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Hi, Alexey,

Thanks for the detail example. It explains the existing mechanism of vertex
creation well.

So it looks like each resource type will have a primary datasource, e.g.
nova.host for nova.host, nova.intance for nova.instance, that holds full
details. Is that correct?

Not sure that you remember the long discussion in static driver review[1]
or not. At last, we agreed on a unified entity definition for both
`nova.host` and `switch`, no extra key to indicate it is "external" (should
create a placeholder).

If I understand it correctly, no placeholder will be created in this case.
Because we can not distinguish them from the static configuration. And the
properties of `nova.host` resource shall to be merged from `static` and
nova.host` datasources. Is that so?

[1]: https://review.openstack.org/#/c/405354/

On Wed, Dec 14, 2016 at 5:40 PM Weyl, Alexey (Nokia - IL) <
alexey.weyl at nokia.com> wrote:

> Hi Yujun,
>
>
>
> This is a good question, and let me explain for you how it works.
>
> Lets say we are supposed to get 2 entities from nova, nova.host called
> host1 and nova.instance called vm1 and vm1 is supposed to be connected to
> host1.
>
> The nova.host driver and nova.instance driver are working simultaneously
> and thus we don’t know the order in which those events will arrive.
>
> We have 2 use cases:
>
> 1.       Host1 event arrives before vm1.
>
> In this case the processor will call the transformer of nova.host and will
> create a vertex for host1 in the graph with the full details of host1.
>
> Then, vm1 event will arrive, the processor will create the vm1 vertex in
> the graph, it will update the host1 properties in the graph (because the
> host1 details that are created in nova.instance are only its basic details
> such as: category, type, id, is_deleted, is_placeholder they host1
> properties won’t be changed in the graph because those details are basic
> and can’t be changed), and then it will create an edge between vm1 and
> host1 (only the nova.instance knows to which nova.host it is connected and
> not vice versa).
>
> 2.       Vm1 event arrives before host1.
>
> In this case the processor will add vm1 to the graph, then it will add the
> host1 placeholder to the graph (so we will know to which host vm1 is
> connected) and then add the edge between them.
>
> Then when the processor will handle with the host1 event, it will just add
> some properties of the host1 vertex, and of course will change the
> is_placeholder property of host1 to false.
>
> We also has the consistency service that runs every 10 minutes (its
> configurable with the snapshot_interval) and checks if there are vertices
> that are is_placeholder=True and are in the graph more then
> 2*snapshot_interval time then it means that such a vertex of host1 for
> example doesn’t really exists and it deletes it from the graph).
>
> In addition, when we understand that we need to delete some entity from
> the graph, upon delete notification for example, then we don’t delete it
> right away, we change the is_deleted property of that entity to True, and
> we will delete it on the next run of the consistency service. The reason we
> do that, is because we need it for the evaluator, because it runs a
> subgraph matching algorithm on the graph, and if the entity that is
> supposed to be there doesn’t appear then it won’t work correctly.
>
>
>
> The best way to create a placeholder is to call the placeholder method of
> the correct transformer using the transformers array that each transformer
> class has.
>
>
>
> I hope this helps.
>
>
>
> Alexey
>
>
>
> *From:* Yujun Zhang [mailto:zhangyujun+zte at gmail.com]
> *Sent:* Wednesday, December 14, 2016 10:55 AM
> *To:* OpenStack Development Mailing List (not for usage questions)
> *Subject:* [ALU] [openstack-dev] [vitrage] how to use placeholder vertex
>
>
>
> Hi, root causers (assuming this nickname has been agreed)
>
>
>
> It seems the placeholder is created for every neighbor of an entity[1].
> I'm not sure what will happen if the neighbor entity has been created
> before.
>
>
>
> Will the graph utility handle it? Or we need to check the existence in
> transformer and deal with it?
>
>
>
> Similar question for how to create a vertex when there is already a
> placeholder with same ID
>
>
>
> [1]:
> https://github.com/openstack/vitrage/blob/master/vitrage/datasources/static_physical/transformer.py#L114
>
>
> --
>
> Yujun
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