Plugin Development Guide

DeepaMehta is made to be extensible by 3rd-party developers. Developers extend DeepaMehta by developing plugins (resp. "modules" resp. "applications" which is all synonymous). This guide teaches you how to develop DeepaMehta plugins.

Build DeepaMehta from source

The best way to develop DeepaMehta plugins is to build DeepaMehta from source first. This way you get a hot-deploy environment, that is DeepaMehta redeploys your plugin automatically once you compile it. This is very handy while plugin development.

Requirements:

• Java 6 (newer versions might work as well, older versions do not work)
• Maven 3 (older versions do not work)
• Git

Build DeepaMehta from source:

$ git clone git://github.com/jri/deepamehta.git
$ cd deepamehta
$ mvn install -P all

This builds all components of the DeepaMehta Standard Distribution and installs them in your local Maven repository. You'll see a lot of information logged, cumulating in:

...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 53.515s
...

The plugin turn-around cycle

This section illustrates how to begin a plugin project, how to build and how to deploy a plugin, and how to redeploy the plugin once you made changes in its source code. In other words, this section illustrates the plugin development turn-around cycle.

Let's start with a very simple plugin called DeepaMehta 4 Tagging. This plugin will just create a new topic type called Tag. Once the plugin is activated the topic type will appear in the DeepaMehta Webclient's Create menu, so you can create tag topics and associate them with arbitrary topics. And you will be able to fulltext search for tags.

Developing a plugin whose only purpose is to provide new topic type definitions requires no Java or JavaScript? coding. All is declarative, mainly in JSON format.

Of course the topic type could be created interactively as well, by using the DeepaMehta Webclient's type editor. However, being packaged as a plugin means you can distribute it. When other DeepaMehta users install your plugin they can use your type definitions.

Begin a plugin project

From the developer's view a DeepaMehta plugin is just a directory on your hard disc. The directory can have an arbitrary name and exist at an arbitrary location. By convention the plugin directory begins with dm4- as it is aimed to the DeepaMehta 4 platform. The directory content adheres to a certain directory structure and file name conventions. The files are text files (xml, json, properties, java, js, css) and resources like images.

To create the DeepaMehta 4 Tagging plugin setup a directory structure as follows:

dm4-tagging/
    pom.xml
    src/
        main/
            resources/
                migrations/
                    migration1.json
                plugin.properties

Create the file pom.xml with this content:

<project>
    <modelVersion>4.0.0</modelVersion>

    <name>DeepaMehta 4 Tagging</name>
    <groupId>org.mydomain.dm4</groupId>
    <artifactId>tagging</artifactId>
    <version>0.1-SNAPSHOT</version>
    <packaging>bundle</packaging>

    <parent>
        <groupId>de.deepamehta</groupId>
        <artifactId>deepamehta-plugin-parent</artifactId>
        <version>4.1.1-SNAPSHOT</version>
    </parent>
</project>

Create the file migration1.json:

{
    topic_types: [
        {
            value: "Tag",
            uri: "domain.tagging.tag",
            data_type_uri: "dm4.core.text",
            index_mode_uris: ["dm4.core.fulltext"],
            view_config_topics: [
                {
                    type_uri: "dm4.webclient.view_config",
                    composite: {
                        dm4.webclient.show_in_create_menu: true
                    }
                }
            ]
        }
    ]
}

Create the file plugin.properties:

requiredPluginMigrationNr=1
importModels=de.deepamehta.webclient

Setup for Hot-Deployment

In order to let DeepaMehta hot-deploy the plugin you must include it in DeepaMehta's hot-deployment list.

In DeepaMehta's pom.xml: add the plugin's target directory (here: /home/myhome/deepamehta-dev/dm4-tagging/target) to the felix.fileinstall.dir property's CDATA section. Important: don't forget to append a comma to the previous line:

<project>
    ...
    <felix.fileinstall.dir>
        <![CDATA[
            ${project.basedir}/modules/dm4-core/target,
            ${project.basedir}/modules/dm4-webservice/target,
            ${project.basedir}/modules/dm4-webclient/target,
            ...
            ${project.basedir}/modules/dm4-storage-neo4j/target,
            /home/myhome/deepamehta-dev/dm4-tagging/target
        ]]>
    </felix.fileinstall.dir>
    ...
</project>

Now start DeepaMehta. In the directory deepamehta (where you've build):

$ mvn pax:run

This starts DeepaMehta in development mode, that is with hot-deployment activated. You'll see a lot of information logged, cumulating with:

...
Apr 6, 2013 11:21:20 PM de.deepamehta.core.impl.PluginManager checkAllPluginsActivated
INFO: ### Bundles total: 32, DeepaMehta plugins: 16, Activated: 16
Apr 6, 2013 11:21:20 PM de.deepamehta.core.impl.PluginManager activatePlugin
INFO: ########## All Plugins Activated ##########
Apr 6, 2013 11:21:20 PM de.deepamehta.plugins.webclient.WebclientPlugin allPluginsActive
INFO: ### Launching webclient (url="http://localhost:8080/de.deepamehta.webclient/")
...

