Code Review Guide for Nova

OpenStack has a general set of code review guidelines:

What follows is a very terse set of points for reviewers to consider when looking at nova code. These are things that are important for the continued smooth operation of Nova, but that tend to be carried as “tribal knowledge” instead of being written down. It is an attempt to boil down some of those things into nearly checklist format. Further explanation about why some of these things are important belongs elsewhere and should be linked from here.


When making a change to the nova API, we should always follow the API WG guidelines rather than going for “local” consistency. Developers and reviewers should read all of the guidelines, but they are very long. So here are some key points:

  • Terms

    • project should be used in the REST API instead of tenant.

    • server should be used in the REST API instead of instance.

    • compute should be used in the REST API instead of nova.

  • Naming Conventions

    • URL should not include underscores; use hyphens (‘-’) instead.

    • The field names contained in a request/response body should use snake_case style, not CamelCase or Mixed_Case style.

  • HTTP Response Codes

    • Synchronous resource creation: 201 Created

    • Asynchronous resource creation: 202 Accepted

    • Synchronous resource deletion: 204 No Content

    • For all other successful operations: 200 OK

Config Options


The central place where all config options should reside is the /nova/conf/ package. Options that are in named sections of nova.conf, such as [serial_console], should be in their own module. Options that are in the [DEFAULT] section should be placed in modules that represent a natural grouping. For example, all of the options that affect the scheduler would be in the file, and all the networking options would be moved to


A config option should be checked for:

  • A short description which explains what it does. If it is a unit (e.g. timeouts or so) describe the unit which is used (seconds, megabyte, mebibyte, …).

  • A long description which explains the impact and scope. The operators should know the expected change in the behavior of Nova if they tweak this.

  • Descriptions/Validations for the possible values.

    • If this is an option with numeric values (int, float), describe the edge cases (like the min value, max value, 0, -1).

    • If this is a DictOpt, describe the allowed keys.

    • If this is a StrOpt, list any possible regex validations, or provide a list of acceptable and/or prohibited values.

Previously used sections which explained which services consume a specific config option and which options are related to each other got dropped because they are too hard to maintain:

Third Party Tests

Any change that is not tested well by the Jenkins check jobs must have a recent +1 vote from an appropriate third party test (or tests) on the latest patchset, before a core reviewer is allowed to make a +2 vote.

Virt drivers

At a minimum, we must ensure that any technology specific code has a +1 from the relevant third party test, on the latest patchset, before a +2 vote can be applied. Specifically, changes to nova/virt/driver/<NNNN> need a +1 vote from the respective third party CI. For example, if you change something in the VMware virt driver, you must wait for a +1 from the VMware CI on the latest patchset, before you can give that patch set a +2 vote.

This is important to ensure:

  • We keep those drivers stable

  • We don’t break that third party CI


Please note:

  • Long term, we should ensure that any patch a third party CI is allowed to vote on, can be blocked from merging by that third party CI. But we need a lot more work to make something like that feasible, hence the proposed compromise.

  • While its possible to break a virt driver CI system by changing code that is outside the virt drivers, this policy is not focusing on fixing that. A third party test failure should always be investigated, but the failure of a third party test to report in a timely manner should not block others.

  • We are only talking about the testing of in-tree code. Please note the only public API is our REST API, see: Development policies

Should I run the experimental queue jobs on this change?

Because we can’t run all CI jobs in the check and gate pipelines, some jobs can be executed on demand, thanks to the experimental pipeline. To run the experimental jobs, you need to comment your Gerrit review with “check experimental”.

The experimental jobs aim to test specific features, such as LXC containers or DVR with multiple nodes. Also, it might be useful to run them when we want to test backward compatibility with tools that deploy OpenStack outside Devstack (e.g. TripleO, etc). They can produce a non-voting feedback of whether the system continues to work when we deprecate or remove some options or features in Nova.

The experimental queue can also be used to test that new CI jobs are correct before making them voting.

Database Schema

  • Use the utf8 charset only where necessary. Some string fields, such as hex-stringified UUID values, MD5 fingerprints, SHA1 hashes or base64-encoded data, are always interpreted using ASCII encoding. A hex-stringified UUID value in latin1 is 1/3 the size of the same field in utf8, impacting performance without bringing any benefit. If there are no string type columns in the table, or the string type columns contain only the data described above, then stick with latin1.

Microversion API

If a new microversion API is added, the following needs to happen:

  • A new patch for the microversion API change in both python-novaclient and in python-openstackclient should be submitted before the microversion change in Nova is merged. See Adding support for a new microversion in python-novaclient for more details. See also Add support for ‘server group create –rule’ parameter patch as example how to support a new microversion in the openstack client.

  • If the microversion changes the response schema, a new schema and test for the microversion must be added to Tempest. The microversion change in Nova should not be merged until the Tempest test is submitted and at least passing; it does not need to be merged yet as long as it is testing the Nova change via Depends-On. The Nova microversion change commit message should reference the Change-Id of the Tempest test for reviewers to identify it.


Release Notes

A release note is required on changes that have upgrade impact, security impact, introduce a new feature, fix Critical bugs, or fix long-standing bugs with high importance. See Release Notes for details on how to create a release note, each available section and the type of content required.