There are a number of skills that make a great Neutron developer: writing good code, reviewing effectively, listening to peer feedback, etc. The objective of this document is to describe, by means of examples, the pitfalls, the good and bad practices that ‘we’ as project encounter on a daily basis and that make us either go slower or accelerate while contributing to Neutron.
By reading and collaboratively contributing to such a knowledge base, your development and review cycle becomes shorter, because you will learn (and teach to others after you) what to watch out for, and how to be proactive in order to prevent negative feedback, minimize programming errors, writing better tests, and so on and so forth...in a nutshell, how to become an effective Neutron developer.
The notes below are meant to be free-form and brief by design. They are not meant to replace or duplicate OpenStack documentation <http://docs.openstack.org>, or any project-wide documentation initiative like peer-review notes <http://docs.openstack.org/infra/manual/developers.html#peer-review> or the team guide <http://docs.openstack.org/project-team-guide/>. For this reason, references are acceptable and should be favored, if the shortcut is deemed useful to expand on the distilled information. We will try to keep these notes tidy by breaking them down into sections if it makes sense. Feel free to add, adjust, remove as you see fit. Please do so, taking into consideration yourself and other Neutron developers as readers. Capture your experience during development and review and add any comment that you believe will make your life and others’ easier.
Happy hacking!
Document common pitfalls as well as good practices done during database development.
first() does not raise an exception.
Do not get an object to delete it. If you can delete() on the query object. Read the warnings for more details about in-python cascades.
If you add a relationship to a Neutron object that will be referenced in the majority of cases where the object is retrieved, be sure to use the lazy=’joined’ parameter to the relationship so the related objects are loaded as part of the same query. Otherwise, the default method is ‘select’, which emits a new DB query to retrieve each related object adversely impacting performance. For example, see this patch which resulted in a significant improvement since router retrieval functions always include the gateway interface.
Conversely, do not use lazy=’joined’ if the relationship is only used in corner cases because the JOIN statement comes at a cost that may be significant if the relationship contains many objects. For example, see this patch which reduced a subnet retrieval by ~90% by avoiding a join to the IP allocation table.
When writing extensions to existing objects (e.g. Networks), ensure that they are written in a way that the data on the object can be calculated without additional DB lookup. If that’s not possible, ensure the DB lookup is performed once in bulk during a list operation. Otherwise a list call for a 1000 objects will change from a constant small number of DB queries to 1000 DB queries. For example, see this patch which changed the availability zone code from the incorrect style to a database friendly one.
...
Sometimes in code we use the following structures:
def create():
with context.session.begin(subtransactions=True):
create_something()
try:
_do_other_thing_with_created_object()
except Exception:
with excutils.save_and_reraise_exception():
delete_something()
def _do_other_thing_with_created_object():
with context.session.begin(subtransactions=True):
....
The problem is that when exception is raised in _do_other_thing_with_created_object it is caught in except block, but the object cannot be deleted in except section because internal transaction from _do_other_thing_with_created_object has been rolled back. To avoid this nested transactions should be used. For such cases help function safe_creation has been created in neutron/db/common_db_mixin.py. So, the example above should be replaced with:
_safe_creation(context, create_something, delete_someting,
_do_other_thing_with_created_object)
Where nested transaction is used in _do_other_thing_with_created_object function.
The _safe_creation function can also be passed the ``transaction=False argument to prevent any transaction from being created just to leverage the automatic deletion on exception logic.
Document common pitfalls as well as good practices done when invoking system commands and interacting with linux utils.
Document common pitfalls as well as good practices done when using eventlet and monkey patching.
Document common pitfalls as well as good practices done when writing tests, any test. For anything more elaborate, please visit the testing section.
Document common pitfalls as well as good practices done when extending the RPC Interfaces.
Document common pitfalls as well as good practices done when writing code that needs to process a lot of data.
Document common pitfalls as well as good practices done when instrumenting your code.
Document common pitfalls as well as good practices done when writing code that is used to interface with other projects, like Keystone or Nova.
Document common pitfalls as well as good practices done when writing docstrings.
Document common nits and pedantic comments to watch out for.
Document common pitfalls as well as good practices done when writing commit messages. For more details see Git commit message best practices <https://wiki.openstack.org/wiki/GitCommitMessages.
Document common pitfalls as well as good practices done when dealing with OpenStack CI.