[openstack-dev] [nova] Consistency, efficiency, and safety of NovaObject.save()

Matthew Booth mbooth at redhat.com
Thu Nov 13 10:09:17 UTC 2014

On 13/11/14 08:52, Nikola Đipanov wrote:
> On 11/13/2014 02:45 AM, Dan Smith wrote:
>>> I’m not sure if I’m seeing the second SELECT here either but I’m less
>>> familiar with what I’m looking at.     compute_node_update() does the
>>> one SELECT as we said, then it doesn’t look like
>>> self._from_db_object() would emit any further SQL specific to that
>>> row.
>> I don't think you're missing anything. I don't see anything in that
>> object code, or the other db/sqlalchemy/api.py code that looks like a
>> second select. Perhaps he was referring to two *queries*, being the
>> initial select and the following update?
> FWIW - I think an example Matt was giving me yesterday was block devices
> where we have:
> @require_context
> def block_device_mapping_update(context, bdm_id, values, legacy=True):
>     _scrub_empty_str_values(values, ['volume_size'])
>     values = _from_legacy_values(values, legacy, allow_updates=True)
>     query =_block_device_mapping_get_query(context).filter_by(id=bdm_id)
>     query.update(values)
>     return query.first()
> which gets called from object save()

Yes, this is one example, another is Aggregate. I already had a big list
in the post and didn't want a second one.

Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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