Faster API list responses

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The Neutron API currently performs a policy check for each attribute of each element in a list response; this is penalizing response times.

Problem description

Slowness in API responses has been reported in a few bug reports; it has also been observed recently with several plugins. The root cause of this issue is that a policy engine check is performed for every attribute of every resource returned in a response. A list response can contain a lot of items; this is true in particular for ports, especially when the API requests are executed with admin credentials without filters. In these cases the huge number of policy checks performed becomes a non-negligible scalability limiting factor. For a list operation, post-plugin operations (ie: policy checks) take about 50% of the time spent in the plugin.

While this problem is not difficult to solve, its solution might require a change which can result in a slightly different policy engine behaviour. The API layer for list responses puts every item through policy checks to check which ones should not be visible to the user at all. However it also puts every attribute through a policy check to exclude those which should not be visible to the user, such as provider attributes for regular users. Doing this for every resource might make sense only if the visibility of an attribute depends on the data in the resource itself. For instance a policy that shows port binding attributes could be defined for all the ports whose name is “ernest”. This might appear as great flexibility, but however it does not make sense at all. A list operation indeed should return the same set of attributes for each resource in the response.

Proposed change

Policy checks should be used to determine the list of attributes to show only once for every response; this attribute list should then be used for every resource to be included in the response for the list operation.

In order for this to work there should not be any attribute-level policy which relies on the resource value; this limitation seems to be fair.


Alternative #1: Move attribute-level checks back in the plugin. This was how the software worked before the Havana release. This does not feel right at all as this will allow, in theory, to have deployments whose API behaviour differs according to the plugin they’re running. All the authZ work should be performed in the common API layer.

Alternative #2: Iterate over items and run policy check only if the attribute’s policy is dependent on the resource data. This will require a level of introspection in the policy engine that we currently do not have and will probably never have. Also, it does not feel right that different items in the same list response might end up having different attributes. A list response is like a table, with a fixed number of columns.

Data model impact

No Data model change.

REST API impact

No API change.

Security impact

This blueprint will not change the logic of authZ processing. However, deployments that have defined custom policies for attributes whose evaluation result depends on the value of the resource being analyses might be affected.

This should be documented as it might result in attributes being returned in responses whereas the deployer’s wish was to hide them.

Notifications impact

No impact.

Other end user impact

No further interaction.

Performance Impact

An estimated 33% improvement on list responses. Prototype code for this blueprint has been tested with list responses to 4,000 items.

Other deployer impact

Deployers should be aware that attribute-level policies dependent on resurce values (eg.: field checks) should not be performed anymore.

Developer impact

No developer impact. Limited chance of merge conflicts.



Primary assignee:


Work Items

This will be implemented with a single, relatively small, patch.


No dependency.


The code being altered is already covered by existing unit and integration tests. New unit tests might be defined as needed. New integration tests should not be needed.

Documentation Impact

Guidelines for managing Neutron authZ policies should be updated.