Revision as of 04:36, 10 January 2013

Architecture

Comparison Process

This is the overview of the comparison process as a whole. Each of the six phases of the comparison process of EMF Compare are briefly defined on the Overview, and a much more in-depth explanation will be given below, in our explanations of the default behavior of EMF Compare.

Project Architecture

EMF Compare is built on top of the Eclipse platform. We depend on the Eclipse Modeling Framework (EMF), the Eclipse Compare framework and, finally, Eclipse Team, the framework upon which the repository providers (EGit, CVS, Subversive...) are built.

The EMF Compare extensions target specific extensions of the modeling framework : UML, the Graphical Modeling Framework (and its own extensions, papyrus, ecoretools, ...).

Whilst we are built atop bricks that are tightly coupled with the eclipse platform, it should be noted that the core of EMF Compare can be run in a standalone application with no runtime dependencies towards Eclipse; as can EMF itself.

The Comparison Model

PENDING describe the metamodel concepts

Core Concepts

Proxy Resolution

PENDING why does EMF Compare avoid proxy resolving, how?

Equality Helper

PENDING what's this?

Longest Common Subsequence

PENDING description of the algorithm, why do we use it, references

Default behavior and extensibility

All main components of EMF Compare have been designed for extensibility. Some are only extensible when comparing models through your own actions, some can be customized globally for a given kind of model or metamodel... We'll outline the customization options of all 6 comparison phases in this section. (Any dead link? Report them on the forum!)

Model Resolving

PENDING description of the phase, extensibility (use of the modelProviders extension point, custom ext point of compare)

Match

Before we can compute differences between two versions of a same Object, we must determine which are actually the "same" Object. For example, let's consider that my first model contains a Package P1 which itself contains a class C1; and that my second model contains a package P1 which contains a class C1. It may seem obvious for a human reader that "P1" and "C1" are the same object in both models. However, since their features might have changed in-between the two versions (for example, the "C1" might now be abstract, or it could have been converted to an Interface), this "equality" is not that obvious for a computer.

The goal of the "Match" phase is to discover which of the objects from model 2 match with which objects of model 1. In other words, this is when we'll say that two objects are one and the same, and that any difference between the two sides of this couple is actually a difference that should be reported as such to the user.

By default, EMF Compare browses through elements that are within the scope, and matches them through their identifier if they have one, r through a distance mechanism for all elements that have none. If the scope contains resources, EMF Compare will first match those two-by-two before browsing through all of their contained objects.

EMF Compare "finds" the identifier of given object through a basic function that can be found in IdentifierEObjectMatcher.DefaultIDFunction. In short, if the object is a proxy, its identifier is its URI fragment. Otherwise its functional ID (in ecore, an attribute that serves as an identifier) takes precedence over its XMI ID (the identifier it was given in the XMI file). If the object is not a proxy and has neither functional nor XMI identifier, then the default behavior will simply pass that object over to the proximity algorithms so that it can be matched through its distance with other objects.

PENDING : brief description of the proximity algorithm

This behavior can be customized in a number of ways.

Overriding the Match engine

The most powerful (albeit most cumbersome) customization you can implement is to override the match engine EMF Compare uses. To this end you can either implement the whole contract, IMatchEngine, in which case you will have to carefully follow the javadoc's recommandations, or extend the default implementation, DefaultMatchEngine.

A custom match engine can be used for your model comparison needs :

IMatchEngine customMatchEngine = new MyMatchEngine(...);
EMFCompare.builder().setMatchEngine(customMatchEngine).build().compare(scope);

Changing how resources are matched

By default, the logic EMF Compare uses to match resources together is very simple : if two resources have the same name (strict equality on the name, without considering folders), they match. When this is not sufficient, EMF Compare will look at the XMI ID of the resources' root(s). If the two resources share at least one root with an equal XMI ID, they match.

This can be changed only by implementing your own subclass of the DefaultMatchEngine and overriding its resource matcher. The method of interest here is DefaultMatchEngine#createResourceMatcher().

Defining custom identifiers

In some cases, there might be ways to identify your objects via the use of "identifiers" that cannot be identified as such by the default mechanism. For example, you might want each of your objects to be matched through their name alone, or through the composition of their name and their type... This can be achieved through code by simply redefining the function EMF Compare uses to find the ID of an object. The following code will tell EMF Compare that the identifier of all "MyEObject" elements is their name, and that any other element should go through the default behavior.

Function<EObject, String> idFunction = new Function<EObject, String>() {
	public String apply(EObject input) {
		if (input instanceof MyEObject) {
			return ((MyEObject)input).getName();
		}
		// a null return here tells the match engine to fall back to the other matchers
		return null;
	}
};
// Using this matcher as fall back, EMF Compare will still search for XMI IDs on EObjects
// for which we had no custom id function.
IEObjectMatcher fallBackMatcher = DefaultMatchEngine.createDefaultEObjectMatcher(UseIdentifiers.WHEN_AVAILABLE);
IEObjectMatcher customIDMatcher = new IdentifierEObjectMatcher(fallBackMatcher, idFunction);
 
IComparisonFactory comparisonFactory = new DefaultComparisonFactory(new DefaultEqualityHelperFactory());
 
IMatchEngine matchEngine = new DefaultMatchEngine(customIDMatcher, comparisonFactory);
EMFCompare.builder().setMatchEngine(matchEngine).build().compare(scope);

