undefined, we will copy some of its contents to separate variables to avoid having to write (inv__data|default({})) for all accesses. This will be done in the 02-copy-metadata.yml inventory file, using the constructed plugin. Since it is working in non-strict mode, the various variables will not be generated if inv__data doesn't exist.

plugin: constructed
strict: false

  inv__component: inv__data.component
  inv__fostack: inv__data.fostack
  inv__instance: inv__data.instance
  inv__service: inv__data.service
  inv__subcomponent: inv__data.subcomponent

It will be necessary to enable the constructed plugin in the Ansible configuration for this to work :

enable_plugins = constructed, community.general.proxmox, yaml

Should this VM be managed?

The next file, 03-check-managed.yml, will create an _inv__managed variable if the metadata includes a service name and an instance name, and if the first network interface is connected. When it is defined, this variable will always contain an empty string. This allows us to use it while defining other variables or groups. If it exists, adding its contents to some variable will have no effect. If it doesn't, Jinja evaluation will fail, causing group or variable creation to be skipped.

In order to do this, we need to use Jinja conditionnals in addition to expressions. The constructed plugin's compose block normally doesn't allow that, but it is possible to do it anyway. In fact, Ansible simply prefixes the expression with {{ and suffixes it with }}, so it is possible to terminate these expressions and add conditionnals.

Here is the 03-check-managed.yml file, which implements that.

plugin: constructed
strict: false

  _inv__managed: >-
    ( inv__instance and inv__service ) | ternary( '' , '' )
    }}{% if proxmox__net0.link_down | default("0") == "1"
    %}{{   this_variable_does_not_exist_and_so_inv_managed_will_not_be_created
    }}{% endif
    %}{{ ''

The first line of the definition relies on the fact that trying to use inv__instance or inv__service in an expression will cause the variable to be skipped if any of them is missing.

The second line exits the expression so a conditional can be used. It is however necessary to re-enter an expression and provide something valid, which is done by the last line.

Finally, the very long variable name in the expression references an undefined variable, and will only be executed if the condition is right, causing the definition for _inv__managed to be skipped.

Basic groups

At this point, we are about ready to start adding our VMs to the network, environment and failover stack groups. We will create a new inventory file called 04-env-fo-net-groups.yml to handle that using the constructed plugin.

First, we will use a lookup table to determine which network the VM is on based on its net0 interface's VLAN tag:

plugin: constructed
strict: false

  inv__network: >
      "30": "infra",
      "31": "dmz",
      "32": "pubapps",
      "33": "intapps",
      "34": "users",
      "60": "dev",
    }[ proxmox__net0.tag | default("") ]
    | default( "unknown" )
    ~ _inv__managed

The last line uses the _inv_managed variable to prevent the variable from being defined if the VM should not be managed. Since the variable normally contains an empty string, using it has no other effect.

At that point, we can create the network-based group:

  - prefix: net
    key: inv__network
    parent_group: by_network

The environment can be computed by checking for a environment field in the original metadata. Failing that, the VM will be assigned to the prod environment if its instance name is prod, or to the dev environment if it isn't. We also need to reference _inv__managed to prevent unmanaged VMs from being added to the group.

  # ...
  inv__environment: >-
      | default(
          ( inv__instance == "prod" ) | ternary( "prod", "dev" )
    ~ _inv__managed

  # ...
  - prefix: env
    key: inv__environment
    parent_group: by_environment

The last basic group to generate is based on the HA stack the VM is a part of, if any. Note the default("") used in the ternary to prevent it from referencing an undefined variable.

  # ...
  _inv__fostack_group: >-
    ( inv__fostack is defined )
      | ternary(
          "fostack_" ~ inv__fostack | default("") ,
    ~ _inv__managed

  # ...
  - prefix: ''
    key: _inv__fostack_group
    parent_group: by_failover_stack

Generating empty intermediary groups

At this point we need to start working on creating the intermediary groups for the service itself and for its components if that feature is being used.

The main problem is that these groups must be created empty - we don't want our VMs to be added directly to them as it would cause variable precedence problems when we try to use them for actual configuration.

Sadly, neither the constructed plugin, which we used above, nor the generator plugin (documented here) can be used to generate the empty groups we need, as both always add a host to the groups that are created. In addition, generator doesn't process the layer names through Jinja. We need to write a custom plugin to generate the groups we need.

