Handling Reshaped Provider Trees


Virt drivers need to be able to change the structure of the provider trees they expose. When moving existing resources, existing allocations need to be moved along with the inventories. And this must be done in such a way as to avoid races where a second entity can create or remove allocations against the moving inventories.

Problem description

Use Cases

  • The libvirt driver currently inventories VGPU resources on the compute node provider. In order to exploit provider trees, libvirt needs to create one child provider per physical GPU and move the VGPU inventory from the compute node provider to these GPU child providers. In a live deployment where VGPU resources are already allocated to instances, the allocations need to be moved along with the inventories.

  • Drivers wishing to model NUMA must similarly create child providers and move inventory and allocations of several classes (processor, memory, VFs on NUMA-affined NICs, etc.) to those providers.

  • A driver is using a custom resource class. That resource class is added to the standard set (under a new, non-CUSTOM_ name). In order to use the standard name, the driver must move inventory and allocations from the old name to the new.

These are just example cases that may exist now or in the future. We’re describing a generic pivot system here.

Proposed change

The overall flow is as follows. The parts in red only happen when a reshape is needed. This represents the happy path on compute startup only.


Note that, for Fast-Forward Upgrades, the Resource Tracker lane is actually the Offline Upgrade Script.


A new SchedulerReportClient method shall be implemented:

def get_allocations_for_provider_tree(self, context, nodename):
    """Retrieve allocation records associated with all providers in the
    provider tree.

    :param context: The security context
    :param nodename: The name of a node for whose tree we are getting
    :returns: A dict, keyed by consumer UUID, of allocation records:
            { $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,

A consumer isn’t always an instance (it may be a “migration” - or other things not created by Nova, in the future), so we can’t just use the instance list as the consumer list.

We can’t get all allocations for associated sharing providers because some of those will belong to consumers on other hosts.

So we have to discover all the consumers associated with the providers in the “local” tree (identified by nodename):

for each "local" provider:
    GET /resource_providers/{provider.uuid}/allocations

We can’t use just those allocations because we would miss allocations for sharing providers. So we have to get all the allocations for just the consumers discovered above:

for each consumer in ^:
    GET /allocations/{consumer.uuid}


We will still miss data if all of a consumer’s allocations live on sharing providers. I don’t have a good way to close that hole. But that scenario won’t happen in the near future, so it’ll be noted as a limitation via a code comment.

Return a dict, keyed by the {consumer.uuid}, of the resulting allocation records. This is the form of the new Allocations Parameter expected by update_provider_tree and update_from_provider_tree), and return it.

ReshapeNeeded exception

A new exception, ReshapeNeeded, will be introduced. It is used as a signal from update_provider_tree to indicate that a reshape must be performed. This is for performance reasons so that we don’t get_allocations_for_provider_tree unless it’s necessary.

ReshapeFailed exception

A new exception, ReshapeFailed, will be introduced. It is raised from update_from_provider_tree only when reshaping is needed, attempted, and unsuccessful (i.e. when the Placement POST /reshaper call fails). This is so we can trap it explicitly in _update_available_resource_for_node and kill the compute service.

Changes to update_provider_tree()

Allocations Parameter

A new allocations keyword argument will be added to update_provider_tree():

def update_provider_tree(self, provider_tree, nodename, allocations=None):

If None, the upgrade_provider_tree() method must not perform a reshape. If it decides a reshape is necessary, it must raise the new ReshapeNeeded exception.

When not None, the allocations argument is a dict, keyed by consumer UUID, of allocation records of the form:

      # NOTE: 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,
      # ...except for this, which is coming in bp/add-consumer-generation
      "consumer_generation": $CONSUMER_GEN,

If update_provider_tree() is moving allocations, it must edit the allocations dict in place.


I don’t love the idea of the method editing the dict in place rather than returning a copy, but it’s consistent with how we’re handling the provider_tree arg.

Virt Drivers

Virt drivers currently overriding update_provider_tree() will need to change the signature to accomodate the new parameter. That work will be done within the scope of this blueprint.

