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| <td width="100%"><h1>Overview</h1></td> |
| <td align="right" valign="middle" nowrap> <a href="users/index.html" title="Forward to User's Guide"><img src="../images/forward.png" border="0"></a></td> |
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| <p class="author">Author: Eike Stepper</p> |
| <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"> |
| |
| |
| <h2><a name="Functionality"></a>1 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>Security</b> |
| <dd>The data in a repository can be secured through pluggable <a href="../../org.eclipse.net4j.util.doc/javadoc/org/eclipse/net4j/util/security/IAuthenticator.html" title="Interface in org.eclipse.net4j.util.security"><code>authenticators</code></a> and |
| <a href="../javadoc/org/eclipse/emf/cdo/server/IPermissionManager.html" title="Interface in org.eclipse.emf.cdo.server"><code>permission managers</code></a>. A default security model is provided on top of these low-level |
| components. The model comprises the concepts of users, groups, roles and extensible permissions. |
| <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 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 stand-alone deployments or in various kinds of containers such as JEE web or application servers. |
| <p> |
| The following chapters give an overview about the architectures of applications and repositories, respectively. |
| |
| <h3><a name="Client"></a>2.1 Client 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/2.11/org/eclipse/emf/ecore/EObject.html" target="_blank" title="Interface in org.eclipse.emf.ecore"><code>EObjects</code></a>. While CDO's basic functionality integrates nicely and |
| transparently with EMF's extension mechanisms some of the more advanced functions may require to add direct |
| dependencies 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><b>See Also:</b></p> |
| <ul> |
| <li><a href="programmers/client/Doc01_Architecture.html" title="Article in CDO Model Repository Documentation">Understanding the Architecture of a Client Application</a></li> |
| </ul> |
| |
| |
| <h3><a name="Repository"></a>2.2 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/2.11/org/eclipse/emf/ecore/EObject.html" target="_blank" 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><b>See Also:</b></p> |
| <ul> |
| <li><a href="programmers/server/Architecture.html" title="Article in CDO Model Repository Documentation">Understanding the Architecture of a Repository</a></li> |
| </ul> |
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| <div class="copyright">Copyright (c) 2014 Eike Stepper (Berlin, 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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