Then a browser windows opens automatically and displays the DeepaMehta Webclient.

The terminal is now occupied by the Gogo shell. Press the return key some times and you'll see its g! prompt.

Type the lb command to get the list of activated bundles:

g! lb

The output looks like this:

START LEVEL 6
   ID|State      |Level|Name
    0|Active     |    0|System Bundle (3.2.1)
   ...
   14|Active     |    5|DeepaMehta 4 Help (4.1.1.SNAPSHOT)
   15|Active     |    5|DeepaMehta 4 Topicmaps (4.1.1.SNAPSHOT)
   16|Active     |    5|DeepaMehta 4 Webservice (4.1.1.SNAPSHOT)
   17|Active     |    5|DeepaMehta 4 Files (4.1.1.SNAPSHOT)
   18|Active     |    5|DeepaMehta 4 Geomaps (4.1.1.SNAPSHOT)
   19|Active     |    5|DeepaMehta 4 Storage - Neo4j (4.1.1.SNAPSHOT)
   20|Active     |    5|DeepaMehta 4 Core (4.1.1.SNAPSHOT)
   21|Active     |    5|DeepaMehta 4 Access Control (4.1.1.SNAPSHOT)
   22|Active     |    5|DeepaMehta 4 Webclient (4.1.1.SNAPSHOT)
   23|Active     |    5|DeepaMehta 4 Webbrowser (4.1.1.SNAPSHOT)
   24|Active     |    5|DeepaMehta 4 Type Search (4.1.1.SNAPSHOT)
   25|Active     |    5|DeepaMehta 4 Workspaces (4.1.1.SNAPSHOT)
   26|Active     |    5|DeepaMehta 4 Notes (4.1.1.SNAPSHOT)
   27|Active     |    5|DeepaMehta 4 Type Editor (4.1.1.SNAPSHOT)
   28|Active     |    5|DeepaMehta 4 Contacts (4.1.1.SNAPSHOT)
   29|Active     |    5|DeepaMehta 4 Facets (4.1.1.SNAPSHOT)
   30|Active     |    5|DeepaMehta 4 File Manager (4.1.1.SNAPSHOT)
   31|Active     |    5|DeepaMehta 4 Icon Picker (4.1.1.SNAPSHOT)

The DeepaMehta 4 Tagging plugin does not yet appear in that list as it is not yet build.

Build the plugin

In another terminal:

$ cd dm4-tagging
$ mvn clean package

This builds the plugin. After some seconds you'll see:

...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.988s
...

Once build, DeepaMehta hot-deploys the plugin automatically. In the terminal where you've started DeepaMehta the logging informs you about plugin activation:

Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl readConfigFile
INFO: Reading config file "/plugin.properties" for plugin "DeepaMehta 4 Tagging"
Apr 6, 2013 11:38:40 PM de.deepamehta.core.osgi.PluginActivator start
INFO: ========== Starting plugin "DeepaMehta 4 Tagging" ==========
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl createPluginServiceTrackers
INFO: Tracking plugin services for plugin "DeepaMehta 4 Tagging" ABORTED -- no consumed services declared
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl addService
INFO: Adding DeepaMehta 4 core service to plugin "DeepaMehta 4 Tagging"
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl addService
INFO: Adding Web Publishing service to plugin "DeepaMehta 4 Tagging"
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl registerWebResources
INFO: Registering Web resources of plugin "DeepaMehta 4 Tagging" ABORTED -- no Web resources provided
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl registerRestResources
INFO: Registering REST resources of plugin "DeepaMehta 4 Tagging" ABORTED -- no REST resources provided
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl registerRestResources
INFO: Registering provider classes of plugin "DeepaMehta 4 Tagging" ABORTED -- no provider classes provided
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl addService
INFO: Adding Event Admin service to plugin "DeepaMehta 4 Tagging"
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginManager activatePlugin
INFO: ----- Activating plugin "DeepaMehta 4 Tagging" -----
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl createPluginTopicIfNotExists
INFO: Installing plugin "DeepaMehta 4 Tagging" in the database
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.MigrationManager runPluginMigrations
INFO: Running 1 migrations for plugin "DeepaMehta 4 Tagging" (migrationNr=0, requiredMigrationNr=1)
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.MigrationManager$MigrationInfo readMigrationConfigFile
INFO: Reading migration config file "/migrations/migration1.properties" ABORTED -- file does not exist
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.MigrationManager runMigration
INFO: Running migration 1 of plugin "DeepaMehta 4 Tagging" (runMode=ALWAYS, isCleanInstall=true)
Apr 6, 2013 11:38:40 PM de.deepamehta.core.util.DeepaMehtaUtils readMigrationFile
INFO: Reading migration file "/migrations/migration1.json"
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.MigrationManager runMigration
INFO: Completing migration 1 of plugin "DeepaMehta 4 Tagging"
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.MigrationManager runMigration
INFO: Updating migration number (1)
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl registerListeners
INFO: Registering listeners of plugin "DeepaMehta 4 Tagging" at DeepaMehta 4 core service ABORTED -- no listeners implemented
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginImpl registerPluginService
INFO: Registering OSGi service of plugin "DeepaMehta 4 Tagging" ABORTED -- no OSGi service provided
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginManager activatePlugin
INFO: ----- Activation of plugin "DeepaMehta 4 Tagging" complete -----
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginManager checkAllPluginsActivated
INFO: ### Bundles total: 33, DeepaMehta plugins: 17, Activated: 17
Apr 6, 2013 11:38:40 PM de.deepamehta.core.impl.PluginManager activatePlugin
INFO: ########## All Plugins Activated ##########
Apr 6, 2013 11:38:40 PM de.deepamehta.plugins.webclient.WebclientPlugin allPluginsActive
INFO: ### Launching webclient (url="http://localhost:8080/de.deepamehta.webclient/") ABORTED -- already launched
...

When you type again lb in the DeepaMehta terminal you'll see the DeepaMehta 4 Tagging plugin now appears in the list of activated bundles:

START LEVEL 6
   ID|State      |Level|Name
    0|Active     |    0|System Bundle (3.2.1)
   ...
   30|Active     |    5|DeepaMehta 4 File Manager (4.1.1.SNAPSHOT)
   31|Active     |    5|DeepaMehta 4 Icon Picker (4.1.1.SNAPSHOT)
   32|Active     |    5|DeepaMehta 4 Tagging (0.1.0.SNAPSHOT)

Try out the plugin

Now you can try out the plugin. In the DeepaMehta Webclient login as user "admin" and leave the password field empty. The Create menu appears and when you open it you'll see the new type Tag listed. Thus, you can create tags now. Additionally you can associate tags to your content topics, search for tags, and navigate along the tag associations, just as you do with other topics.

The result so far: the DeepaMehta 4 Tagging plugin provides a new topic type definition or, in other words: a data model. All the active operations on the other hand like create, edit, search, delete, associate, and navigate are provided by the DeepaMehta Webclient at a generic level, and are applicable to your new topic type as well.

Redeploy the plugin

Once you've made any changes to the plugin files, you have to build the plugin again. Just like before in the plugin terminal:

$ mvn clean package

Once building is complete the changed plugin is redeployed automatically. You'll notice activity in the DeepaMehta terminal:

