Driver composition reform

This spec suggests revamping how we name and compose drivers from interfaces. It will allow having one vendor driver with options configurable per node instead of many drivers for every vendor.

Problem description

Our driver interface matrix has become increasingly complex. To top it all off nowadays we have many interfaces that can be used for every driver. To name a few:

  • boot: most drivers support PXE and iPXE, while some also support virtual media; support for petitboot bootloader is proposed.
  • deploy: two deploy approaches are supported: write image via iSCSI or write it directly from within the agent.
  • inspect: there is generic inspection using ironic-inspector, but some drivers also allow out-of-band inspection. This feature is optional, so we should provide a way to disable it.

Currently we’ve ended up with a complex and really confusing naming scheme. For example:

  • pxe_ipmitool uses PXE or iPXE for boot and iSCSI for deploy.
  • agent_ipmitool actually also uses PXE or iPXE, but it does not use iSCSI.
  • To top it all, pxe_ipmitool is actually using agent!
  • To reflect all the possibilities, the names would have to be more like pxe_iscsi_ipmitool, ipxe_iscsi_ipmitool, pxe_direct_ipmitool, ipxe_direct_ipmitool, etc.
  • Now repeat the same with every power driver we have.

Proposed change


The following concepts are used in this spec:

The driving force behind a specific driver. It can be hardware vendors or the ironic team itself in case of generic drivers, such as IPMI. This also includes out-of-tree drivers.
hardware interface (or just interface)
A notion replacing the term “driver interface” - a set of functionality dealing with some aspect of bare metal provisioning in a vendor-specific way. For example, right now we have power, deploy, inspect, boot and a few more interfaces.
hardware type
A family of hardware supporting the same set of interfaces from the ironic standpoint. This can be as wide as all hardware supporting the IPMI protocol or as narrow as several hardware models supporting some specific interfaces.
A thin object containing links to hardware interfaces. Before this spec driver meant roughly the same as hardware type means in this spec.
classic driver
An ironic driver from before this spec: a class with links to interfaces hardcoded in the Python code.
dynamic driver
A driver created at run time with links to interfaces generated based on information in the node record (including hardware type and interfaces).

With this spec we are going to achieve the following goals:

  • Make vendors in charge of defining a set of supported interface implementations in priority order.
  • Allow vendors to guarantee that unsupported interface implementations will not be used with hardware types they define. This is done by having a hardware type list all interfaces it supports.
  • Allow 3rd parties to create out-of-tree hardware types that allow them to maximize their reuse of the in-tree interface implementations.
  • Make hardware type definition as declarative as possible.
  • Allow a user to switch hardware type for a node, just as it was possible to change a driver before this spec.
  • Allow a user to switch between interface implementations supported by a hardware type for a node via the bare metal API.


  • A hardware type is defined as a Python class - see Hardware Types for details. An entry point is created to provide a simple name for each hardware type, for example:

    ironic.hardware.types =
        generic-ipmi = ironic.hardware.ipmi:GenericIpmiHardware
        ilo-gen8 = ironic.hardware.ilo:iLOGen8Hardware
        ilo-gen9 = ironic.hardware.ilo:iLOGen9Hardware
  • The list of hardware interfaces is still hardcoded in the Python code and cannot be extended by plugins. The interfaces are implemented in the same way as before this spec: by subclassing an appropriate abstract class from ironic.drivers.base.

  • For each hardware interface, all implementations get their own entrypoint and a unique name, for example:

    ironic.hardware.interfaces.power =
        ipmitool = ironic.drivers.modules.ipmitool:IpmitoolPower
  • Compatibility between hardware types and hardware interface implementations is expressed in the Python code - see Hardware Types for details.

  • Create a new configuration option enabled_hardware_types with a list of enabled hardware types. This will not include classic drivers which are enabled by the existing enabled_drivers option.

  • Create a family of configuration options default_<INTERFACE>_interface that allows an operator to explicitly set a default interface for new nodes upon creation, if one is not specified in the creation request.

    Here <INTERFACE> is a type of interface: power, management, etc.

  • Create a family of configuration options enabled_<INTERFACE>_interfaces with a list of enabled implementations of each hardware interface that are available for use in the ironic deployment.

    The default value, if provided by the default_<INTERFACE>_interface, must be in this list, otherwise a conductor will fail to start.

