This provides details on the ComputeDriver abstract method update_provider_tree for developers implementing this method in their own virt drivers.


In the movement towards using placement for scheduling and resource management, the virt driver method get_available_resource was initially superseded by get_inventory (now gone), whereby the driver could specify its inventory in terms understood by placement. In Queens, a get_traits driver method was added. But get_inventory was limited to expressing only inventory (not traits or aggregates). And both of these methods were limited to the resource provider corresponding to the compute node.

Developments such as Nested Resource Providers necessitate the ability for the virt driver to have deeper control over what the resource tracker configures in placement on behalf of the compute node. This need is filled by the virt driver method update_provider_tree and its consumption by the resource tracker, allowing full control over the placement representation of the compute node and its associated providers.

The Method

update_provider_tree accepts the following parameters:

  • A nova.compute.provider_tree.ProviderTree object representing all the providers in the tree associated with the compute node, and any sharing providers (those with the MISC_SHARES_VIA_AGGREGATE trait) associated via aggregate with any of those providers (but not their tree- or aggregate-associated providers), as currently known by placement. This object is fully owned by the update_provider_tree method, and can therefore be modified without locking/concurrency considerations. In other words, the parameter is passed by reference with the expectation that the virt driver will modify the object. Note, however, that it may contain providers not directly owned/controlled by the compute host. Care must be taken not to remove or modify such providers inadvertently. In addition, providers may be associated with traits and/or aggregates maintained by outside agents. The update_provider_tree method must therefore also be careful only to add/remove traits/aggregates it explicitly controls.

  • String name of the compute node (i.e. ComputeNode.hypervisor_hostname) for which the caller is requesting updated provider information. Drivers may use this to help identify the compute node provider in the ProviderTree. Drivers managing more than one node (e.g. ironic) may also use it as a cue to indicate which node is being processed by the caller.

  • Dictionary of allocations data of the form:

          # The shape of each "allocations" dict below is identical
          # to the return from GET /allocations/{consumer_uuid}
          "allocations": {
              $RP_UUID: {
                  "generation": $RP_GEN,
                  "resources": {
                      $RESOURCE_CLASS: $AMOUNT,
          "project_id": $PROJ_ID,
          "user_id": $USER_ID,
          "consumer_generation": $CONSUMER_GEN,

    If None, and the method determines that any inventory needs to be moved (from one provider to another and/or to a different resource class), the ReshapeNeeded exception must be raised. Otherwise, this dict must be edited in place to indicate the desired final state of allocations. Drivers should only edit allocation records for providers whose inventories are being affected by the reshape operation. For more information about the reshape operation, refer to the spec.

The virt driver is expected to update the ProviderTree object with current resource provider and inventory information. When the method returns, the ProviderTree should represent the correct hierarchy of nested resource providers associated with this compute node, as well as the inventory, aggregates, and traits associated with those resource providers.


Despite the name, a ProviderTree instance may in fact contain more than one tree. For purposes of this specification, the ProviderTree passed to update_provider_tree will contain:

  • the entire tree associated with the compute node; and

  • any sharing providers (those with the MISC_SHARES_VIA_AGGREGATE trait) which are associated via aggregate with any of the providers in the compute node’s tree. The sharing providers will be presented as lone roots in the ProviderTree, even if they happen to be part of a tree themselves.

Consider the example below. SSP is a shared storage provider and BW1 and BW2 are shared bandwidth providers; all three have the MISC_SHARES_VIA_AGGREGATE trait:

         CN1                 SHR_ROOT               CN2
        /   \       agg1    /   /\     agg1        /   \
   NUMA1     NUMA2--------SSP--/--\-----------NUMA1     NUMA2
  /     \   /    \            /    \         /     \   /    \
PF1    PF2 PF3   PF4--------BW1   BW2------PF1    PF2 PF3   PF4
                     agg2             agg3

When update_provider_tree is invoked for CN1, it is passed a ProviderTree containing:

         CN1 (root)
        /   \       agg1
   NUMA1     NUMA2-------SSP (root)
  /     \   /    \
PF1    PF2 PF3   PF4------BW1 (root)

Driver implementations of update_provider_tree are expected to use public ProviderTree methods to effect changes to the provider tree passed in. Some of the methods which may be useful are as follows:

  • new_root: Add a new root provider to the tree.

  • new_child: Add a new child under an existing provider.

  • data: Access information (name, UUID, parent, inventory, traits, aggregates) about a provider in the tree.

  • remove: Remove a provider and its descendants from the tree. Use caution in multiple-ownership scenarios.

  • update_inventory: Set the inventory for a provider.

  • add_traits, remove_traits: Set/unset virt-owned traits for a provider.

  • add_aggregates, remove_aggregates: Set/unset virt-owned aggregate associations for a provider.


There is no supported mechanism for update_provider_tree to effect changes to allocations. This is intentional: in Nova, allocations are managed exclusively outside of virt. (Usually by the scheduler; sometimes - e.g. for migrations - by the conductor.)

Porting from get_inventory

Virt driver developers wishing to move from get_inventory to update_provider_tree should use the ProviderTree.update_inventory method, specifying the compute node as the provider and the same inventory as returned by get_inventory. For example:

def get_inventory(self, nodename):
    inv_data = {
        'VCPU': { ... },
        'MEMORY_MB': { ... },
        'DISK_GB': { ... },
    return inv_data

would become:

def update_provider_tree(self, provider_tree, nodename, allocations=None):
    inv_data = {
        'VCPU': { ... },
        'MEMORY_MB': { ... },
        'DISK_GB': { ... },
    provider_tree.update_inventory(nodename, inv_data)

When reporting inventory for the standard resource classes VCPU, MEMORY_MB and DISK_GB, implementors of update_provider_tree may need to set the allocation_ratio and reserved values in the inv_data dict based on configuration to reflect changes on the compute for allocation ratios and reserved resource amounts back to the placement service.

Porting from get_traits

To replace get_traits, developers should use the ProviderTree.add_traits method, specifying the compute node as the provider and the same traits as returned by get_traits. For example:

def get_traits(self, nodename):
    traits = ['HW_CPU_X86_AVX', 'HW_CPU_X86_AVX2', 'CUSTOM_GOLD']
    return traits

would become:

def update_provider_tree(self, provider_tree, nodename, allocations=None):
        nodename, 'HW_CPU_X86_AVX', 'HW_CPU_X86_AVX2', 'CUSTOM_GOLD')

Taxonomy of traits and capabilities

There are various types of traits:

  • Some are standard (registered in os-traits); others are custom.

  • Some are owned by the compute service; others can be managed by operators.

  • Some come from driver-supported capabilities, via a mechanism which was introduced to convert them to standard traits on the compute node resource provider. This mechanism is documented in the configuration guide.

This diagram may shed further light on how these traits relate to each other and how they are managed.

Venn diagram showing taxonomy of traits and capabilities