Apr 8, 2013 1:10:40 AM de.deepamehta.core.osgi.PluginActivator stop
INFO: ========== Stopping plugin "DeepaMehta 4 Tagging" ==========
Apr 8, 2013 1:10:40 AM de.deepamehta.core.impl.PluginImpl removeService
INFO: Removing DeepaMehta 4 core service from plugin "DeepaMehta 4 Tagging"
Apr 8, 2013 1:10:40 AM de.deepamehta.core.impl.PluginImpl removeService
INFO: Removing Web Publishing service from plugin "DeepaMehta 4 Tagging"
Apr 8, 2013 1:10:40 AM de.deepamehta.core.impl.PluginImpl removeService
INFO: Removing Event Admin service from plugin "DeepaMehta 4 Tagging"
...
...
Apr 8, 2013 1:10:44 AM de.deepamehta.core.osgi.PluginActivator start
INFO: ========== Starting plugin "DeepaMehta 4 Tagging" ==========
...
...
Apr 8, 2013 1:10:44 AM de.deepamehta.core.impl.PluginManager activatePlugin
INFO: ----- Activating plugin "DeepaMehta 4 Tagging" -----
Apr 8, 2013 1:10:44 AM de.deepamehta.core.impl.PluginImpl createPluginTopicIfNotExists
INFO: Installing plugin "DeepaMehta 4 Tagging" in the database ABORTED -- already installed
Apr 8, 2013 1:10:44 AM de.deepamehta.core.impl.MigrationManager runPluginMigrations
INFO: Running migrations for plugin "DeepaMehta 4 Tagging" ABORTED -- everything up-to-date (migrationNr=1)
...
...
Apr 8, 2013 1:10:44 AM de.deepamehta.core.impl.PluginManager activatePlugin
INFO: ----- Activation of plugin "DeepaMehta 4 Tagging" complete -----
Apr 8, 2013 1:10:44 AM de.deepamehta.core.impl.PluginManager checkAllPluginsActivated
INFO: ### Bundles total: 33, DeepaMehta plugins: 17, Activated: 17
Apr 8, 2013 1:10:44 AM de.deepamehta.core.impl.PluginManager activatePlugin
INFO: ########## All Plugins Activated ##########
Apr 8, 2013 1:10:44 AM de.deepamehta.plugins.webclient.WebclientPlugin allPluginsActive
INFO: ### Launching webclient (url="http://localhost:8080/de.deepamehta.webclient/") ABORTED -- already launched
...

In contrast to the initial build of the plugin you can recognize some differences in this log:

• The old version of the plugin currently deployed is stopped.
• The new version of the plugin is deployed (that is started and activated) right away.
• The plugin is not installed again in the database as already done while initial build.
• The migration is not run again as already done while initial build.

To ensure the DeepaMehta Webclient is aware of the changed plugin press the browser's reload button.

Stopping DeepaMehta

To stop DeepaMehta, in the Gogo shell type:

g! stop 0

This stops all bundles, shuts down the webserver, and the database.

Migrations

A migration is a sequence of database operations that is executed exactly once in the lifetime of a particular DeepaMehta installation. You as a developer are responsible for equipping your plugin with the required migrations. Migrations serve several purposes:

1. Define the plugin's data model. That is, storing new topic type definitions and association type definitions in the database. E.g. a Books plugin might define the types Book, Title, and Author.

2. A newer version of your plugin might extend or modify the data model defined by the previous version of your plugin. The migration of the updated plugin change the stored type definitions and transforms existing content if necessary.

3. The application logic of a newer version of your plugin changes in a way it is not compatible anymore with the existing database content. The migration must transform the existing content then.

So, the purpose expressed in points 2. and 3. is to make your plugin upgradable. That is, keeping existing database content in-snyc with the plugin logic. By providing the corresponding migrations you make your plugin compatible with the previous plugin version.

The migration machinery

Each plugin comes with its own data model. For each plugin DeepaMehta keeps track what data model version is currently installed. It does so by storing the version of the installed data model in the database as well. The data model version is an integer number that starts at 0 and is increased consecutively: 0, 1, 2, and so on. Each version number (except 0) corresponds with a particular migration. The migration with number n is responsible for transforming the database content from version n-1 to version n.

You as the developer know 2 things about your plugin: a) Which plugin version relies on which data model version, and b) How to transform the database content in order to advance from a given data model version to the next. So, when you ship your plugin you must equip it with 2 things:

• The information what data model version the plugin relies on.
• All the migrations required to update to that data model version.

The relationship between plugin version and data model version might look as follows:

Plugin Version	Data Model Version
0.1	2
0.2	5
0.2.1	5
0.3	6

If e.g. version 0.1 of the plugin is currently installed, the database holds "2" as the current data model version. When the user updates to version 0.3 of the plugin, DeepaMehta's migration machinery will recognize that data model version 2 is present but version 6 is required. As a consequence DeepaMehta will consecutively run migrations 3 through 6. Once completed, the database holds "6" as the current data model version.

Thus, the users database will always be compatible with the installed version of the plugin. Furthermore, the user is free to skip versions when upgrading the plugin.

Plugin configuration

If your plugin comes with its own data model you must tell DeepaMehta the data model version it relies on. To do so, set the requiredPluginMigrationNr configuration property in the plugin.properties file, e.g.:

requiredPluginMigrationNr=2

DeepaMehta's migration machinery takes charge of running the plugin's migrations up to that configured number. If your plugin comes with no data model, you can specify 0 resp. omit the requiredPluginMigrationNr property as 0 is its default value.

