Example Spec - The title of your blueprint

Include the URL of your launchpad blueprint:


Introduction paragraph – why are we doing anything? A single paragraph of prose that operators can understand. The title and this first paragraph should be used as the subject line and body of the commit message respectively.

Some notes about the nova-spec and blueprint process:

  • Not all blueprints need a spec. For more information see https://docs.openstack.org/nova/latest/contributor/blueprints.html#specs

  • The aim of this document is first to define the problem we need to solve, and second agree the overall approach to solve that problem.

  • This is not intended to be extensive documentation for a new feature. For example, there is no need to specify the exact configuration changes, nor the exact details of any DB model changes. But you should still define that such changes are required, and be clear on how that will affect upgrades.

  • You should aim to get your spec approved before writing your code. While you are free to write prototypes and code before getting your spec approved, its possible that the outcome of the spec review process leads you towards a fundamentally different solution than you first envisaged.

  • But, API changes are held to a much higher level of scrutiny. As soon as an API change merges, we must assume it could be in production somewhere, and as such, we then need to support that API change forever. To avoid getting that wrong, we do want lots of details about API changes upfront.

Some notes about using this template:

  • Your spec should be in ReSTructured text, like this template.

  • Please wrap text at 79 columns.

  • The filename in the git repository should match the launchpad URL, for example a URL of: https://blueprints.launchpad.net/nova/+spec/awesome-thing should be named awesome-thing.rst

  • Please do not delete any of the sections in this template. If you have nothing to say for a whole section, just write: None

  • For help with syntax, see http://sphinx-doc.org/rest.html

  • To test out your formatting, build the docs using tox and see the generated HTML file in doc/build/html/specs/<path_of_your_file>

  • If you would like to provide a diagram with your spec, ascii diagrams are required. http://asciiflow.com/ is a very nice tool to assist with making ascii diagrams. The reason for this is that the tool used to review specs is based purely on plain text. Plain text will allow review to proceed without having to look at additional files which can not be viewed in gerrit. It will also allow inline feedback on the diagram itself.

  • If your specification proposes any changes to the Nova REST API such as changing parameters which can be returned or accepted, or even the semantics of what happens when a client calls into the API, then you should add the APIImpact flag to the commit message. Specifications with the APIImpact flag can be found with the following query:


Problem description

A detailed description of the problem. What problem is this blueprint addressing?

Use Cases

What use cases does this address? What impact on actors does this change have? Ensure you are clear about the actors in each use case: Developer, End User, Deployer etc.

Proposed change

Here is where you cover the change you propose to make in detail. How do you propose to solve this problem?

If this is one part of a larger effort make it clear where this piece ends. In other words, what’s the scope of this effort?

At this point, if you would like to just get feedback on if the problem and proposed change fit in nova, you can stop here and post this for review to get preliminary feedback. If so please say: Posting to get preliminary feedback on the scope of this spec.


What other ways could we do this thing? Why aren’t we using those? This doesn’t have to be a full literature review, but it should demonstrate that thought has been put into why the proposed solution is an appropriate one.

Data model impact

Changes which require modifications to the data model often have a wider impact on the system. The community often has strong opinions on how the data model should be evolved, from both a functional and performance perspective. It is therefore important to capture and gain agreement as early as possible on any proposed changes to the data model.

Questions which need to be addressed by this section include:

  • What new data objects and/or database schema changes is this going to require?

  • What database migrations will accompany this change.

  • How will the initial set of new data objects be generated, for example if you need to take into account existing instances, or modify other existing data describe how that will work.

REST API impact

Each API method which is either added or changed should have the following

  • Specification for the method

    • A description of what the method does suitable for use in user documentation

    • Method type (POST/PUT/GET/DELETE)

    • Normal http response code(s)

    • Expected error http response code(s)

      • A description for each possible error code should be included describing semantic errors which can cause it such as inconsistent parameters supplied to the method, or when an instance is not in an appropriate state for the request to succeed. Errors caused by syntactic problems covered by the JSON schema definition do not need to be included.

    • URL for the resource

      • URL should not include underscores, and use hyphens instead.