  • If no interface implementation is explicitly requested by a user in a node creation request, use the value calculated as follows:

    • If default_<INTERFACE>_interface is set, use its value. Return an error if it is not supported by the hardware type of the node.
    • Otherwise choose the first available interface implementation from an intersection of the enabled_<INTERFACE>_interfaces as defined in the deployment’s configuration and the hardware_type‘s priority ordered list of supported_<INTERFACE>_interfaces. Return an error, if this intersection is empty.

    This calculated default will be stored in the database entry for the node upon creation.

  • Change how we load drivers instead of one singleton instance of a driver, we’ll have an instance of dynamic driver per node, containing links to hardware interface implementations (just like today).

    However, interface implementations themselves will stay singletons, and will be preloaded during the start up and stored in the conductor.

    Conductor will fail to start if any enabled hardware types or interface implementations cannot be loaded (e.g. due to missing dependencies).


    While it’s technically possible to enable interfaces that are not used in any of enabled hardware types, they will not get loaded in this case.

    The classic drivers will be loaded exactly as before.

  • Modify the periodic tasks collection code to also collect periodic tasks for enabled interfaces of every enabled hardware type.

  • Conductor will fail to start if there is a name clash between a classic driver and a hardware type.

Database and Rest API

  • Allow the node driver field to accept the hardware types as well. This will work in all API versions.


    There are two reasons for that:

    • Consistency: we never prevented new drivers to be used with old API versions, and dynamic drivers will look mostly like new drivers to users.
    • Usability: we plan on eventually deprecating the classic drivers. When we remove them, all clients will need to specify the hardware types when enrolling nodes. To allow older clients to continue interacting with the API service, even as they use new driver names (hardware types), we must continue to use the same field name and API semantics.
  • For each interface create a new field on the node table named <interface_name>_interface. A migration will be needed each time we add a new interface (which hopefully won’t happen too often).

    For hardware types setting <interface_name>_interface field to None means using the calculated default value as described in Configuration.

    Trying to set any of these fields to a value other than None will result in an error if the driver field is set to a classic driver. Similarly, all these fields are reset to None if the driver field is set to a classic driver.

  • Every time driver and/or any of the interface fields is updated, the conductor checks that the hardware type supports all the resulting interfaces (except when driver is set to a classic driver).

    To change between two incompatible sets of interfaces, all changes should come in one API call. E.g. for a node with the ilo-gen8 hardware type and vmedia_ilo boot interface the following JSON patch will be allowed:

        {"op": "replace", "path": "/boot_interface", "value": "ipxe"},
        {"op": "replace", "path": "/driver", "value": "generic-ipmi"}

    but the following patch will fail because of incompatible boot interface:

        {"op": "replace", "path": "/driver", "value": "generic-ipmi"},


    RFC 6902 requires a JSON patch to be atomic, because an HTTP PATCH operation must be atomic. Meaning, it’s possible for some operations to end up with an inconsistent object as long as the end result is consistent.

    The validation will be conducted on the API service side by checking the new conductor_hardware_interfaces database table.

  • If for some reason the existing interface becomes invalid for a node (e.g. it was disabled after the node was enrolled), it will be signalized via the usual node validation API. The validation for this interface won’t pass with an appropriate error message. On the programming level, the driver attribute for this interface (e.g. task.driver.deploy) will be set to None.

  • Update GET /v1/drivers to also list enabled hardware types. This change is not affected by API versioning, because we allow old API versions to use hardware types with the driver field.

  • Allow GET /v1/drivers to filter only hardware types or only classic drivers.

    Update GET /v1/drivers/<HW TYPE> to report the hardware type information, including the list of enabled hardware interfaces.

    This feature is guarded by an API version bump (as usual).

  • Allow filtering nodes by <interface_name>_interface fields in the node list API.

    This feature is guarded by an API version bump (as usual).

  • Create a new table conductor_hardware_interfaces to hold the relationship between conductors, hardware types and available interfaces. A warning will be issued on conductor start up, if it detects that other conductors have a different set of interfaces for the same enabled hardware type. This will also track the default interface for each hardware type and interface type combination.

    This situation is inevitable during live upgrades, so it must not result in an error. However, we will document that all conductors should have the same set of interfaces for the same enabled hardware types.

    This table will not be exposed in the HTTP API for now.


We are not planning to deprecate and remove the support for classic drivers in the V1 API.

We are planning to deprecate and remove the classic drivers which exist in-tree. The deprecation procedure may be tricky and will be covered by a follow-up spec.


  • We could put interfaces under a new JSON key on a node. However, we’re trying to move away from informally defined JSON keys. It would also prevent us from being able to implement the filtering of nodes based on a particular interface.