As virt drivers begin to model resources in nested providers, their implementations will need to:

  • determine whether a reshape is necessary and raise ReshapeNeeded as appropriate;

  • perform the reshape by processing provider inventories and the specified allocations.

That work is outside the scope of this blueprint.

Changes to update_from_provider_tree()

The SchedulerReportClient.update_from_provider_tree() method is changed to accept a new parameter allocations:

def update_from_provider_tree(self, context, new_tree, allocations):
    """Flush changes from a specified ProviderTree back to placement.


    :param allocations: A dict, keyed by consumer UUID, of allocation records
            of the form returned by GET /allocations/{consumer_uuid}. The
            dict must represent the comprehensive final picture of the
            allocations for each consumer therein. A value of None indicates
            that no reshape is being performed.

When allocations is None, the behavior of update_from_provider_tree() is as it was previously (in Queens).

Changes to Resource Tracker _update()

The _update() method will get a new parameter, startup, which is percolated down from update_available_resource().

Where update_provider_tree and update_from_provider_tree are currently invoked, the code flow will be changed to approximately:

    self.driver.update_provider_tree(prov_tree, nodename)
except exception.ReshapeNeeded:
    if not startup:
        # This isn't supposed to happen during periodic, so raise
        # it up; the compute manager will treat it specially,
        # killing the compute service.
    LOG.info("Performing resource provider inventory and "
             "allocation data migration during compute service "
             "startup or FFU.")
    allocs = reportclient.get_allocations_for_provider_tree()
    self.driver.update_provider_tree(prov_tree, nodename,
reportclient.update_from_provider_tree(context, prov_tree, allocs)

Changes to _update_available_resource_for_node()

This is currently where all exceptions for the Resource Tracker _update periodic task are caught, logged, and otherwise ignored. We will add new except conditions for reshape-related exceptions that will actually blow up the compute service (i.e. not log-and-otherwise-ignore). These exceptions should only legitimately reach this method on startup.

Placement POST /reshaper

In a new placement microversion, a new POST /reshaper operation will be introduced. The payload is of the form:

  "inventories": {
    $RP_UUID: {
      # This is the exact payload format for
      # PUT /resource_provider/$RP_UUID/inventories.
      # It should represent the final state of the entire set of resources
      # for this provider. In particular, omitting a $RC dict will cause the
      # inventory for that resource class to be deleted if previously present.
      "inventories": { $RC: { <total, reserved, etc.> } }
      "resource_provider_generation": <gen of this RP>,
    $RP_UUID: { ... },
  "allocations": {
    # This is the exact payload format for POST /allocations
      "project_id": $PROJ_ID,
      "user_id": $USER_ID,
      # This field is part of the consumer generation series under review,
      # not yet in the published POST /allocations payload.
      "consumer_generation": $CONSUMER_GEN,
      "allocations": {
        $RP_UUID: {
          "resources": { $RC: $AMOUNT, ... }
        $RP_UUID: { ... }
    $CONSUMER_UUID: { ... }

In a single atomic transaction, placement replaces the inventories for each $RP_UUID in the inventories dict; and replaces the allocations for each $CONSUMER_UUID in the allocations dict.

Return values:

  • 204 No Content on success.

  • 409 Conflict on any provider or consumer generation conflict; or if a concurrent transaction is detected. Appropriate error codes should be used for at least the former so the caller can tell whether a fresh GET is necessary before recalculating the necessary reshapes and retrying the operation.

  • 400 Bad Request on any other failure.

Direct Interface to Placement

To make the Offline Upgrade Script possible, we need to make placement accessible by importing Python code rather than as a standalone service. The quickest path forward is to use wsgi-intercept to allow HTTP interactions, using the requests library, to work with only database traffic going over the network. This allows client code to make changes to the placement data store using the same API, but without running a placement service.

An implementation of this, as a context manager called PlacementDirect, is merged. The context manager accepts an oslo config, populated by the caller. This allows the calling code to control how it wishes to discover configuration settings, most importantly the database being used by placement.

This implementation provides a quick solution to the immediate needs of offline use of Placement POST /reshaper while allowing options for prettier solutions in the future.

