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Re: [] Overload resolution and dynamic dispatch patch

Hi Axel

Ok, so we misunderstood the application domain.

The IA is targetted at huge models of which only large sub-models are loaded in memory.

This now makes sense of your 'what are all the meta-models' questions. If the loaded submodels consist of A objects and the unloaded submodels might contain X objects, you need to known whether an X::a relationship exists, so that when an A changes you can propagate impact to an X. This has three problems: a) you need to know all the meta-models of all the unloaded parts of the models to discover that there is an X::a relationship
b) you need to be able to identify all X objects in the unloaded submodels
c) you need to identify which X objects have an X::a relationship targetting the impacted A object

a) is soluble by requiring a closed universe of declared meta-models
b) is only soluble with some collaboration; if the unloaded objects are in a repository, the repository may partition by class and so may be able to support allInstances() without a total scan since the result of allInstances() is cached as repository control state c) is similarly soluble via a repository's allInstances() cache - but only via an inefficient large scan for a huge model

If the repository maintains an allInstances() cache, the the repository can certainly provide an allClasses()/allPackages() response to enable all meta-models to be identified for a).

I see no way of doing hidden opposites from unloaded resources without cached state resulting from a transient load of the whole model.

The problem of dynamic dispatch is much the same as hidden opposites. Any function computes using modeled state, so the source type is deducible whenever the function uses fully navigable relationships. The problem is that if the function uses forward relationships, then you cannot follow in reverse. This is the hidden opposites impact problem.


So if we have a large partial model in memory, in order to propagate impact to the unloaded part, there are three options:
a) don't - it is an unsupported capability
b) use a type-based allInstances() search of those objects that might be hidden opposites c) use an instance-based cache that supports navigation of the hidden opposite relationships

a) is easy and useful for large loadable models.

Both b) and c) rely on cached knowledge of the whole system, so if you're going to have cached knowledge surely you should provide the cache that supports efficient instance-based navigation rather than dubiously scalable type-based searches?

The instance-based cache could take a variety of forms. To support huge unloaded models, it is probably a good idea for objects to have co-objects that contain their hidden opposites. The co-objects can be unloaded and maintained in the repository, so the in-memory cost is proportional to the size of the in-memory model and the number of actual hidden opposites per object. Whenever impact spreads, you can load the impacted objects and their co-objects. The IA can of course maintain the co-object relationships automatically.

Some classes, such as OCLExpression, have a large number of hidden opposites, so the 'coOCLExpression' probably wants to use a list of (feature,object) pairs for the typically one hidden opposite that actually exists.


Once co-objects support navigation of all relationships of all loaded objects, the active system consists of unloaded (co-)objects and loaded (co-)objects, some of which are on the boundary because they have one or more relationships to unloaded objects. To prime the impact propagation, dependency analysis from all loaded objects populates the notifier chain. If at run-time an impact reaches a border object, the dynamic types of the unloaded neighbours are examined to see if impact propagation is possible and if necessary, neighbouring objects are loaded, analyzed for dependencies, notifiers installed, and the impact continues to propagate, loading only impacted unloaded objects and co-objects.


        Ed Willink

On 07/05/2012 20:44, Axel Uhl wrote:
Ok, understood. This conflicts with the way the current IA approach works. It is geared towards non-loaded resources and large, scalable repositories. The design assumes that it's impractical to load all relevant resources into memory, analyze and then memorize all dependencies for all OCL expressions for all context elements. Instead, it is analyzing the expression structure up-front, then deriving the change listeners which obviously need to be registered only on objects loaded into memory because only for those changes can be detected at all. Once a change occurs, the change is analyzed by traversing the element graph "backwards" using the expression structure. This also works for even the largest repositories because the memory required for backwards traversal doesn't grow with repository size.

It would be good if we knew which classes were generally available in the scope of a ResourceSet so as to scan their operations during backwards traversal ("traceback"). For the "traceback" approach this would consider all operations known/loaded at that time. The NavigationStep approach which is pre-computing the traceback paths would not work this way because it wouldn't know operations loaded through metamodels that become available after the IA has pre-computed the navigation steps.

What would be a good way to scan all existing operations and be notified when new subclasses with redefinitions / overrides become available? Wasn't there an open EMF bug requesting a notification mechanism for EPackage loads?

-- Axel

On 5/7/2012 9:06 PM, Ed Willink wrote:

a) unloaded Resources

In your earlier example with resource Y (unloaded) so that propagation
of 42 through resource X failed to impact:

Using my dependency analysis perspective, if the Y resource is not
loaded, I don't analyze it, I don't detect any dependencies, so I don't
install any listeners/notifiers to cause the Y resource elements to
react to changes. So I agree that it doesn't work. It's nothing to do
with EMF-based; it's just common sense.

not so common sense after all if a reference path leads through Y back
to X. Which resources from a repository an editor loads into a
ResourceSet is fairly random. JDT wouldn't be the same if errors in
Java resources not currently open in an editor remained undetected
until the resource is loaded.

Exactly. JDT knows how to open all files, and in order to report errors
it either loads everything or exploits some sort of memento of previous

It seems to be mandatory to load all resources for which any form of
analysis is to be performed. (It may be that they are unloaded retaining
only a memento, but they must be loaded to create the memento.)

Consider a doctor's patient list and the impact of a policy change to
invite all patients over 50 rather than over 60 to have a free flu
vaccination for the winter. The derived property of the number of
required injections can only be determined by examining each patients
records. If a patient's record is not loaded into the system/looked at
by a human, the accurate answer cannot be determined. It seems
unreasonable to expect to have impact or dependencies for unloaded

b) polymorphic calls

It seems that your traceback does not use the source object and so needs to consider all possible operations. This seems inefficient and may lead
to fat notifications that can be filtered. The dependency perspective
knows the source object and so need only consider the relevant

I don't understand your "dependency perspective." Maybe you can
explain using the example I gave earlier?

For the derived property

context X::derivedProperty : Integer = self.m().i

on y1:Y, which is loaded so its dependencies are analyzed.
- it is a Y, so Y::m() which is self.b.a so depends on
-- the object identity at self.b, which is currently a B
-- the object value at self.b, which is b.a, so depends on
--- the object identity at self.b.a, which is currently an A
--- the object value at self.b.a which is i, so depends on
[---- the object identity i is not relevant since it is a data type]
---- the datatype value i

Each of the dependencies needs a direct or transitive notifier chain to
cause the change to propagate and in the case of identity changes, the
notifier chain to be adjusted.



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