Usually each plugin has its own plugin.properties file. It allows the developer to configure certain aspects of the plugin. The name of the plugin.properties file and its path within the plugin directory is fixed:

dm4-myplugin/src/main/resources/plugin.properties

If no plugin.properties file is present, the default configuration values apply.

The two kinds of migrations

As you've already learned, migrations serve different (but related) purposes: some just create new type definitions and others modify existing type definitions and/or transform existing database content. To support the developer with these different tasks DeepaMehta offers two kinds of migrations:

• A Declarative Migration is a JSON file that declares 4 kinds of things: topic types, association types, topics, associations. Use a declarative migration to let DeepaMehta create new types and instances in the database. Use a declarative migration to let your plugin setup the initial type definitions.

With a declarative migration you can only create new things. You can't modify existing things. All you do with a declarative migration you could achieve with an imperative migration as well, but as long as you just want create new things, it is more convenient to do it declaratively.

• An Imperative Migration is a Java class that has access to the DeepaMehta Core Service. Thus, you can perform arbitrary database operations like creation, retrieval, update, deletion. Use an imperative migration when (a later version of) your plugin needs to modify existing type definitions and/or transform existing database content.

The developer can equip a plugin with an arbitrary number of both, declarative migrations and imperative migrations.

Directory structure

In order to let DeepaMehta find the plugin's migration files, you must adhere to a fixed directory structure and file names. Each migration file must contain its number, so DeepaMehta can run them consecutively.

A declarative migration must be named migration<nr>.json and must be located in the plugin's src/main/resources/migrations/ directory.

An imperative migration must be named Migration<nr>.java and must be located in the plugin's src/main/java/<your plugin package>/migrations/ directory.

Example:

dm4-myplugin/
    src/
        main/
            java/
                org/
                    mydomain/
                        deepamehta4/
                            myplugin/
                                migrations/
                                    Migration2.java
                                    Migration5.java
            resources/
                migrations/
                    migration1.json
                    migration3.json
                    migration4.json
                    migration6.json
                plugin.properties

This example plugin would have set requiredPluginMigrationNr to 6 (configured in plugin.properties), so 6 migrations are involved. 4 are declarative and 2 are imperative here.

Important: for each number between 1 and requiredPluginMigrationNr exactly one migration file must exist. That is either a declarative migration file or an imperative migration file.

It would be invalid if for a given number a) no migration file exists, or b) two migration files exist (one declarative and one imperative). In these cases the DeepaMehta migration machinery throws an error and the plugin is not activated.

Writing a declarative migration

A declarative migration is a JSON file with exactly one JSON Object in it. In a declarative migration you can define 4 things: topic types, association types, topics, associations. The general format is:

{
    topic_types: [
        ...
    ],
    assoc_types: [
        ...
    ],
    topics: [
        ...
    ],
    associations: [
        ...
    ]
}

Each of the 4 sections is optional.

As an example see the (simplified) migration that defines the Note topic type. This migration is part of the DeepaMehta 4 Notes plugin:

{
    topic_types: [
        {
            value: "Title",
            uri: "dm4.notes.title",
            data_type_uri: "dm4.core.text",
            index_mode_uris: ["dm4.core.fulltext"]
        },
        {
            value: "Text",
            uri: "dm4.notes.text",
            data_type_uri: "dm4.core.html",
            index_mode_uris: ["dm4.core.fulltext"]
        },
        {
            value: "Note",
            uri: "dm4.notes.note",
            data_type_uri: "dm4.core.composite",
            assoc_defs: [
                {
                    child_type_uri:        "dm4.notes.title",
                    child_cardinality_uri: "dm4.core.one",
                    assoc_type_uri:        "dm4.core.composition_def"
                },
                {
                    child_type_uri:        "dm4.notes.text",
                    child_cardinality_uri: "dm4.core.one",
                    assoc_type_uri:        "dm4.core.composition_def"
                }
            ],
            view_config_topics: [
                {
                    type_uri: "dm4.webclient.view_config",
                    composite: {
                        dm4.webclient.icon: "/de.deepamehta.notes/images/yellow-ball.png",
                        dm4.webclient.show_in_create_menu: true
                    }
                }
            ]
        }
    ]
}

As you see, this migration defines 3 topic types (and no other things): Title and Text are 2 simple types, and Note is a composite type. A Note is composed of one Title and one Text.

Writing an imperative migration

An imperative migration is a Java class that is derived from de.deepamehta.core.service.Migration and that overrides the run() method. The run() method is called by DeepaMehta to run the migration.