Ignoring identifiers

There are some cases where you do not want the identifiers of your elements to be taken into account when matching the objects. This can easily be done when calling for comparisons programmatically :

Through code

IEObjectMatcher matcher = DefaultMatchEngine.createDefaultEObjectMatcher(UseIdentifiers.NEVER);
IComparisonFactory comparisonFactory = new DefaultComparisonFactory(new DefaultEqualityHelperFactory());
 
IMatchEngine matchEngine = new DefaultMatchEngine(matcher , comparisonFactory);
EMFCompare.builder().setMatchEngine(matchEngine).build().compare(scope);

From the user interface

PENDING : preference page

Refine the default Match result

If you are happy with most of what the default behavior does, but would like to refine some of it, you can do so by post-processing the result of the match phase. The original models are only used when matching, and will never be queried again afterwards. All remaining phases are incremental refinings of the "Comparison" model that's been created by the matching phase.

As such, you can impact all of the differencing process through this. Within this post-processing implementation, you can :

• Remove Match elements

no difference will be detected on those : neither additions, nor deletions, nor conflicts... They'll simply be entirely ignored by the remaining process. Do note that elements for which we have no match will be considered "distinct" by the innards of EMF Compare : if a couple "B<->B'" references a couple "C<->C'" through one of their references, but you have removed the Match "C<->C'", we will considered that this reference has been "changed" from C to C' and this difference within the references of B will be shown as such.

• Add new Match element

the new couples of elements will be considered by the remaining comparison process and difference may be detected on them.

• Change existing Match elements

unmatched elements have two or three associated Match objects. For example if you are comparing three version of a model which all contain a different version of a given package, and all three version change the name of this package : version 1 has package "P1", version 2 has package "P2" and version three has package "P3". This package is actually the same, but EMF Compare did not manage to match it. We will thus have three Match objects : one that references "P1" as left, one that references "P2" as right and one that references "P3" as origin.

You may remove two of those three elements and change the third one so that it references P1 as left, P2 as right and P3 as origin. In such a case, those three will be considered to Match for the remainder of the comparison process. Make sure that there are not two different Match referencing the same object though, as this would yield unspecified results.

Defining a custom post-processor requires you to implement IPostProcessor and registering this sub-class against EMF Compare. The latter can be done via either an extension point, in which case it will be considered for all comparisons on models that match its enablement, or programmatically if you only want it active for your own actions :

Through code

The following registers a post-processor for all UML models. This post-processor will not be triggered if there are no UML models (matching the given namespace URI) within the compared scope. Take note that the NsURI provided here is treated as a regular expression.

IPostProcessor customPostProcessor = new CustomPostProcessor();
 
PostProcessorRegistry registry = new PostProcessorRegistry();
registry.addPostProcessor(new PostProcessorDescriptor("http://www.eclipse.org/uml2/\\d\\.0\\.0/UML", null,
		"my.custom.post.processor.id", customPostProcessor ));
Comparison comparison = EMFCompare.builder().setPostProcessorRegistry(registry).build().compare(scope);

Through extension point

This accomplishes the exact same task, but it registers the post-processor globally. Any comparison through EMF Compare on a scope that contains models matching the given namespace URI will trigger that post-processor.

<extension point="org.eclipse.emf.compare.postProcessor">
      <postProcessor class="my.package.CustomPostProcessor">
         <nsURI value="http://www.eclipse.org/uml2/\\d\\.0\\.0/UML">
         </nsURI>
      </postProcessor>

Diff

Now that the Matching phase has finished completed and that we know how our objects are coupled together, EMF Compare no longer requires the two (or three) input models. It will no longer iterate over them or the comparison's input scope. From this point onward, only the result of our comparison, the Comparison object, will be refined through the successive remaining phases, starting by the Diff.

The goal of this phase is to iterate over all of our Match elements, be they unmatched (only one side has this object), couples (two of the three sides contain this object) or trios (all three sides have this object) and compute any difference that may appear between the sides. For example, an object that is only on one side of the comparison is an object that has been added, or deleted. But a couple might also represent a deletion : during three way comparisons, if we have an object in the common ancestor (origin) and in the left side, but not in the right side, then it has been deleted from the right version. However, this latter example might also be a conflict : we have determined that the object has been removed from the right side... but there might also be differences between the original version and the "left" version.

The differencing phase does not care about conflicts though : all it does is refine the comparison to tell that this particular Match has n diffs : one DELETE difference on the right side, and n differences on the left. Detecting conflicts between these differences will come at a later time, during the conflict resolution phase.

There are a little fewer customization options for this phase.

Overriding the Diff engine

PENDING use of EMFCompare

Changing the Diff Processor

PENDING constructor of DefaultDiffEngine

Refine the default Diff result

PENDING post-process

Equivalences

PENDING description of the phase, extensibility options (post-process)

Requirements

PENDING description of the phase, extensibility options (post-process)

Conflicts

PENDING description of the phase, extensibility options (post-process)

Merging

PENDING how to provide custom mergers, override existing ones?

User Interface

PENDING customize display of custom differences, add custom menu entries, add groups, add filters, add export options, provide custom content viewer

Using The Compare APIs

Compare two models

PENDING programmatci use of class EMFCompare

Query the differences

PENDING all differences, differences on an object, non-conflicting diffs, diffs on side, use of EMFComparePredicates

Merge differences

PENDING how to re-implement copyDiff and copyAllNonConflicting

Open a compare editor

PENDING description of the need (dialog and editor), link to appropriate page