Architecture Overview

DeepaMehta features an OSGi-based modular software architecture that shares features with a service-oriented architecture (SOA). Each module's service can be consumed as an OSGi service. At the same time DeepaMehta can publish a module's service as a RESTful web service without requiring the module developer to do extra coding.

In a distributed (client-server) DeepaMehta installation the OSGi container runs at the server-side. Clients access the server as a RESTful web service (via HTTP). The representation format is JSON.

A central part of DeepaMehta is its plugin development framework. A DeepaMehta plugin is an OSGi bundle. However the DeepaMehta plugin developer must not know about the OSGi intrinsics. Instead she can rely on the facilities and conventions rooted in the DeepaMehta Core and its high-level API.

DeepaMehta Core

Features of the DeepaMehta Core:

• The DeepaMehta Core mediates between the DeepaMehta plugins and the underlying OSGi environment. In particular the DeepaMehta Core ...
  • registers the plugin's service as an OSGi service.
  • publishes the plugin's service as a RESTful web service via the OSGi HTTP service.
  • publishes the plugin's web resources (images, scripts, CSS) via the OSGi HTTP service.
  • tracks the plugin services the plugin want to consume.

• The DeepaMehta Core Service provides transactional access to the DeepaMehta database. There are high-level methods for dealing with the basic DeepaMehta application objects (Topics, Associations, Topic Types, Association Types) and with complex composite structures. The DeepaMehta Core Service also provides basic query capabilities (including fulltext) and traversal capabilities.

Every DeepaMehta plugin has access to the DeepaMehta Core Service. External applications can access the DeepaMehta Core Service as a RESTful web service (via HTTP).

• DeepaMehta Core Events are fired in particular situations, e.g. before and after a topic is created in the database. By implementing event listeners DeepaMehta plugins can react upon these events.

• The Migration Manager ensures the database model and content is compatible with the installed plugins. It does so by running database migrations when a plugin is installed or updated.

• The low-level Storage Layer access methods are specified by a DB-agnostic service provider interface. 3rd-party developers can deploy any database system as the DeepaMehta storage by implementing that interface.

The DeepaMehta Core is an OSGi bundle.

DeepaMehta Plugins

DeepaMehta can be extended with domain-specific data models, application logic and presentation logic by the means of plugins. In the diagram above you can see that a DeepaMehta plugin (colored green) can have both, a server-side portion and a client-side portion.

A DeepaMehta plugin can have several parts:

• A data model. The data model is a collection of domain-specific topic types and/or association types.

• Server-side logic. Server-side logic comes in form of a) service methods, and/or b) event handlers. Server-side logic is written in Java.

• Client-side logic. Client-side logic runs inside the DeepaMehta Webclient. Client-side logic comes in form of a) custom renderers, b) event handlers, and/or c) public API methods. Client-side logic is written in JavaScript.

A DeepaMehta plugin is an OSGi bundle.

DeepaMehta Webclient

The DeepaMehta Webclient is a HTML5/JavaScript/AJAX-based single-page web application. It features the known graph-based DeepaMehta GUI along with the detail panel and main toolbar. In particular it provides the standard topicmap renders and detail panel renderers, most notably the HTML5 Canvas-based default topicmap renderer.

The HTML5 History API is used to fight the problems inherent to single-page AJAX applications: a) each application resource get an unique URL which appears in the browser's address bar, and b) the browser's back and forward buttons work as usual.

The DeepaMehta Webclient is itself a DeepaMehta plugin. It consist mainly of web resources (scripts, images, CSS).

What is special about the DeepaMehta Webclient plugin is that it establishes a client-side application framework along with an event mechanism. This enables other DeepaMehta plugins to hook-in custom code in order to extend the DeepaMehta Webclient. Thus DeepaMehta plugins can e.g. provide custom renderers and widgets, add controls to the toolbar, or add commands to the context menus and the detail panel. Furthermore a DeepaMehta plugin can provide a domain-specific public client-side API (JavaScript), callable by other DeepaMehta plugins.

Storage Implementation

The standard storage implementation is based on Neo4j and Apache Lucene. Neo4j and Lucene are embedded and accessed via the respective Java API.

A DeepaMehta storage implementation is an OSGi bundle.