Within the migration you have access to the DeepaMehta Core Service through the dms object. By the means of the DeepaMehta Core Service you can perform arbitrary database operations. Typically this involves importing further objects from the de.deepamehta.core API.

As an example see a migration that comes with the DeepaMehta 4 Topicmaps plugin:

package de.deepamehta.plugins.topicmaps.migrations;

import de.deepamehta.core.TopicType;
import de.deepamehta.core.model.AssociationDefinitionModel;
import de.deepamehta.core.service.Migration;

public class Migration3 extends Migration {

    @Override
    public void run() {
        TopicType type = dms.getTopicType("dm4.topicmaps.topicmap", null);
        type.addAssocDef(new AssociationDefinitionModel("dm4.core.composition_def",
            "dm4.topicmaps.topicmap", "dm4.topicmaps.state", "dm4.core.one", "dm4.core.one"));
    }
}

Here an association definition is added to the Topicmap type subsequently.

The server side

What a DeepaMehta plugin can do at the server side:

• Listen to DeepaMehta Core events. In particular situations the DeepaMehta Core fires events, e.g. before and after it creates a new topic in the database. Your plugin can listen to these events and react in its own way. Thus, the DeepaMehta 4 Workspaces plugin e.g. ensures that each new topic is assigned to a workspace.

• Providing a service. Your plugin can make its business logic, that is its service methods, accessible by other plugins (via OSGi) and/or by external applications (via HTTP/REST). Example: the service provided by the DeepaMehta 4 Topicmaps plugin includes methods to add a topic to a topicmap or to change the topic's coordinates within a topicmap.

• Consuming services provided by other plugins. Example: in order to investigate a topic's workspace assignments and the current user's memberships the DeepaMehta 4 Access Control plugin consumes the service provided by the DeepaMehta 4 Workspaces plugin.

• Access the DeepaMehta Core Service. The DeepaMehta Core Service provides the basic database operations (create, retrieve, update, delete) to deal with the DeepaMehta Core objects: Topics, Associations, Topic Types, Association Types.

Weather a DeepaMehta plugin has a server side part at all depends on the nature of the plugin. Plugins without a server side part include those which e.g. just define a data model or just provide a custom (JavaScript?) renderer.

The plugin main file

You must write a plugin main file if your plugin needs to a) listen to DeepaMehta Core events and/or b) provide a service. The plugin main file contains the event handlers resp. the service implementation then.

The plugin main file must be located directly in the plugin's src/main/java/<your plugin package>/ directory. By convention the plugin main class ends with Plugin.

Example:

dm4-mycoolplugin/
    src/
        main/
            java/
                org/
                    mydomain/
                        deepamehta4/
                            mycoolplugin/
                                MyCoolPlugin.java

Here the plugin package is org.mydomain.deepamehta4.mycoolplugin and the plugin main class is MyCoolPlugin.

A plugin main file is a Java class that is derived from de.deepamehta.core.osgi.PluginActivator. The smallest possible plugin main file looks like this:

package org.mydomain.deepamehta4.mycoolplugin;

import de.deepamehta.core.osgi.PluginActivator;

public class MyCoolPlugin extends PluginActivator {
}

3 things are illustrated here:

• The plugin should be packaged in an unique namespace.
• The PluginActivator class needs to be imported.
• The plugin main class must be derived from PluginActivator and must be public.

Furthermore when writing a plugin main file you must add 2 entries in the plugin's pom.xml:

1. a <dependencies> element to include the deepamehta-core dependency. This brings you the PluginActivator class.
2. a <build> element to configure the Maven Bundle Plugin. It needs to know what your plugin main class is. You must specify the fully-qualified class name.

<project>
    <modelVersion>4.0.0</modelVersion>

    <name>My Cool Plugin</name>
    <groupId>org.mydomain.dm4</groupId>
    <artifactId>my-cool-plugin</artifactId>
    <version>0.1-SNAPSHOT</version>
    <packaging>bundle</packaging>

    <parent>
        <groupId>de.deepamehta</groupId>
        <artifactId>deepamehta-plugin-parent</artifactId>
        <version>4.1.1-SNAPSHOT</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>de.deepamehta</groupId>
            <artifactId>deepamehta-core</artifactId>
            <version>4.1.1-SNAPSHOT</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <configuration>
                    <instructions>
                        <Bundle-Activator>
                            org.mydomain.deepamehta4.mycoolplugin.MyCoolPlugin
                        </Bundle-Activator>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>