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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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