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FAQ for CDO and Net4j



How can I enable tracing?

If running as Eclipse launch config it's easy: just turn on tracing on the tracing page of the launch config (plus all net4j and cdo plugins in the list).

If running standalone:



Why does CDO complain that my objects can't be cast to InternalCDOObject?

Looks as if you are not generating your model for CDO. Have you already read Preparing EMF Models for CDO?

How can I modify the behaviour of generated getters and setters?

If you want to modify the behaviour of generated getters and setters (or have already done so in existing models) you might want to try dynamic feature delegation (introduced in EMF 2.5). With this pattern, the reflective methods like eGet still call your generated method like getX() and then that calls the dymamic reflective method like eDynamicGet. It effectively produces the same behavior as "Refective" delegating but does so by delegating through your generated accessors allowing you to specialize those as you could when you used "None"...

What's the best way to convert an XMIResource to a CDOResource?

You just need to copy/move the root objects over to the CDOResource. There's a good example in ImportResourceAction.doRun():

  protected void doRun() throws Exception
    CDOTransaction transaction = getTransaction();
    // Source ResourceSet
    ResourceSet sourceSet = new ResourceSetImpl();
    Map<String, Object> map = sourceSet.getResourceFactoryRegistry().getExtensionToFactoryMap();
    map.put("*", new XMIResourceFactoryImpl());
    // Source Resource
    Resource source = sourceSet.getResource(sourceURI, true);
    List<EObject> sourceContents = new ArrayList<EObject>(source.getContents());
    // Target Resource
    Resource target = transaction.createResource(targetPath);
    EList<EObject> targetContents = target.getContents();
    // Move contents over
    for (EObject root : sourceContents)

How can I react to remote changes to my objects?

CDO 2.0 has several mechansims to catch up with remote changes. They differ in the amount of change information being delivered and thus in the burden put on the network:

  • CDOSessionInvalidationEvent is the oldest and simplest form. The event is emitted to registered IListeners of a CDOSession *if* PassiveUpdate is enabled for the session. See
  • CDOViewInvalidationEvent is emitted to registered IListeners of a CDOView *if* PassiveUpdate is enabled for the session. See
  • CDOInvalidationNotification is a *custom* EMF notification that is emitted to adapters of the EObjects in a CDOView *if* InvalidationNotification is enabled for the view. Since the notifications are constructed out of the information in a CDOSessionInvalidationEvent (i.e. CDOIDs) they don't carry detailed change deltas. All the Notification methods related to change deltas throw UOEs. See
  • CDOChangeSubscriptionPolicy is the newest form and the one that delivers the most detailed information about the actual change. The change information is delivered to object adapters. To compensate the bandwidth burden of the change info per object you must exactly specify for which objects it is provides. With one of the shipped policies or an own one you can do this per view. See

Can we change the underlying data base to use something like Objectivity instead of relational DB?

Yes, Simon McDuff (CDO team member) is working on exactly that. Basically we are waiting for some IP clearance stuff to make it (the CDO storage adapter) open source. That's the beauty that the persistence vehicle is complete orthogonal. So you can put any type of backend behind it.

How is CDO doing the object-relational mapping? Is there JPA or JDO underneath or homebrewn?

The backend integration is completely decoupled from the repository framework. The default integration is our proprietary DBStore. Together with Martin Taal from the Teneo project we developed a CDO Hibernate store. I heard that Teneo supports JPA, so yes, a CDO repository can work with JPA. For object-oriented backends you need no mapping at all. See the preceding question about Objectivity. And by implementing a handful of interfaces you can plug in your own backend integration.

Any advice for a mature open source ODB?

CDO ships with support for DB4O and MongoDB.

Can I have multiple communication plugins available, i.e. AMQP?

Eike is working to separate out Net4J to better support other communication mechanisms. It would be nice to closely work together with extenders of these new APIs to ensure that they meet concrete expectations. Apart from this effort it should also be possible to integrate new transport technologies via Net4j. Fo this purpose Net4j has a strong separation of the physical transport layer and the application protocl layer (here: CDO protocol).

Is it possible to tunnel through HTTP, so HTTP on port 80 can go through firewalls for communication?

Yes, Net4j comes with an (experimental) HTTP transport layer implementation that supports asynchronous bidirectional communication through http connections.

Am I correct in understanding that a ViewSet is mainly useful when connecting to multiple different repositories?

Yes, we introduced the explicit notion of a CDOViewSet when we supported external references in the framework, i.e. references from one repository to another repository or completely different resource types. Each EMF RescourceSet always has an associated CDOViewSet (when used with CDO) but in the standard case this ViewSet will contain only a single CDO view or transaction.

Does each change increment the version of an object?

