Respecting Nova Scheduler Filters

Problem description

This specification continues ideas of nova-policies blueprint. One of global business requirements to Watcher is a requirement to respect Nova Scheduler filters at least in context of affinity/anti-affinity filters. Many corporate customers build HA architecture for their cloud applications and they want to respect virtual machine’s policies, which are represented as Nova Scheduler filters. There are the following typical filters which can be used in the VMs booting process: * To hold some VMs on different hosts (to achieve HA compatibility). * To place VMs inside of specified host aggregates or availability zones. * To place VMs only on enabled and core-compatible hosts.

As of now, Nova doesn’t expose an open API to Nova Scheduler’s select_destinations method. This method is available for internal Nova’s usage and it isn’t good architectural practice to access this method from a third-party service like Watcher.

Use Cases

As an openstack operator, I want to execute audit’s with taking into account Nova Scheduler filters.

Proposed change

The main proposition is to add Watcher Filters pluggable system, that would use set of Nova-compatible filters in auditing process. We can’t import them directly (cause OpenStack projects can import only project clients to interact to each other), so we need to implement each filter in the way Nova does. Nova Scheduler use internal objects host_state and spec_obj that represents host and instance attributes respectively. We can replace these objects with appropriate nodes from CDM. CDM consumes notifications via notification bus system. Extending attributes in CDM nodes will depend on specific filter. If we can consume notifications of changing availability zones or host aggregates, we can add them to CDM as host attributes. Otherwise, we can get them by API calls.

Primary logic should be saved to save compatibility with appropriate Nova’s filters. Let’s take a look at Nova’s DifferentHost filter:

def host_passes(self, host_state, spec_obj):
    affinity_uuids = spec_obj.get_scheduler_hint('different_host')
    if affinity_uuids:
        overlap = utils.instance_uuids_overlap(host_state, affinity_uuids)
        return not overlap
    # With no different_host key
    return True

There should be created filter subfolder in decision_engine folder. It will contain and filters/ folder with filter plugins. Filter Manager will load enabled plugins and pass virtual machine along with destination host through the list of the loaded plugins. Instance of FilterManager class should be added to BaseStrategy class as class property to let each strategy filtering VMs. Strategies should be reworked to get support for filters. Method self.verify_destination(instance, host) can be added to BaseStrategy class to simplify its usage for other strategies. There cannot be common point for using filters in strategies because of unique workflow of each one.

In the current situation it’s a trade-off between architectural design and business requirements. I recommend to take it as temporary solution until we find the better one.


Continue to use Watcher scope to segregate resources of cloud cluster.

Another solution is enhance-watcher-applier-engine blueprint. Instead of making exact action, we make all the probable actions and give the decision to Nova Scheduler.

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact

Watcher config file should contain filters option with dict of enabled filters by strategies like:

{"basic": "AvailabilityZoneFilter, DifferentHostFilter",
 "zone_migration": "AvailabilityZoneFilter"}.

By default there should be AllHostsFilter associated with every strategy to pass every virtual machine without limitations.

Developer impact




Primary assignee:


Work Items

  • Add documentation page regarding filters

  • Update documentation for strategy developers

  • Add filter manager to watcher/decision_engine/

  • Add affinity/anti-affinity, host aggregates and availability zone filters

  • Add unit and functional tests

  • Update configuration file to support filters option.




Appropriate unit and functional tests should be added.

Documentation Impact

  • Add documentation page regarding filters

  • Update documentation for strategy developers