  • We could create a new API endpoint for updating the interfaces. This will be inconsistent with how we update the driver field though.

    We could then create a new API version, preventing updating driver via the regular node update API, but that would be a breaking change.

  • We could create a new field hardware_type instead of having the existing driver field accept a hardware type. This was a part of the proposal previously, but we found that it complicates things substantially without clear benefits.

  • We could create a whole new family of API endpoints instead of reusing /v1/drivers, e.g. /v1/hardware-types. However, it would require us to replicate all driver-related functionality nearly intact, for example driver vendor passthru. So users would have to somehow figure out which vendor passthru endpoint to use based on what kind of a driver is in the driver field.

Data model impact

  • For each interface, create a new node field <interface_name>_interface initially set to NULL.

  • Create a new internal table conductor_hardware_interfaces:

    conductor_id - conductor ID (foreign key to conductors table),

    hardware_type VARCHAR(255) - hardware type entrypoint name,

    interface_type VARCHAR(16) - interface type name (e.g. deploy),

    interface_name VARCHAR(255) - interface implementation entry point name.

    default TINYINT(1) - boolean which denotes if this interface_name is

    the default for a given hardware_type and interface_type combination.

    This table will get populated on conductor start up and purged on deleting the conductor record. On conductor startup, during init_host(), the conductor will fetch the list of hardware interfaces supported by all registered conductors and compare to its own configuration. If the same hardware type is enabled on two conductors with a different set of enabled_interfaces, this will result in a WARNING log message. The enabled hardware types themselves do not have to match (just like today, different conductors can have different set of drivers).

State Machine Impact


REST API impact

  • Update GET /v1/drivers:

    Return both classic drivers and hardware types no matter which API version is used.

    New URL parameters:

    • type (string, one of classic, dynamic, optional) - if provided, limit the resulting driver list to only classic drivers or hardware types accordingly.

    New response field:

    type whether the driver is dynamic or classic.

    This change is guarded by a new API version.

  • Update GET /v1/drivers/<NAME>:

    New response field:

    type whether the driver is dynamic or classic.

    New response fields that are not None only for hardware types:


    the entrypoint name of the calculated default implementation for a given interface.


    the list of entrypoint names of enabled implementations for a given interface.

  • Update GET /v1/drivers/<NAME>/properties and GET /v1/drivers/<NAME>/vendor_passthru/methods and the actual driver vendor passthru call implementation:

    When requested for a dynamic driver, assume the calculated defaults for the vendor interface implementation as described in Configuration. We will need to support non-default implementations as well, but it goes somewhat beyond the scope of this already big spec.

Client (CLI) impact

“ironic” CLI

  • Update the node creation command to accept one argument per interface. Example:

    ironic node-create --driver=ilo-gen9 --power-interface=redfish

    The same change is applied to the OSC plugin.

  • Extend the output of the driver-list command with the Type column.

  • Extend the driver-list command with --type argument, which, if supplied, limits the driver list to only classic drivers (classic value) or hardware types (dynamic value).

  • Extend the output of the driver-show command with the newly introduced fields.

“openstack baremetal” CLI

Similar changes to what’s in “ironic” CLI are applied here.

RPC API impact

  • No impact on the hash ring, as both hardware types and classic drivers are used in the same field.

Driver API impact

Hardware Types

  • Create a new AbstractHardwareType class as an abstract base class for all hardware types. Here is a simplified example implementation, using only power, deploy and inspect interfaces:

    import abc, six
    class AbstractHardwareType(object):
        def supported_power_interfaces(self):
        def supported_deploy_interfaces(self):
        def supported_inspect_interfaces(self):
            return [NoopInspect]

    Note that some interfaces (power, deploy) are mandatory, while the other (inspect) are not. A dummy implementation will be provided for all optional interfaces. Depending on the specific call it will either do nothing or raise an error. For user-initiated calls (e.g. start inspection), an error will be returned. For internal calls (e.g. attach cleaning ports), no action will be taked.

  • Create a new GenericHardwareType class which most of the actual hardware type classes will want to subclass. This class will insert generic implementations for some interfaces:

    class GenericHardwareType(AbstractHardwareType):
        supported_deploy_interfaces = [AgentDeploy]
        supported_inspect_interfaces = [NoopInspect, InspectorInspect]

    Note that all properties contain classes, not instances. Also note that order matters: in this example NoopInspect will be the default, if both implementations are enabled in the configuration.

