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<td width="100%"><h1>Collaborating in Real-Time</h1></td>
<td align="right" valign="middle" nowrap><a href="Doc07_UsingModels.html" title="Backward to Working with Models and Model Elements"><img src="../../images/backward.png" border="0"></a> <a href="Doc09_TechnicalBackground.html" title="Forward to Understanding the Technical Background"><img src="../../images/forward.png" border="0"></a></td>
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<p class="author">Author: Eike Stepper</p>
<p>
CDO supports real-time collaboration on models by transferring the changes that one user
<a href="../../javadoc/org/eclipse/emf/cdo/transaction/CDOUserTransaction.html#commit(org.eclipse.core.runtime.IProgressMonitor)" title="Method in org.eclipse.emf.cdo.transaction.CDOUserTransaction"><code>commits</code></a> to the repository to all
other users connected to the same repository and transparently weaving those changes into their model copies.
<p align="center"><img src="collaborating.png">
<p>
With CDO the local model copies (in particular with <a href="Doc04_CheckingOut.html#Doc_TransactionalCheckouts" title="Chapter in CDO Model Repository Documentation">online transactional checkouts</a>) do not need to be
<i>updated</i> manually; they are automatically updated (almost) at the time they are changed by other users.
<p>
As real-time collaboration relies on committing a <a href="../../javadoc/org/eclipse/emf/cdo/transaction/CDOTransaction.html" title="Interface in org.eclipse.emf.cdo.transaction"><code>transaction</code></a> it applies only to
online transactional checkouts and the <a href="Doc07_UsingModels.html#Doc_EditingModelElementsEditor" title="Chapter in CDO Model Repository Documentation">editors</a>
opened on online transactional models. Saving a model editor commits the underlying transaction.
<p>
The data integrity of the models and model elements in a repository is guaranteed by <a href="../../../org.eclipse.net4j.util.doc/javadoc/org/eclipse/net4j/util/concurrent/IRWLockManager.LockType.html#WRITE" title="Field in org.eclipse.net4j.util.concurrent.IRWLockManager.LockType"><code>write locks</code></a>
that are acquired <b>per model element</b>. <a href="../../../org.eclipse.net4j.util.doc/javadoc/org/eclipse/net4j/util/concurrent/IRWLockManager.LockType.html#READ" title="Field in org.eclipse.net4j.util.concurrent.IRWLockManager.LockType"><code>Read locks</code></a> and <a href="../../../org.eclipse.net4j.util.doc/javadoc/org/eclipse/net4j/util/concurrent/IRWLockManager.LockType.html#OPTION" title="Field in org.eclipse.net4j.util.concurrent.IRWLockManager.LockType"><code>write options</code></a>,
as well as <a href="../../javadoc/org/eclipse/emf/cdo/view/CDOView.html#enableDurableLocking()" title="Method in org.eclipse.emf.cdo.view.CDOView"><code>durable locks</code></a> are supported by the core-level APIs
but not by the CDO Explorer's user interface.
<p>
<b>Table of Contents</b> <p>
<table border="0">
<tr><td>1 </td><td class="te" colspan="4"><a href="Doc08_Collaborating.html#Doc_OptimisticLocking" title="Chapter in CDO Model Repository Documentation">Optimistic Locking</a></td></tr>
<tr><td></td><td>1.1 </td><td class="te" colspan="3"><a href="Doc08_Collaborating.html#Doc_EarlyConflictDetection" title="Chapter in CDO Model Repository Documentation">Early Conflict Detection</a></td></tr>
<tr><td></td><td>1.2 </td><td class="te" colspan="3"><a href="Doc08_Collaborating.html#Doc_AutomaticConflictResolution" title="Chapter in CDO Model Repository Documentation">Automatic Conflict Resolution</a></td></tr>
<tr><td></td><td>1.3 </td><td class="te" colspan="3"><a href="Doc08_Collaborating.html#Doc_InteractiveConflictResolution" title="Chapter in CDO Model Repository Documentation">Interactive Conflict Resolution</a></td></tr>
<tr><td>2 </td><td class="te" colspan="4"><a href="Doc08_Collaborating.html#Doc_PessimisticLocking" title="Chapter in CDO Model Repository Documentation">Pessimistic Locking</a></td></tr>
<tr><td></td><td>2.1 </td><td class="te" colspan="3"><a href="Doc08_Collaborating.html#Doc_TreeLocking" title="Chapter in CDO Model Repository Documentation">Tree Locking</a></td></tr>
<tr><td>3 </td><td class="te" colspan="4"><a href="Doc08_Collaborating.html#Doc_AutomaticLocking" title="Chapter in CDO Model Repository Documentation">Automatic Locking</a></td></tr>
<tr><td>4 </td><td class="te" colspan="4"><a href="Doc08_Collaborating.html#Doc_AutomaticCommitting" title="Chapter in CDO Model Repository Documentation">Automatic Committing</a></td></tr>
</table>
</p>
<h2><a name="Doc_OptimisticLocking"></a>1 Optimistic Locking</h2>
<p>
By default model elements are locked optimistically, that is, the CDO server <i>implicitly</i> acquires and releases locks while executing
a commit operation. These implicit locks are not visible to the committing user or any other user of the same repository.
