Add action execution interface

Add analogy to datasource driver for services that can execute actions.

Problem description

Datasource drivers pull information from cloud services, allowing Congress to read the current state of the cloud (e.g. the list of all servers). Some cloud services also have the capacity to change the state of the cloud (e.g. create a new server). Today there is no way for the policy engine to ask for changes to be made in the state of the cloud. Congress needs an interface for cloud services that allows it to execute actions that change the state of the cloud.

Proposed change

This change will create a new (mix-in) class called ExecutionDriver that has one main function: execute(name, positional-args, named-args). That interface takes the name of the action, a list of its positional arguments, and a dictionary of its named arguments and executes that action on the cloud service.

This change will also overload the functionality of




so that a call to ‘execute’ by the policy engine will send a DSE message to the appropriate service to execute the ExecutionDriver:execute() function. This will be a unicast message from the policy engine to the appropriate service on the DSE. That service will be included in the name of the action, e.g. ‘nova:disconnectNetwork’ is an action that is sent to the ‘nova’ service.

As a starting point, this change will include an implementation of the ExecutionDriver mixin within datasources/ (or another service that the primary assignee chooses).


An alternative is to make the ExecutionDriver class not a mix-in but a standalone class, thereby having two separate files for each cloud service, e.g. and

Treating the ability to execute a service as an interface is beneficial because in reality services typically have some functionality that allows them to read the state of the cloud and other functionality that allows them to change the state of the cloud. Treating them as different files makes them seem to be separate services.

Another alternative choice in the design is to use publish/subscribe to communicate action execution between the policy engine and the instance of ExecutionDriver. Publish/subscribe has the drawback that it could result in multiple services subscribing to the same action (perhaps by accident) and a single ‘execute’ command producing several API calls modifying the state of the cloud. The unicast choice eliminates this as a potential problem (though admittedly eliminates the possibility of a use case where there are legitimately 2 different services that need to be informed of the action).



Policy Actions

This is a step toward reactive enforcement.

Data Sources

Eventually all data sources will want to implement the ExecutionDriver interface.

Data model impact


REST API impact

We will want to expose the policy/ command via the API.

Question: Should we rename the “data-sources” in the API to “cloud-services”?

If so, then we’d have…

Execute an action on a specified service.

POST v1/cloud-services/nova?action=execute -d {'name': 'disconnectNetwork',
                                               'args': ['vm123', 'net456'],
                                               'options': {}}

Execute an action as if the given policy dictated it should be executed.

POST v1/policies/alice?action=execute -d {'name': 'nova:disconnectNetwork',
                                          'args': ['vm123', 'net456'],
                                          'options': {}}

These API calls will have an empty response (HTTP code 204 perhaps) or an appropriate error response if there was an error raised before the API call was actually made. These API calls will return before the call we are executing returns.

Security impact

This change gives anyone with the ability to write policy the ability to execute API calls using the same rights Congress has been granted. Since we typically run Congress with administrative rights, we need to secure Congress authentication properly. But that is already accomplished because we use the standard Keystone authentication system.

Notifications impact


Other end user impact

python-congressclient would need a new endpoint if we expose the execute() functionality via the API.

Performance impact

Because the DatasourceDriver and ExecutionDriver will most often be implemented as part of the same DSE instance, a poorly written ExecutionDriver could impair the ability of that driver to properly read information via the DatasourceDriver interface. The two pieces of functionality run in a single thread, i.e. either one runs or the other runs. This is actually beneficial because we would not want the driver to be pulling new data at the same time it executes a change in that data.

Other deployer impact


Developer impact




Primary assignee:

<launchpad-id or None>

Other contributors:

<launchpad-id or None>

Work items

  • Rename folder congress/datasources to congress/cloudservices and change etc/congress/datasources.conf.sample appropriately. There may be other places that reference ‘datasources’ explicitly.

  • Add congress/cloudservices/ to include the mixin class ExecutionDriver

  • Implement policy/ to send a unicast message to the appropriate service or raise an error if that service does not exist on the bus.

  • Implement the ExecutionDriver for an existing service, e.g. Nova.




  • Unit tests that ensure a call to DseRuntime.execute() invokes the appropriate ExecutionDriver.execute().

  • Tempest tests that invoke execute() and ensure the proper change actually happens.

Documentation impact

Need to add description of actions and action format to docs, along with documentation for execute().