Empty group generator

What we need is a relatively simple inventory plugin that will generate groups with templated names and templated parents. It could be configured using a list of groups, each described by a dictionary with a name entry containing a Jinja template, and a parents key containing a list of Jinja templates (one for each parent group). Each invidual template could return an empty string; in the name part it would cause the group to be skipped, and in the parents list it would simply be ignored.

We will not cover the actual writing of the plugin here, but we will comment some parts of the code. It can be found in the repository's inventory_plugins/group_creator.py file.

It starts by the "documentation", which Ansible uses to validate the plugin's configuration data and set defaults to the various options.

Following that, we define the plugin's class. Its main method, parse(), is shown below :

    def parse(self, inventory, loader, path, cache=False):
        super(InventoryModule, self).parse(inventory, loader, path, cache=cache)
        strict = self.get_option("strict")
        for host in inventory.hosts:
            host_vars = self.inventory.get_host(host).get_vars()
            for group in self.get_option("groups"):
                name = self._get_group_name(host, group['name'], host_vars, strict)
                if not name:
                for ptmpl in group.get("parents"):
                    parent = self._get_group_name(host, ptmpl, host_vars, strict)
                    if parent:
                        self.inventory.add_child(parent, name)

It goes through all known inventory hosts, and tries to generate groups based on each of these host's facts. It then computes the parent groups' names, ensures the parent groups exist, and registers the new group as a child. The _get_group_name method will simply apply the templates, either returning an empty string or causing an exception if a problem occurs, depending on the strict option's value.

The plugin must be added to the enabled plugins in the Ansible configuration as well.

enable_plugins = constructed, community.general.proxmox, group_creator, yaml

Note: at this point, testing with ansible-inventory really requires the --playbook-dir . option, as the tool will not find the plugin if it is not present.

Creating the groups

We can create a new file in the inventory for the intermediary groups' creation. For all Ansible-managed VMs, we must ensure that the service group exists. We must also create component groups if components are defined, and sub-component groups if both components and sub-components are defined.

Creating the service group is pretty straightforward:

plugin: group_creator
strict: true


  - name: >-
      {{ 'svc_' ~ inv__service ~ _inv__managed }}
    - managed

Component groups are about as straightforward. If no components are defined for the current service, groups will not be created as the inv__component variable will fail to evaluate.

  - name: >-
        'svcm_' ~ inv__service
        ~ '_' ~ inv__component
        ~ _inv__managed
    - 'svc_{{ inv__service }}'

Finally, if sub-components are in use, their groups must also be created. Doing it at this point will free us from having to specify the correct parent groups in the next step. Sub-component groups should be created if the VM is managed and has both a component and sub-component.

  - name: >-
        'svcm_' ~ inv__service
        ~ '_' ~ inv__subcomponent
        ~ _inv__managed
        ~ ( inv__component | ternary('','') )
    - 'svcm_{{ inv__service }}_{{ inv__component }}'

Testing at that point should show the various groups that have been created. They should not contain any hosts.

  |  |--@svc_ldap:
  |  |  |--@svcm_ldap_front:
  |  |  |--@svcm_ldap_ldap:
  |  |  |  |--@svcm_ldap_roldap:
  |  |  |  |--@svcm_ldap_rwldap:

Assigning VMs to service groups

We can proceed with assigning VMs to service groups using the constructed plugin. This is done in the 06-hosts-in-service-groups.yml file.

First we will add the hosts to instance groups under the main service groups. As usual, _inv__managed ensures that we only create groups from VMs we actually need to and can manage.


  _inv__instance_group: >-
    inv__service ~ '_' ~ inv__instance ~ _inv__managed


  - prefix: svin
    key: _inv__instance_group
    parent_group: "svc_{{ inv__service }}"

Next we need to add the VM to the group which corresponds to the component or sub-component of the service. This should only be done if there is a component. We do not need to specify a parent_group in the group definition as the hierarchy has already been defined by the group creation.


  _inv__component_group: >-
    inv__service ~ '_' ~ inv__subcomponent | default( inv__component )
    ~ _inv__managed


  - prefix: svcm
    key: _inv__component_group


This setup creates the structure we needed to create. However, achieving this is quite convoluted (7 YAML files and a Python plugin), and has to rely on quite a few hacks and side-effects - the "Jinja injection" used for things that Ansible expects to be a single Jinja expression being quite dirty. Given the complexity involved, it would probably be worth it to replace all steps following the Proxmox fetch with a single Python plugin that handles the whole process.