Flag object to be ignore during a diff

[dgarros]

Environment

• DSync version: master

Proposed Functionality

Add a flag per DSyncModel object to indicate that a specific object should be ignored during the diff/sync.

Use Case

We have a situation right now for the network-importer where the netbox adapter is getting some cables from netbox and for various reasons these cables should be ignore because they can't be touched ...
Currently we don't have a way to solve this situation, if the objects exist in the network adapter but are removed from the netbox adapter, they will be flag as MISSING and the sync will try to create them.
Adding a flag per object that indicate if the object should be ignore all together would solve this situation and I can imagine other use cases where it will be useful to explicitly ignore an object.

[glennmatthews]

Related to #9.

[dgarros]

@glennmatthews I've been thinking about how to implement this one and I think we should add a generic Flag system at the DSyncModel level, somewhat similar to what was introduced in #10 for the Diff object.

The idea is to capture how each DSyncModel/DiffElement should be handled during a diff/sync:

• Explicitly disable CREATE, UPDATE or DELETE on a given object
• IGNORE an object all together from the diff/sync (initial scope of this issue)
• Control how children should be managed on CREATE and/or DELETE (do we need to delete children if the parent has been deleted already, do we need to delete children before or after the parent ...)
  It should be possible to have multiple flags defined together as there are multiple behaviors to capture.

We don't need to support them all right away but it would be good if we can get a system in place that will support that in the future.

[glennmatthews]

Makes sense. So thinking out loud we could potentially have the following flags as a starting point:

NO_CREATE
NO_UPDATE
NO_DELETE
IGNORE_COMPLETELY = NO_CREATE | NO_UPDATE | NO_DELETE
SKIP_CHILDREN_ON_DELETE
DELETE_CHILDREN_FIRST

As I see it these would be instance attributes of a DSyncModel (which could also be set to default values on a class-by-class basis, e.g. if we know that all instances of ModelX should have SKIP_CHILDREN_ON_DELETE) which would then need to be propagated into a corresponding DiffElement when it's created.

One tricky point is that some of these flags need to be set when there isn't a DSyncModel instance (e.g., NO_CREATE would only be meaningful when there isn't an instance in the target system) so we may need to make sure that we can have class-level flag settings and have a DiffElement pick those up as defaults when there's no model instance....

[glennmatthews]

Keeping things simple for the first implementation, it seems like we currently need just two flags:

• SKIP_CHILDREN_ON_DELETE or CHILDREN_ARE_AUTODELETED - when deleting this object, its children are auto-deleted, so we don't need to explicitly delete them (Example: deleting a NetBox Interface automatically deletes the IPAddress associated with it).
  • This is currently managed via error-handling (we try to delete the child IPAddress and get a "does not exist" response, and treat that as good enough to consider a successful deletion) but it would be more elegant and efficient to not even attempt to delete the child object(s) in the first place.
• IGNORE - when rendering diffs, do not display any diffs for this object or its children; when syncing, do not attempt any changes to this element or its children. (Example: particular device cables, as in the original description of this issue above)