Revision as of 10:15, 9 January 2013

Architecture

Comparison Process

The comparison process of EMF Compare is explained in depth on the Overview.

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.

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

Match

Before we can compute difference 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 be matched through distance algorithms.

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.

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);

Using the Compare Services

Introduction

All the following examples will use a simple (some would say "stupid") domain model used to keep personal information, here 

contact lists.

Here is the diagram:

The whole comparison process is divided in two phases :

1.  matching both versions of the elements and providing a match model.
2. computing the diff (delta) model from this match.

When comparing (using the user interface) two models conformed to the AddressBook metamodel you'll get:

This is an emfdiff model displayed using the emf compare editor. Both match and delta information are kept in this model, let's have 

a further look on the match part:

The match model weave elements from both versions of the model.

From this match model EMF compare is able to compute the delta (diff):

Getting the differences of two models

Getting these differences using some code is easy, here is the snippet::

private DiffModel getDiff(AddressBook v1, AddressBook v2) throws Exception {
        Map options = new HashMap();
        MatchModel match = MatchService.doContentMatch(v1, v2, options);
        // ...

And you'll get the Match model which you can browse like any other EMF model.

Let's now produce the diff from this match information::

  DiffModel diff =  DiffService.doDiff(match);
        return diff;
    }

Both Diff and Match services leverage the Eclipse architecture to find the best match/diff engine from the file extension.

But EMF Compare can be used to get the differences of two models within Eclipse or out of Eclipse (standalone mode) and using it in standalone mode you'll loose the ability of "auto-picking" the right match/diff engine considering the file extension : you'll have to call the engines yourself. Here the generic one::

private DiffModel getDiff(AddressBook v1, AddressBook v2) throws Exception {
        Map options = new HashMap();
        MatchModel match = new GenericMatchEngine().contentMatch(v1,v2,options);        
        DiffModel diff =  new GenericDiffEngine().doDiff(match);
        return diff;
    }

The generic match engine can be parameterized thanks to some options available in the MatchOptions interface:

  please note that all these calls can be done with 3 models, left, right and the ancestor one. Then you'll be able to detect conflicts.

Merging the differences

Once you get the differences you can merge them. You can merge every detected delta from the diff model using the merge service::

AddModelElement add = DiffHelper.isAdded(alice, changes);
MergeService.merge(add, false);

Here is a quick view of all the diff elements:

!images/DiffMM.jpg!

Using EMF Compare in standalone mode

You can setup your environment to use EMF Compare in standalone mode. The will looks like this ::

/**
	 * This application will try and launch an headless model comparison.
	 * 
	 * @author Cedric Brun <a href="mailto:cedric.brun@obeo.fr">cedric.brun@obeo.fr</a>
	 */
	public final class ExampleLauncher {
		/**
		 * This class doesn't need to be instantiated.
		 */
		private ExampleLauncher() {
			// prevents instantiation
		}
	
		/**
		 * Launcher of this application.
		 * 
		 * @param args
		 *            Arguments of the launch.
		 */
		public static void main(String[] args) {
			if (args.length == 2 && new File(args[0]).canRead() && new File(args[1]).canRead()) {
				// Creates the resourceSet where we'll load the models
				final ResourceSet resourceSet = new ResourceSetImpl();
				// Register additionnal packages here. For UML2 for instance :
	//			Resource.Factory.Registry.INSTANCE.getExtensionToFactoryMap().put(UMLResource.FILE_EXTENSION,
	//					UMLResource.Factory.INSTANCE);
	//			resourceSet.getPackageRegistry().put(UMLPackage.eNS_URI, UMLPackage.eINSTANCE);
	
				try {
					System.out.println("Loading resources.\n"); //$NON-NLS-1$
					// Loads the two models passed as arguments
					final EObject model1 = ModelUtils.load(new File(args[0]), resourceSet);
					final EObject model2 = ModelUtils.load(new File(args[1]), resourceSet);
	
					// Creates the match then the diff model for those two models
					System.out.println("Matching models.\n"); //$NON-NLS-1$
					final MatchModel match = MatchService.doMatch(model1, model2, Collections
							.<String, Object> emptyMap());
					System.out.println("Differencing models.\n"); //$NON-NLS-1$
					final DiffModel diff = DiffService.doDiff(match, false);
					
					System.out.println("Merging difference to args[1].\n"); //$NON-NLS-1$
					final List<DiffElement> differences = new ArrayList<DiffElement>(diff.getOwnedElements());
					// This will merge all references to the right model (second argument).
					MergeService.merge(differences, true);
	
					// Prints the results
					try {
						System.out.println("MatchModel :\n"); //$NON-NLS-1$
						System.out.println(ModelUtils.serialize(match));
						System.out.println("DiffModel :\n"); //$NON-NLS-1$
						System.out.println(ModelUtils.serialize(diff));
					} catch (IOException e) {
						e.printStackTrace();
					}
	
					// Serializes the result as "result.emfdiff" in the directory this class has been called from.
					System.out.println("saving emfdiff as \"result.emfdiff\""); //$NON-NLS-1$
					final ModelInputSnapshot snapshot = DiffFactory.eINSTANCE.createModelInputSnapshot();
					snapshot.setDate(Calendar.getInstance().getTime());
					snapshot.setMatch(match);
					snapshot.setDiff(diff);
					ModelUtils.save(snapshot, "result.emfdiff"); //$NON-NLS-1$
				} catch (IOException e) {
					// shouldn't be thrown
					e.printStackTrace();
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			} else {
				System.out.println("usage : ExampleLauncher <Model1> <Model2>"); //$NON-NLS-1$
			}
		}
	}

Tutorials

• Setting up a development environment for EMF Compare
• Comparing XML files with EMF Compare
• Comparing pom.xml files with EMF Compare

You'll find a few more tutorials on the Eclipse Online Help once you installed EMF Compare

• Architecture
• Using the Compare Services
• Adapting the Comparison Process to your Ecore Model
• Adding new actions to the export menu

Additional UI APIs:

• Create a viewer from the result of a model comparison (ComparisonSnapshot) to show:
  • The structural differences
  • The effect of filtering and grouping parameters
• Request the viewer to retrieve only the visible differences, according to the viewer filtering configuration
• Retrieve the compared graphical models (resource set), filled by the difference decorators
• Open an EMF Compare editor from two model versions (CompareEditorInput) and select the structural differences related to the model object identifiers in parameters
• Using EMF Compare to preview changes
• Create an action to compare Objects from an editor
• Handling editor's reload conflicts using EMF Compare

Feel free to add any tutorial or documentation on the wiki, we'll integrate them back in the online help.