No. The version of an object is incremented at the end of a successful commit operation on a the underlying CDOTransaction or CDOXATransaction. In fact the version is part of the primary key of a CDORevision, so a new version is assigned with each new revision of an object.

When does the view acquire the latest version of an object? The first time you access the object, or every time you access the object?

Basically the first time you access an object. Sometimes earlier (configurable pre-fetching) and sometimes later (after remote invalidation).

What happens in case of a conflict (two transactions changing the same object)?

The most important reaction is that the model repository reliantly rejects the second commit attempt. Depending on the local configuration (session.options.setPassiveUpdateEnabled) you might receive early notification about pending commit failure (conflict state of some objects). All this is about detecting conflicts. Quite new is a pluggable mechanism for custom conflict resolvers. As an example we offer a resolver that reduces the conflict granularity down from object to structural features of an object. See transaction.options.addConflictResolver.

Can I access the same repository through different sessions too?

Yes, you can open arbitrary number of sessions to a single repository, even from a single client. But keep in mind that each session maintains a revision cache and should be considered somewhat expensive. Each session can have arbitrary numbers of views / transactions. The objects (empty data wrappers) provided by these views all share the revision instances of the session cache.

Did I understand Eike correctly that changing an object requires a Transaction, which is the only read-write view?


Is Audit a mode of a repository or a view on the repository?

Both. A CDOAudit is a special CDOView on the client side that enables to look at consistent historical states of the whole object graph. As such it is a client side concept. For this to be possible the repository must be configured to support it. As such it is a mode of the repository. Third, the chosen backend (plus integration) must support this repository configuration. As such it is a capability of the storage implementation.

If I disable the audit mode, then no historical data is stored anymore, right?

Yes, if auditing is disabled in the repository, then no historical revisions are stored. Only current revisions. Please note that each storage implementation can have different capabilities with respect to switching auditing on or off. New: the DBStore can switch it off now!

So when audit is disabled in the server configuration and request an audit object, then what do I get? null or a view on the current revision (the only one available)?

If the repository is not configured to support auditing you will get an exception when you try to open an audit view on the client side. So you’ll never be able to hold references to old revisions.

Can I have custom views on the client editor based on different packages (models) I have in the repo? I assume there is a generic editor to handle all the different models I can have in the repo. I need to have different views on the editor based on different models.

Yes and yes. CDO supports multiple packages and multiple resources both in the client and in the repository. You can mix them as you want to. And the generic CDOEditor supports all of this, too.

Do you have references of production apps based on CDO?

Yes, we have. Although I am often surprised to learn that many companies use CDO for a long time before I even get aware of it through some newsgroup questions. I am planning to open a public poll to gather more feedback, stay tuned. We know of many applications in the modeling industry (tooling) as well as in space and aeronautics, automotive, defense, asset management, and so on. If you know of any applications please tell us about!

Why is the server config file not an EMF model?

The main reson is that formerly we had no other EMF requirement on the server side of CDO. This will change in the near future to better support custom EdataTypes and Eoperations in server-side queries. Together with our new Definition Framework ( this will make a perfect fit to configure a CDO server.

Any parallels/differences to MS Oslo Repository (i.e. in philosophy/objectives)?

Ed Merks talked about Oslo in this blog: . The idea of CDO is to support any repository, not just one specific one. I think that can make some aspects of Oslo much slicker.

Does the CDO user interface use standard properties viewer? So the Databinding frameworks can sit on top to create custom UIs and Views?

Yes, CDO is pretty much invisible for the rest of the application. Standards should always be usable.

What is the state of CDO Browser project?

We've recently added Common Navigator and Team Provider support. There is also EFS integration on the way (contribution). It is the main focus of the UI sub component to deliver high-quality and re-usable user interface components that make perfect use of the scalable core components of CDO.

In the demo, when you change something in one client, and it appears on the other client, what's the API like for handling that? Do you have to use listeners? Or does it just look like the model was changed by another thread?

Both, you receive EMF Notifications (depending on the local configuration with or without change deltas) and you also can listen to some special events sent by the server. See for details.

Wow! Team Provider, cool! It would be kind of natural to have something like the compare view for conflicts. Can team integration be used to handle conflicts?

Team integration is very new and work in progress. But yes, it is our plan to make all the respective CDO core functionality available through the well-known Team support of the Eclipse IDE, including compare and merge. Consider also that while CVS has file level granularity, CDO can handle two clients updating different properties of an individual object. You can't get more fine grained than this...

I saw that a CDOResource can have a „resource path“. Does that mean the "/folder/company" string is processed and treated as an IPath or something anagolous?

A special obejct hierarchy is defined for that, CDOResourceNode (abstract), CDOResourceFolder and CDOResource. They form kind of a virtual file system for resources within a model repository. They can be navigated and queried just like normal objects (in fact they are EObjects). Each such CDOResourceNode has a unique path, but it is always presented as a String. You can convert it easily to an IPath.