Offline Upgrade Script

To facilitate Fast Forward Upgrades, we will provide a script that can perform this reshaping while all services (except databases) are offline. It will look like:

nova-manage placement migrate_compute_inventory

…and operate as follows, for each nodename (one, except for ironic) on the host:

  • Spin up a SchedulerReportClient with a Direct Interface to Placement.

  • Retrieve a ProviderTree via SchedulerReportClient.get_provider_tree_and_ensure_root().

  • Instantiate the appropriate virt driver.

  • Perform the algorithm noted in Resource Tracker _update, as if startup is True.

We may refer to https://review.openstack.org/#/c/501025/ for an example of an upgrade script that requires a virt driver.


Reshaper API

Alternatives to Placement POST /reshaper were discussed in the mailing list thread, the etherpad, IRC, hangout, etc. They included:

  • Don’t have an atomic placement operation - do the necessary operations one at a time from the resource tracker. Rejected due to race conditions: the scheduler can schedule against the moving inventories, based on incorrect capacity information due to the moving allocations.

  • “Lock” the moving inventories - either by providing a locking API or by setting reserved = total - while the resource tracker does the reshape. Rejected because it’s a hack; and because recovery from partial failures would be difficult.

  • “Merge” forms of the new placement operation:

    • PATCH (or POST) with RFC 6902-style "operation", "path"[, "from", "value"] instructions.

    • PATCH (or POST) with RFC 7396 semantics. The JSON payload would look like a sparse version of that described in Placement POST /reshaper, but with only changes included.

  • Other payload formats for the placement operation (see the etherpad). We chose the one we did because it reuses existing payload syntax (and may therefore be able to reuse code) and it provides a full specification of the expected end state, which is RESTy.

Direct Placement

Alternatives to the wsgi-intercept model for the Direct Interface to Placement:

  • Directly access the object methods (with some refactoring/cleanup). Rejected because we lose things like schema validation and microversion logic.

  • Create cleaner, pythonic wrappers around those object methods. Rejected (in the short term) for the sake of expediency. We might take this approach longer-term as/when the demand for direct placement expands beyond FFU scripting.

  • Use wsgi-intercept but create the pythonic wrappers outside of the REST layer. This is also a long-term option.

Reshaping Via update_provider_tree()

  • We considered passing allocations to update_provider_tree every time, but gathering the allocations will be expensive, so we needed a way to do it only when necessary. Enter ReshapeNeeded exception.

  • We considered running the check-and-reshape-if-needed algorithm on every periodic interval, but decided we should never need to do a reshape except on startup.

Data model impact


REST API impact

See Placement POST /reshaper.

Security impact


Notifications impact


Other end user impact

See Upgrade Impact.

Performance Impact

The new Placement POST /reshaper operation has the potential to be slow, and to lock several tables. Its use should be restricted to reshaping provider trees. Initially we may use the reshaper from update_from_provider_tree even if no reshape is being performed; but if this is found to be problematic for performance, we can restrict it to only reshape scenarios, which will be very rare.

Gathering allocations, particularly in large deployments, has the potential to be heavy and slow, so we only do this at compute startup, and then only if update_provider_tree indicates that a reshape is necessary.

Other deployer impact

See Upgrade Impact.

Developer impact

See Virt Drivers.

Upgrade impact

Live upgrades are covered. The Resource Tracker _update flow will run on compute start and perform the reshape as necessary. Since we do not support skipping releases on live upgrades, any virt driver-specific changes can be removed from one release to the next.

The Offline Upgrade Script is provided for Fast-Forward Upgrade. Since code is run with each release’s codebase for each step in the FFU, any virt driver-specific changes can be removed from one release to the next. Note, however, that the script must always be run since only the virt driver, running on a specific compute, can determine whether a reshape is required for that compute. (If no reshape is necessary, the script is a no-op.)



Work Items

See Proposed change.



Functional test enhancements for everyone, including gabbi tests for Placement POST /reshaper.

Live testing in Xen (naichuans) and libvirt (bauzas) via their VGPU work.

Documentation Impact




Release Name