    • Parameters which can be passed via the url

    • JSON schema definition for the request body data if allowed

      • Field names should use snake_case style, not CamelCase or MixedCase style.

    • JSON schema definition for the response body data if any

      • Field names should use snake_case style, not CamelCase or MixedCase style.

  • Example use case including typical API samples for both data supplied by the caller and the response

  • Discuss any policy changes, and discuss what things a deployer needs to think about when defining their policy.

Example JSON schema definitions can be found in the Nova tree http://git.openstack.org/cgit/openstack/nova/tree/nova/api/openstack/compute/schemas

Note that the schema should be defined as restrictively as possible. Parameters which are required should be marked as such and only under exceptional circumstances should additional parameters which are not defined in the schema be permitted (eg additionaProperties should be False).

Reuse of existing predefined parameter types such as regexps for passwords and user defined names is highly encouraged.

Security impact

Describe any potential security impact on the system. Some of the items to consider include:

  • Does this change touch sensitive data such as tokens, keys, or user data?

  • Does this change alter the API in a way that may impact security, such as a new way to access sensitive information or a new way to login?

  • Does this change involve cryptography or hashing?

  • Does this change require the use of sudo or any elevated privileges?

  • Does this change involve using or parsing user-provided data? This could be directly at the API level or indirectly such as changes to a cache layer.

  • Can this change enable a resource exhaustion attack, such as allowing a single API interaction to consume significant server resources? Some examples of this include launching subprocesses for each connection, or entity expansion attacks in XML.

For more detailed guidance, please see the OpenStack Security Guidelines as a reference (https://wiki.openstack.org/wiki/Security/Guidelines). These guidelines are a work in progress and are designed to help you identify security best practices. For further information, feel free to reach out to the OpenStack Security Group at openstack-security@lists.openstack.org.

Notifications impact

Please specify any changes to notifications. Be that an extra notification, changes to an existing notification, or removing a notification.

Consider proposing changes to the versioned notifications:

  • When the feature adds or removes fields to the API responses. For example when the feature adds a new field to the GET /servers API response consider adding similar information to the payload of the instance action notifications

  • When the feature adds a new action to the existing API entities. For example adding a new action to the server might mean you want to emit a corresponding new instance action notification

  • When the feature adds a new resource (noun) to the REST API consider adding new notifications about the creation and deletion of such resource

Other end user impact

Aside from the API, are there other ways a user will interact with this feature?

  • Does this change have an impact on python-novaclient and openstack client? What does the user interface there look like?

Performance Impact

Describe any potential performance impact on the system, for example how often will new code be called, and is there a major change to the calling pattern of existing code.

Examples of things to consider here include:

  • A periodic task might look like a small addition but if it calls conductor or another service the load is multiplied by the number of nodes in the system.

  • Scheduler filters get called once per host for every instance being created, so any latency they introduce is linear with the size of the system.

  • A small change in a utility function or a commonly used decorator can have a large impacts on performance.

  • Calls which result in a database queries (whether direct or via conductor) can have a profound impact on performance when called in critical sections of the code.

  • Will the change include any locking, and if so what considerations are there on holding the lock?

Other deployer impact

Discuss things that will affect how you deploy and configure OpenStack that have not already been mentioned, such as:

  • What config options are being added? Should they be more generic than proposed (for example a flag that other hypervisor drivers might want to implement as well)? Are the default values ones which will work well in real deployments?

  • Is this a change that takes immediate effect after its merged, or is it something that has to be explicitly enabled?

  • If this change is a new binary, how would it be deployed?

  • Please state anything that those doing continuous deployment, or those upgrading from the previous release, need to be aware of. Also describe any plans to deprecate configuration values or features. For example, if we change the directory name that instances are stored in, how do we handle instance directories created before the change landed? Do we move them? Do we have a special case in the code? Do we assume that the operator will recreate all the instances in their cloud?

Developer impact

Discuss things that will affect other developers working on OpenStack, such as:

  • If the blueprint proposes a change to the driver API, discussion of how other hypervisors would implement the feature is required.