Does CDO provide a Validation mechanism?

At the CDO client side you can apply any validation mechanism that you can get to work with regular EMF models. CDO does not offer own solutions in this area. On the server side you can hook your own vetoable ReadHandlers and WriteHandlers with a particular repository. In the future you will also be able to use EMF based technologies like EMF Query, Validation and OCL on the server side, together with your own validators written in Java.

Are there query facilities for querying the models in the repo?

We have a very flexible query framework that can support any type of query language and deliver query results synchronously as well as asynchronously from the server to the client. You can hook either backend-specific or backend-agnostic language handlers into a repository. We would appreciate the contribution of such query handlers. And we are working on the integration of a common query language (OCL).

Any thoughts on data migration? Data migration and data import from/to XMI?

Import and export to any other persistence style that is supported by EMF is also supported by CDO. We have examples for XML/XMI import and export. Automatic data migration, as needed after model evolution, is currently not supported, although we started discussion on this topic in

How about locking, one client can lock the object so that the other clients not allow to change till it's unlocked? Is there any tree-locking facilities in CDO and/or EMF?

Yes, we have explicit locking on object level. You can acquire read and qrite locks one one or several objects at once. The contained sub tree of objects can be included (as of 4.1). Locks are automatically released when the view/transaction is closed, optionally at commit time. This locking support is part of the CDO core.

Would it be possible to run the CDO server code within an app server - would that make sense at all?

Yes and yes. Each single component of CDO can run in OSGi, standalone or in any imaginable container. Embedding the repository into an JEE server makes particular sense, when the CDO client is a web application in the same container. Then you can use Net4js JVM connector to bypass socket transport. In the near future we will come up with even more direct connection methods (i.e. without Net4j in between).

Though it's called connected data objects, is there some way of using CDO in an offline mode?

Yes, there are two alternative setups:

  1. The master repository can be cloned and the replica can be used quite like a normal repository. The entire history of the master is replicated whenever a network connection to the master is available. This setup requires branching mode enabled in both master and all clones. A cloned repository can either be embedded into each client or multiple clones can be set up as networked repositories to serve multiple clients each.
  2. A CDOWorkspace can be checked out from the master and provide clients with views and transactions that operate on an offline baseline. While the first mode is roughly comparable to Git a CDOWorkspace supports an SVN/CVS like workflow with checkout/update/checkin. Through a special EFS implementation a CDOWorkspace can be integrated into the Eclipse IWorkspace as an IProject with proper (yet simulated on the fly) IFiles.

Speaking of transformation, QVT/ATL over EMF/CDO should be possible, right? Would it perform well?

You should be able to use CDO backed models just like any other "normal" EMF model. CDO is effectively transparent so pretty much every normal EMF thing applies. Regarding performance, there is a lot that you can configure in CDO. See for details.

Which newsgroups to submit thoughts to?

Use the EMF newsgroup and prefix with [CDO]: See

We need to save locks when user is offline, is that possible?

Currently, when a user is offline, the session is terminated (offline mode for a session is work in progress). Maintaining locks beyond the end of a session is supported by CDO 4.1. Locks are automatically released when the view/transaction is closed, optionally at commit time.

Is there dynamic CDO?

Yes. You can register dynamic EPackages with a CDOSession and if you commit a transaction that contains objects of these packages they (the packages) are automatically committed to the repository as well. Note, that you can generally only register top-level packages with a CDO session. Nested packages are registered implicitely if they have a namespace URI.

Do I have to regenerate my models?

Currently yes, or you must use a dynamic form of your model. We are working together with EMF Core on a mechanism to support „legacy models“ (i.e. those that are generated, but not for CDO) in CDO. See for details. Please note that regeneration does not require the Ecore model to be changed! But the regeneration for CDO removes all member fields from the generated classes (reflective delegation) and your hand-written modifications that use these member fields (they should use their accessors!) would not compile anymore.

What about the new EMF index project and server side queries?

EMF Index will work on the EMF side not directly on the servery/repository side. But we are working closely with them to ensure that CDO can nicely fit into an index provider framework.

Are the CDO webinar and the webinar slides available online?

Yes, Lynn will make both available through Eclipse Live: . And I’ll check the slides into the CVS and make them available through the CDO wiki:

Is there kind of transaction Rollback mechanism?

CDOTransaction and CDOXATransaction have rollback() methods.We even have save points which you can roll back to. There's a save point snippet in the webinar slides.

If it was just about storing EMF objects in an RDBMS, Teneo would be sufficient I'd think. So I was wondering what other requirements you could have for the whole "bus" thing?

