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		<td width="100%"><h1>Overview</h1></td>
		<td align="right" valign="middle" nowrap>&nbsp;<a href="programmers/index.html" title="Forward to Programmer's Guide"><img src="../images/forward.png" border="0"></a></td>
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<p>
 CDO is a pure Java <i>model repository</i> for your EMF models and meta models. CDO can also serve as a
 <i>persistence and distribution framework</i> for your EMF based application systems. For the sake of this overview a
 model can be regarded as a graph of application or business objects and a meta model as a set of classifiers that
 describe the structure of and the possible relations between these objects.
 <p>
 CDO supports plentyfold deployments such as embedded repositories, offline clones or replicated clusters. The
 following diagram illustrates the most common scenario: <p align="center"><img src="cdo-overview.png"></p>


<h2><a name="Functionality"></a>1&nbsp;&nbsp;Functionality</h2>
<p>
 The main functionality of CDO can be summarized as follows:
 <dl>
 <dt><b>Persistence</b>
 <dd>Persistence of your models in all kinds of database backends like major relational databases or NoSQL
 databases. CDO keeps your application code free of vendor specific data access code and eases transitions between
 the supported backend types.
 <p>
 <dt><b>Multi User Access</b>
 <dd>Multi user access to your models is supported through the notion of repository sessions. The physical transport
 of sessions is pluggable and repositories can be configured to require secure authentication of users. Various
 authorization policies can be established programmatically.
 <p>
 <dt><b>Transactional Access</b>
 <dd>Transactional access to your models with ACID properties is provided by optimistic and/or pessimistic locking
 on a per object granule. Transactions support multiple savepoints that changes can be rolled back to. Pessimistic
 locks can be acquired separately for read access, write access and the option to reserve write access in the
 future. All kinds of locks can optionally be turned into long lasting locks that survive repository restarts.
 Transactional modification of models in multiple repositories is provided through the notion of XA transactions
 with a two phase commit protocol.
 <p>
 <dt><b>Transparent Temporality</b>
 <dd>Transparent temporality is available through audit views, a special kind of read only transactions that provide
 you with a consistent model object graph exactly in the state it has been at a point in the past. Depending on the
 chosen backend type the storage of the audit data can lead to considerable increase of database sizes in time.
 Therefore it can be configured per repository.
 <p>
 <dt><b>Parallel Evolution</b>
 <dd>Parallel evolution of the object graph stored in a repository through the concept of branches similar to source
 code management systems like Subversion or Git. Comparisons or merges between any two branch points are supported
 through sophisticated APIs, as well as the reconstruction of committed change sets or old states of single objects.
 <p>
 <dt><b>Scalability</b>
 <dd>Scalability, the ability to store and access models of arbitrary size, is transparently achieved by loading
 single objects on demand and caching them <i>softly</i> in your application. That implies that objects that are no
 longer referenced by the application are automatically garbage collected when memory runs low. Lazy loading is
 accompanied by various prefetching strategies, including the monitoring of the object graph's <i>usage</i> and the
 calculation of fetch rules that are optimal for the current usage patterns. The scalability of EMF applications can
 be further increased by leveraging CDO constructs such as remote cross referencing or optimized content adapters.
 <p>
 <dt><b>Thread Safety</b>
 <dd>Thread safety ensures that multiple threads of your application can access and modify the object graph without
 worrying about the synchronization details. This is possible and cheap because multiple transactions can be opened
 from within a single session and they all share the same object data until one of them modifies the graph. Possible
 commit conflicts can be handled in the same way as if they were conflicts between different sessions.
 <p>
 <dt><b>Collaboration</b>
 <dd>Collaboration on models with CDO is a snap because an application can opt in to be notified about remote
 changes to the object graph. By default your local object graph transparently changes when it has changed remotely.
 With configurable change subscription policies you can fine tune the characteristics of your <i>distributed shared
 model</i> so that all users enjoy the impression to collaborate on a single instance of an object graph. The level
 of collaboration can be further increased by plugging custom collaboration handlers into the asynchronous CDO
 protocol.
 <p>
 <dt><b>Data Integrity</b>
 <dd>Data integrity can be ensured by enabling optional commit checks in the repository server such as referential
 integrity checks and containment cycle checks, as well as custom checks implemented by write access handlers.
 <p>
 <dt><b>Fault Tolerance</b>
 <dd>Fault tolerance on multiple levels, namely the setup of fail-over clusters of replicating repositories under
 the control of a fail-over monitor, as well as the usage of a number of special session types such as fail-over or
 reconnecting sessions that allow applications to hold on their copy of the object graph even though the physical
 repository connection has broken down or changed to a different fail-over participant.
 <p>
 <dt><b>Offline Work</b>
 <dd>Offline work with your models is supported by two different mechanisms:
 <ul>
 <li>One way is to create a <b>clone</b> of a complete remote repository, including all history of all branches.
 Such a clone is continuously synchronized with its remote master and can either act as an embedded repository to
 make a single application tolerant against network outage or it can be set up to serve multiple clients, e.g., to
 compensate low latency master connections and speed up read access to the object graph.
 <p>
 <li>An entirely different and somewhat lighter approach to offline work is to check out a single version of the
 object graph from a particular branch point of the repository into a local CDO <b>workspace</b>. Such a workspace
 behaves similar to a local repository without branching or history capture, in particular it supports multiple
 concurrent transactions on the local checkout. In addition it supports most remote functionality that is known from
 source code management systems such as update, merge, compare, revert and check in.
 </ul>
 </dl>