<p>
Optimistic locking provides for the highest possible degree of concurrency but it also comes with a non-zero risk of commit conflicts
that are only detected when a commit operation is executed by the CDO server and, as a consequence, rejected. Because of
<a href="Doc08_Collaborating.html#Doc_EarlyConflictDetection" title="Chapter in CDO Model Repository Documentation">Early Conflict Detection</a> the risk of conflicts that are detected that late in the commit process is generally much lower
than, for example, in pure database-based applications.
<p>
To completely eliminate the risk of commit conflicts <a href="Doc08_Collaborating.html#Doc_PessimisticLocking" title="Chapter in CDO Model Repository Documentation">Pessimistic Locking</a> must be used.
<h3><a name="Doc_EarlyConflictDetection"></a>1.1 Early Conflict Detection</h3>
<p>
As the local model copies of a user are automatically updated (almost) at the time they are changed by other users
CDO can anticipate the conflict potential of the local changes early, in particular before an attempt to commit these changes is even made.
The <a href="Doc07_UsingModels.html#Doc_EditingModelElementsEditor" title="Chapter in CDO Model Repository Documentation">CDO Model Editor</a> decorates such conflicting model elements with a red-colored font,
indicating that the underlying <a href="../../javadoc/org/eclipse/emf/cdo/transaction/CDOTransaction.html" title="Interface in org.eclipse.emf.cdo.transaction"><code>transaction</code></a> can not be successfully committed anymore.
<p align="center"><img src="early-conflict.png">
<p>
<a href="Doc08_Collaborating.html#Doc_AutomaticConflictResolution" title="Chapter in CDO Model Repository Documentation">Automatic Conflict Resolution</a> and <a href="Doc08_Collaborating.html#Doc_InteractiveConflictResolution" title="Chapter in CDO Model Repository Documentation">Interactive Conflict Resolution</a>, if enabled, may have an impact on what exact types
of changes are considered a conflict.
<h3><a name="Doc_AutomaticConflictResolution"></a>1.2 Automatic Conflict Resolution</h3>
<p>
Each time a local transaction is notified of a remote change by the CDO server and local conflicts are detected (see <a href="Doc08_Collaborating.html#Doc_EarlyConflictDetection" title="Chapter in CDO Model Repository Documentation">Early Conflict Detection</a>)
these conflicts are categorized as being either <i>trivial</i> conflicts or <i>non-trivial</i> conflicts. Trivial conflicts are:
<ul>
<li> Changes to multi-valued <a href="http://download.eclipse.org/modeling/emf/emf/javadoc/2.9.0/org/eclipse/emf/ecore/EStructuralFeature.html" target="_blank" title="Interface in org.eclipse.emf.ecore"><code>features</code></a> on both sides (local and remote) of the same model element.
<li> Changes to different single-valued <a href="http://download.eclipse.org/modeling/emf/emf/javadoc/2.9.0/org/eclipse/emf/ecore/EStructuralFeature.html" target="_blank" title="Interface in org.eclipse.emf.ecore"><code>features</code></a> on both sides (local and remote) of the same model element.
</ul>
<p>
Trivial conflicts are merged automatically into the local transaction, i.e., no user interaction is involved.
<p>
When non-trivial changes are detected, i.e., changes to the same single-valued <a href="http://download.eclipse.org/modeling/emf/emf/javadoc/2.9.0/org/eclipse/emf/ecore/EStructuralFeature.html" target="_blank" title="Interface in org.eclipse.emf.ecore"><code>feature</code></a> on both sides (local and remote)
of the same model element, automatic conflict resolution is suspended for all model elements until the next local commit operation.
During this period all incoming change notifications are accumulated and remembered for possible <a href="Doc08_Collaborating.html#Doc_InteractiveConflictResolution" title="Chapter in CDO Model Repository Documentation">Interactive Conflict Resolution</a>
at commit time.
<h3><a name="Doc_InteractiveConflictResolution"></a>1.3 Interactive Conflict Resolution</h3>
<p>
If <a href="Doc08_Collaborating.html#Doc_AutomaticConflictResolution" title="Chapter in CDO Model Repository Documentation">Automatic Conflict Resolution</a> has detected non-trivial conflicts in a local <a href="../../javadoc/org/eclipse/emf/cdo/transaction/CDOTransaction.html" title="Interface in org.eclipse.emf.cdo.transaction"><code>transaction</code></a> and
an attempt is made to commit this transaction the following dialog pops up:
<p align="center"><img src="late-conflict.png">
<p>
The dialog shows an overview of how many local model elements are added, changed, and removed. One of several conflict resolution
actions has to be selected by the user:
<ul>
<li> If the Merge action is selected an EMF Merge editor is opened with the left side showing the remote changes and
the right side showing the local changes:
<p align="center"><img src="merge.png">
<p>
Saving this merge editor commits the merged local transaction. Note that new non-trivial conflicts may have been detected
in the meantime, in which case interactive conflict resolution is triggered again.