For instance a distributed shared model or the client side scalability for really huge models. CDO provides a clever memory usage with garbage collector support and stuff like partial object loading, partial collection loading. In this sense CDO is like the virtual memory of EMF (Steve suggested: „Demand Paged Virtual Modelling!“).

Why is my database log file locked when using HSQLDB?

For some applications, you may need to delete your database files after shutting down your CDO session. However, if you are using HSQLDB, the database will hold locks on the the files until the database itself is shut down. You can shutdown the HSQLDB database by executing a simple SQL command. But first you will need to gain access to a JDBC Connection object. This can be accessed through your session in the following manner (assuming that you have access to your CDO repository):

IDBStore store = (IDBStore)repository.getStore();
Connection c = store.getDBConnectionProvider().getConnection();
Statement s = c.createStatement();

You should now have write access to your database files.

Address in use when starting CDO Server

When you get an 'address already in use' warning when starting the CDO server you have:

  • set the tcp/http port to a port which is already used by another application, change this in the arguments of the launch config and the cdo-server.xml file
  • there is already a CDO server running with the same port definitions, check in the Debug view if you see a CDO server. If you don't see any then check on OS level if you see a CDO server running (on linux: ps -ef | grep cdo). Even if you started a CDO server from Eclipse it does not always remain visible in the Debug view.

IllegalArgumentException: Use CDOFactory to create dynamic object

Usually the CDOPackageRegistry recognizes dynamic packages automatically and replaces the EFactory instance. But you must ensure that you are using that factory. So, after registering the package with the session registry you can create new DynamicCDOObjectImpl instances. You can not use instances with CDO that you created before the package was registered with CDO!

How to write tests for Bugzilla

It would be nice to have a test for any reported bug. Please implement tests using the CDO test framework.

Can I use EContentAdapter with Lazy Loading?

Some applications use EContentAdapters to track model state (the Eclipse UML implementation even attaches one automatically to get its work done!). This will cause the entire content tree to be loaded from the repository, on which such an adapter is attached. If you do need to use EContentAdapters, or any adapter that crawls the content tree in a similar fashion, then consider pre-fetching the contents to avoid unnecessary traffic between client and server. See the CDO performance tips for details.

Also, if you're creating your own EContentAdapter, not using an existing one like the ECrossReferenceAdapter, you might consider basing it on the CDOLazyContentAdapter.

Why do all of the CDOResources that I load have no contents?

If you are trying to load resources from your CDO repository, and you know that they have contents, but they always seem to be empty, check whether you are using a custom resource-set implementation.

When creating a new CDOResource, the CDOResourceFactoryImpl inspects the current thread's stack trace to see whether the resource is being created as a consequence of invocation of the ResourceSet::getResource(...) API. If so, then it initializes the resource's CDOState as PROXY (meaning that it needs to be "filled in" from the repository). Otherwise, the state will be NEW, indicating that it is a new resource to be added to the repository. As an aside, trying to commit the transaction in this case could result in duplicate resource exceptions.

The introspection of the stack trace looks for a stack frames naming the getResource method of a set of known classes implementing the ResourceSet interface. If you have a custom resource set implementation that overrides getResource, then your class name is unknown to CDO and CDO will not find that a getResource invocation is in progress and your resource will incorrectly be created in NEW state.

To remedy this problem, ensure that when you create a CDO transaction using your custom resource set, your install (in the reosurce set's factory registry's protocol-to-factory map) a subclass of CDOResourceFactoryImpl that overrides the isGetResource() method. This is easily done by maintaining state in your resource set that tells your factory whether getResource(...) is in progress.


How can I use authentication?

If you're using any sort of negotiation you must usually ensure that the INegotiators of both client and server side properly match together. INegotiators are currently only supported by IConnectors. Since server-side connectors are created by an IAcceptor you need to configure the acceptor with the needed INegotiator so tat it can pass it down to new connectors. If you do so you need to add a matching negotiator to your client connector before connecting, too.

If you're pulling your client connector out of an IManagedContainer it will already be connected. So you need to register a post processor with the container to inject your negotiator into the connector *before* it is returned:

    container.addPostProcessor(new IElementProcessor()
      public Object process(IManagedContainer container, String productGroup, String factoryType, String description,
          Object element)
        if (element instanceof TCPClientConnector)
          TCPClientConnector connector = (TCPClientConnector)element;
          INegotiator negotiator = createNegotiator();
        return null;
      private INegotiator createNegotiator()
        ResponseNegotiator negotiator = new ResponseNegotiator();
        negotiator.setCredentialsProvider(new PasswordCredentialsProvider("login", "password"));
        // negotiator.setCredentialsProvider(new InteractiveCredentialsProvider());
        return negotiator;

See also: CDO/Net4j Authentication

Wikis: CDO | Net4j | EMF | Eclipse

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