<h2><a name="Architecture"></a>2&nbsp;&nbsp;Architecture</h2>
<p>
 The architecture of CDO comprises applications and repositories. Despite a number of embedding options applications
 are usually deployed to client nodes and repositories to server nodes. They communicate through an application
 level CDO protocol which can be driven through various kinds of physical transports, including fast intra JVM
 connections.
 <p>
 CDO has been designed to take full advantage of the OSGi platform, if available at runtime, but can perfectly be
 operated in standalone deployments or in various kinds of containers such as JEE web or application servers.
 <p>
 The following chapters give an overview about the architecures of applications and repositories, respectively.

<h3><a name="Application"></a>2.1&nbsp;&nbsp;Application Architecture</h3>
<p>
 <p>
 The architecture of a CDO application is characterized by its mandatory dependency on EMF, the Eclipse Modeling
 Framework. Most of the time an application interacts with the object graph of the model through standard EMF APIs
 because CDO model graph objects are <a href="http://download.eclipse.org/modeling/emf/emf/javadoc/2.7.0/org/eclipse/emf/ecore/EObject.html" title="Interface in org.eclipse.emf.ecore"><code>EObjects</code></a>. While CDO's basic functionality integrates nicely and
 transparently with EMF's extension mechansims some of the more advanced functions may require to add direct
 dependendcies on CDO to your application code.
 <p>
 The following diagram illustrates the major building blocks of a CDO application: <p align="center"><img src="programmers/client/application-architecture.png"></p>


<h3><a name="Repository"></a>2.2&nbsp;&nbsp;Repository Architecture</h3>
<p>
 <p>
 The main building block of a CDO repository is split into two layers, the generic repository layer that client
 applications interact with and the database integration layer that providers can hook into to integrate their data
 storage solutions with CDO. A number of such integrations already ship with CDO, making it possible to connect a
 repository to all sorts of JDBC databases, Hibernate, Objectivity/DB, MongoDB or DB4O.
 <p>
 While technically a CDO repository depends on EMF this dependency is not of equal importance as it is in a CDO
 application. In particular the generated application models are not required to be deployed to the server because a
 CDO repository accesses models reflectively and the model objects are not implemented as <a href="http://download.eclipse.org/modeling/emf/emf/javadoc/2.7.0/org/eclipse/emf/ecore/EObject.html" title="Interface in org.eclipse.emf.ecore"><code>EObjects</code></a> on
 the server.
 <p>
 The following diagram illustrates the major building blocks of a CDO repository: <p align="center"><img src="programmers/server/repository-architecture.png"></p>


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<i>Copyright (c) 2004 - 2011 Eike Stepper (Berlin, Germany) and others.</i>
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