  • Here is an example of how hardware types could be created:

    class GenericIpmiHardware(GenericHardwareType):
        supported_power_interfaces = [IpmitoolPower, IpminativePower]
    class iLOGen8Hardware(GenericHardwareType):
        supported_power_interfaces = (
            + [IloPower]
        supported_inspect_interfaces = (
            + [IloInspect]
    class iLOGen9Hardware(iLOGen8Hardware):
        supported_power_interfaces = (
            + [RedfishPower]


These definitions use classes, not entrypoints names. These examples assume the required classes are imported.


The following entrypoints will have to be defined for these examples to work:

ironic.hardware.types =
    generic-ipmi = ironic.hardware.ipmi:GenericIpmiHardware
    ilo-gen8 = ironic.hardware.ilo:iLOGen8Hardware
    ilo-gen9 = ironic.hardware.ilo:iLOGen9Hardware

ironic.hardware.interfaces.power =
    ipmitool = ironic.drivers.modules.ipmitool:IpmitoolPower
    ipminative = ironic.drivers.modules.ipmitool:IpminativePower
    ilo = ironic.drivers.modules.ilo:IloPower
    redfish = ironic.drivers.modules.redfish:RedfishPower

ironic.hardware.interfaces.inspect =
    inspector = ironic.drivers.modules.inspector:InspectorInspect
    ilo = ironic.drivers.modules.ilo:IloInspect

The following configuration will be required to enable everything in these examples:

enabled_hardware_types = generic-ipmi,ilo-gen8,ilo-gen9
enabled_power_interfaces = ipmitool,ipminative,ilo,redfish
enabled_inspect_interfaces = inspector,ilo

Driver Creation

  • At start up time the conductor instantiates all enabled hardware types, as well as all enabled interface implementations for enabled hardware types.
  • Each time the node is created or loaded from the database, a thin BareDriver object is created with all interfaces set on it. This is similar to how network drivers already work. It gets assigned to task.driver, and after that everything works as before this spec.

Nova driver impact


Ramdisk impact


Security impact


Other end user impact

  • End users should switch to hardware types over time.

Scalability impact


Performance Impact

  • A driver instance will be now created per node as opposed to creating one per conductor right now. This will somewhat increase the memory usage per node. We can probably define __slots__ on the driver class to reduce this effect.

Other deployer impact

  • A deployer can set the new enabled_hardware_types option to enable more hardware types. Otherwise only the default hardware types and already enabled classic drivers will be available.
  • A deployer can also set any of new enabled_<INTERFACE>_interfaces options to enable more interfaces for the enabled hardware types.

Developer impact

This spec changes the way we expect the developers to write their drivers.

  • No more new classic drivers will be accepted in-tree as soon as this change lands.
  • Developers should implement hardware types and interfaces to provide new hardware support for Ironic. Built-in interfaces implementations will be available for reuse both in-tree and out-of-tree.



  • Dmitry Tantsur (lp: divius, irc: dtantsur)
  • Jim Rollenhagen (irc: jroll)

Work Items

  • Create base classes supporting hardware types.
  • Create tables for tracking enabled hardware interfaces.
  • Load hardware types on conductor start up and record them in the internal table.
  • Create node fields for interfaces and expose them in the API.
  • Update the drivers API to support hardware types.
  • Create the hardware types for hardware supported directly by the team, i.e. the generic IPMI-compatible hardware. The SSH driver might be removed soon; it won’t get updated in this case.



  • Unit test coverage will obviously be provided.
  • A new gate job will be created, using a dynamic version of the IPMI driver. We will aim to make it the primary approach in the gate over time.
  • Grenade testing for upgrades / migration of existing workloads to new drivers.

Upgrades and Backwards Compatibility

This reform is designed to be backward compatible. The classic drivers will be supported for at least some time. A separate spec will cover the deprecation of the classic drivers.

We will recommend switching to using appropriate dynamic drivers as soon as it’s possible.

Upgrade flow

  1. Ironic is updated to a version supporting dynamic drivers. The API version used by clients is not updated yet.
  2. All nodes are still using classic drivers. On a node driver=x_y.
  3. Users with an old API version:
    • can set driver to a classic driver.
    • can set driver to a hardware type, which will result in using a dynamic driver with the default set of interfaces.
  4. Users with a new API version:
    • can set driver to a hardware type or a classic driver
    • can set non-default interface implementations when driver is set to a real hardware type

Documentation Impact

  • Document switching to dynamic drivers
  • Document creating new hardware types