Upgrade impact

Describe any potential upgrade impact on the system, such as:

  • If this change adds a new feature to the compute host that the controller services rely on, the controller services may need to check the minimum compute service version in the deployment before using the new feature. For example, in Ocata, the FilterScheduler did not use the Placement API until all compute services were upgraded to at least Ocata.

  • While we strive to have feature parity between all virt drivers, it is not uncommon for one virt driver to implement a new feature exposed out of the API before the others. For example, extending the size of an attached volume. Since Nova does not yet have any type of sophisticated capabilities API so a user can know what actions can be performed on a given instance, consider adding a new policy rule to at least let operators that cannot support a virt-specific feature disable it in their cloud which is at least presented to the user in an understandable way by getting a 403 Forbidden error.

  • Nova supports N-1 version nova-compute services for rolling upgrades. Does the proposed change need to consider older code running that may impact how the new change functions, for example, by changing or overwriting global state in the database? This is generally most problematic when making changes that involve multiple compute hosts, like move operations such as migrate, resize, unshelve and evacuate.



Who is leading the writing of the code? Or is this a blueprint where you’re throwing it out there to see who picks it up?

If more than one person is working on the implementation, please designate the primary author and contact.

Primary assignee:

<launchpad-id or None>

Other contributors:

<launchpad-id or None>

Feature Liaison

Ideally feature work is sponsored by a member of the nova core team or other experienced and active nova developer. The purpose of a liaison is to:

  • Mentor developers through the arcana of nova’s development processes.

  • Advocate for (aka “care about”) the feature to the rest of the nova team.

  • Be the initial go-to for reviews.

See the Feature Liaison FAQ for more details.

Feature liaison:

<name and/or nick>

  • Feature liaison is optional. However we suggest to find a liaison for your feature as it will help getting your feature merged. The Feature Liaison FAQ has details about how to find a liaison for your work.

  • If you do not already have agreement from a nova developer to act as your liaison, you may write “Liaison Needed” here and/or in your commit message.

  • If you are a core or experienced nova dev, you need not have a separate liaison; if you wish, you may just assign yourself, or put “None”/”N/A”.

Work Items

Work items or tasks – break the feature up into the things that need to be done to implement it. Those parts might end up being done by different people, but we’re mostly trying to understand the timeline for implementation.


  • Include specific references to specs and/or blueprints in nova, or in other projects, that this one either depends on or is related to.

  • If this requires functionality of another project that is not currently used by Nova (such as the glance v2 API when we previously only required v1), document that fact.

  • Does this feature require any new library dependencies or code otherwise not included in OpenStack? Or does it depend on a specific version of library?


Please discuss the important scenarios needed to test here, as well as specific edge cases we should be ensuring work correctly. For each scenario please specify if this requires specialized hardware, a full openstack environment, or can be simulated inside the Nova tree.

Please discuss how the change will be tested. We especially want to know what tempest tests will be added. It is assumed that unit test coverage will be added so that doesn’t need to be mentioned explicitly, but discussion of why you think unit tests are sufficient and we don’t need to add more tempest tests would need to be included.

Is this untestable in gate given current limitations (specific hardware / software configurations available)? If so, are there mitigation plans (3rd party testing, gate enhancements, etc).

Documentation Impact

Which audiences are affected most by this change, and which documentation titles on docs.openstack.org should be updated because of this change? Don’t repeat details discussed above, but reference them here in the context of documentation for multiple audiences. For example, the Operations Guide targets cloud operators, and the End User Guide would need to be updated if the change offers a new feature available through the CLI or dashboard. If a config option changes or is deprecated, note here that the documentation needs to be updated to reflect this specification’s change.


Please add any useful references here. You are not required to have any reference. Moreover, this specification should still make sense when your references are unavailable. Examples of what you could include are:

  • Links to mailing list or IRC discussions

  • Links to notes from a summit session

  • Links to relevant research, if appropriate

  • Related specifications as appropriate (e.g. if it’s an EC2 thing, link the EC2 docs)

  • Anything else you feel it is worthwhile to refer to


Optional section intended to be used each time the spec is updated to describe new design, API or any database schema updated. Useful to let reader understand what’s happened along the time.


Release Name