# Overview

Author: Eike Stepper

CDO is a pure Java model repository for your EMF models and meta models. CDO can also serve as a persistence and distribution framework 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.

CDO supports plentyfold deployments such as embedded repositories, offline clones or replicated clusters. The following diagram illustrates the most common scenario:

## 1 Functionality

The main functionality of CDO can be summarized as follows:

Persistence
  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.

Multi User Access
  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.

Transactional Access
  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.

Transparent Temporality
  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.

Parallel Evolution
  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.

Scalability
  Scalability, the ability to store and access models of arbitrary size, is transparently achieved by loading single objects on demand and caching them softly 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 usage 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.

Thread Safety
  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.

Collaboration
  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 distributed shared model 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.

Data Integrity
  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.

Security
  The data in a repository can be secured through pluggable `authenticators` and `permission managers`. 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.

Fault Tolerance
  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.

Offline Work
  Offline work with your models is supported by two different mechanisms:
  - One way is to create a clone 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.

  - 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 workspace. 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.

## 2 Architecture

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.

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.

The following chapters give an overview about the architectures of applications and repositories, respectively.

### 2.1 Client Architecture

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 `EObjects`. 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.

The following diagram illustrates the major building blocks of a CDO application:

See Also:

- Understanding the Architecture of a Client Application

### 2.2 Repository Architecture

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.

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 `EObjects` on the server.

The following diagram illustrates the major building blocks of a CDO repository:

See Also:

- Understanding the Architecture of a Repository

Copyright (c) 2014 Eike Stepper (Berlin, Germany) and others.
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