<li> If the Rollback action is selected the local transaction is <a href="../../javadoc/org/eclipse/emf/cdo/transaction/CDOUserTransaction.html#rollback()" title="Method in org.eclipse.emf.cdo.transaction.CDOUserTransaction"><code>rolled back</code></a> and the local model copies
are automatically updated to their latest remote versions. As a result all local changes will be lost and eventually need to be
re-applied and committed again.
</ul>
<h2><a name="Doc_PessimisticLocking"></a>2 Pessimistic Locking</h2>
<p>
Sometimes it seems not desirable to risk commit conflicts as they can occur with <a href="Doc08_Collaborating.html#Doc_OptimisticLocking" title="Chapter in CDO Model Repository Documentation">Optimistic Locking</a>.
In these cases CDO supports the acquisition of <i>explicit</i> locks on selected models (see <a href="Doc08_Collaborating.html#Doc_TreeLocking" title="Chapter in CDO Model Repository Documentation">Tree Locking</a>) and model elements.
<p>
Pessimistic locking support consists of:
<ul>
<li> Lock and unlock actions
<li> Lock state visualization
</ul>
<p>
Whether custom user interface components, such as model editors or views, support local actions and/or lock state visualization depends
on the implementation of those components. The CDO Model Editor's context menu offers lock actions for model elements that are not locked
by anyone and unlock actions for model elements that are locked by the current user. Both the CDO Model Editor and the
<a href="Doc01_UserInterface.html#Doc_ProjectExplorerIntegration" title="Chapter in CDO Model Repository Documentation">Project Explorer Integration</a> support lock state visualization by decorating model elements that are locked by the current user
with a green lock icon (indicating that they can be modified) and model elements that are locked by other users with a red lock icon
(indicating that they can not be modified):
<p align="center"><img src="pessimistic-locking.png">
<p>
Note that a CDO editor generally operates in the context of a separate transaction, in particular not in the context of the
<a href="../../javadoc/org/eclipse/emf/cdo/view/CDOView.html" title="Interface in org.eclipse.emf.cdo.view"><code>read-only view</code></a> of the associated checkout, which explains why, in the screen shot above, both checkouts show the
locked model elements with a red lock icon decoration. In other words, while a model element is locked in a CDO editor it can not be
modified directly in the associated checkout via the Project Explorer.
<h3><a name="Doc_TreeLocking"></a>2.1 Tree Locking</h3>
<p>
Sometimes it is desirable to lock not just a single model element but to atomically lock the tree of model elements rooted at the
selected model element. The CDO Model Editor's context menu offers a Lock Tree action for model elements that are not locked by
anyone and an Unlock Tree action for model elements that are locked by the current user.
<h2><a name="Doc_AutomaticLocking"></a>3 Automatic Locking</h2>
<p>
With automatic locking turned on for a particular <a href="../../javadoc/org/eclipse/emf/cdo/transaction/CDOTransaction.html" title="Interface in org.eclipse.emf.cdo.transaction"><code>transaction</code></a> write locks are automatically acquired
for model elements at the time these model elements are modified the first time.
<p>
Automatic locking is not yet supported for checkouts.
<h2><a name="Doc_AutomaticCommitting"></a>4 Automatic Committing</h2>
<p>
With automatic committing turned on for a particular <a href="../../javadoc/org/eclipse/emf/cdo/transaction/CDOTransaction.html" title="Interface in org.eclipse.emf.cdo.transaction"><code>transaction</code></a> that transaction is automatically committed
each time a model element is modified. This can be very useful when the primary purpose of a repository is to support real-time
collaboration between a number of users.
<p>
On the other hand with automatic committing multiple logically related changes are no longer
isolated in single composed commits. This can be especially undesirable in repositories with auditing or branching support
because the databases of these types of repositories monotonously grow with the number of commits.
<p>
Automatic committing is not yet supported for checkouts.
<p align="right">
<a href="Doc07_UsingModels.html" title="Backward to Working with Models and Model Elements"><img src="../../images/backward.png" border="0"></a> <a href="Doc09_TechnicalBackground.html" title="Forward to Understanding the Technical Background"><img src="../../images/forward.png" border="0"></a></p>
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<div class="copyright">Copyright (c) 2014 Eike Stepper (Loehne, Germany) and others.<br>All rights reserved. This program and the accompanying materials are made available under the terms of the Eclipse Public License v1.0 which accompanies this distribution, and is available at http://www.eclipse.org/legal/epl-v10.html</div>
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