Code Review Guide for Nova

This 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.

Upgrade-Related Concerns

RPC API Versions

• If an RPC method is modified, the following needs to happen:

• The manager-side (example: compute/manager) needs a version bump
• The manager-side method needs to tolerate older calls as well as newer calls
• Arguments can be added as long as they are optional. Arguments cannot be removed or changed in an incompatible way.
• The RPC client code (example: compute/rpcapi.py) needs to be able to honor a pin for the older version (see self.client.can_send_version() calls). If we are pinned at 1.5, but the version requirement for a method is 1.7, we need to be able to formulate the call at version 1.5.
• Methods can drop compatibility with older versions when we bump a major version.

• RPC methods can be deprecated by removing the client (example: compute/rpcapi.py) implementation. However, the manager method must continue to exist until the major version of the API is bumped.

Object Versions

• If a tracked attribute (i.e. listed in fields) or remotable method is added, or a method is changed, the object version must be bumped. Changes for methods follow the same rules as above for regular RPC methods. We have tests to try to catch these changes, which remind you to bump the version and then correct the version-hash in the tests.
• Field types cannot be changed. If absolutely required, create a new attribute and deprecate the old one. Ideally, support converting the old attribute to the new one with an obj_load_attr() handler. There are some exceptional cases where changing the type can be allowed, but care must be taken to ensure it does not affect the wireline API.
• New attributes should be removed from the primitive in obj_make_compatible() if the attribute was added after the target version.
• Remotable methods should not return unversioned structures wherever possible. They should return objects or simple values as the return types are not (and cannot) be checked by the hash tests.
• Remotable methods should not take complex structures as arguments. These cannot be verified by the hash tests, and thus are subject to drift. Either construct an object and pass that, or pass all the simple values required to make the call.
• Changes to an object as described above will cause a hash to change in TestObjectVersions. This is a reminder to the developer and the reviewer that the version needs to be bumped. There are times when we need to make a change to an object without bumping its version, but those cases are only where the hash logic detects a change that is not actually a compatibility issue and must be handled carefully.

Database Schema

• Changes to the database schema must generally be additive-only. This means you can add columns, but you can’t drop or alter a column. We have some hacky tests to try to catch these things, but they are fragile. Extreme reviewer attention to non-online alterations to the DB schema will help us avoid disaster.
• Dropping things from the schema is a thing we need to be extremely careful about, making sure that the column has not been used (even present in one of our models) for at least a release.
• Data migrations must not be present in schema migrations. If data needs to be converted to another format, or moved from one place to another, then that must be done while the database server remains online. Generally, this can and should be hidden within the object layer so that an object can load from either the old or new location, and save to the new one.

REST API

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

Location

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 scheduler.py file, and all the networking options would be moved to network.py.

Implementation

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: http://lists.openstack.org/pipermail/openstack-dev/2016-May/095538.html

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 XenAPI virt driver, you must wait for a +1 from the XenServer 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

Notes

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 python-novaclient side should be submitted.
• 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.

Notifications

• Every new notification type shall use the new versioned notification infrastructure documented in Notifications in Nova

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.