# ComputeDriver.update_provider_tree

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

## Background

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using placement for scheduling and resource management, the virt driver method `get_available_resource` was initially superseded by `get_inventory` (now gone), whereby the driver could specify its inventory in terms understood by placement.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:

      { $CONSUMER_UUID: {
            # 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.

Note

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)
                         agg2

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.

Note

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`, implementers 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):
        provider_tree.add_traits(
            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.

this page last updated: 2026-06-08 12:33:00

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