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 watcher-spec and blueprint process:

  • Not all blueprints need a spec. For more information see http://docs.openstack.org/developer/nova/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/watcher/+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 Watcher 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 Watcher, 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
    • Parameters which can be passed via the url
    • JSON schema definition for the body data if allowed
    • JSON schema definition for the response data if any
  • 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.

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 (e.g., additionalProperties 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.

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-watcherclient? 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? 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.



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>

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 Watcher, 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 Watcher (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 Watcher 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

What is the impact on the docs team of this change? Some changes might require donating resources to the docs team to have the documentation updated. Don’t repeat details discussed above, but please reference them here.


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 for liberty 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 Description
Liberty Introduced