Plugin Development Guide

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

To see how a DeepaMehta plugin fits in the overall DeepaMehta architecture go to Architecture Overview.

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

Naming Conventions

When you are working with git, your new DeepaMehta repo should always start with 'dm4-',
eg. 'dm4-my-fancy-plugin'. The minor-version number is not part of the repo name.

But the Maven Artifact (jar file) should include the minor-version number, to display the
compatibility with a certain DeepaMehta release, eg. dm48-my-fancy-plugin-0.1.jar or 
dm48-my-fancy-plugin-0.2-SNAPSHOT.jar . Usually a dm48- artefact needs adoption, to be 
compatible with the next major version DeepaMehta 4.9.

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</artifactId>
        <version>4.8</version>
    </parent>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <configuration>
                    <instructions>
                        <Bundle-SymbolicName>
                            org.mydomain.dm4-tagging
                        </Bundle-SymbolicName>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>
</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",
                    childs: {
                        dm4.webclient.show_in_create_menu: true
                    }
                }
            ]
        }
    ]
}

Create the file plugin.properties:

dm4.plugin.activate_after=de.deepamehta.webclient
dm4.plugin.model_version=1

Setup for Hot-Deployment

The easiest way to let DeepaMehta hot-deploy the plugin is to develop it within the bundle-dev directory. To do so move the plugin directory on your hard disc into DeepaMehta's hot-deployment folder called bundle-dev. The next step is then to build your plugin.

But lets first start DeepaMehta in development mode, that is with hot-deployment activated.

In the platforms home directory deepamehta:

$ mvn pax:run

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 dm4.plugin.model_version configuration property in the plugin.properties file, e.g.:

dm4.plugin.model_version=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 dm4.plugin.model_version 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 dm4.plugin.model_version 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 dm4.plugin.model_version 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",
                    childs: {
                        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 dm4 object. By the means of the 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.topicmaps.migrations;

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

public class Migration3 extends Migration {

    @Override
    public void run() {
        TopicType type = dm4.getTopicType("dm4.topicmaps.topicmap");
        type.addAssocDef(mf.newAssociationDefinitionModel("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 <parent> element to declare the artifactId deepamehta-plugin. This brings you necessary dependenies and 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</artifactId>
        <version>4.8</version>
    </parent>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <configuration>
                    <instructions>
                        <Bundle-SymbolicName>
                            org.mydomain.dm4.my-cool-plugin
                        </Bundle-SymbolicName>
                        <Bundle-Activator>
                            org.mydomain.deepamehta4.mycoolplugin.MyCoolPlugin
                        </Bundle-Activator>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

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.

Listening to a DeepaMehta Core event means implementing the corresponding listener interface. A listener interface consist of just one method: the listener method. That method is called by the DeepaMehta Core when the event is fired. The listener interfaces are located in package de.deepamehta.core.service.event.

To listen to a DeepaMehta Core event, in the plugin main class you must:

• Import the listener interface.
• Declare the plugin main class implements that interface.
• Implement the listener method. Use the @Override annotation.
• Import the classes appearing in the listener method arguments.

Example:

package org.mydomain.deepamehta4.mycoolplugin;

import de.deepamehta.core.Topic;
import de.deepamehta.core.model.TopicModel;
import de.deepamehta.core.osgi.PluginActivator;
import de.deepamehta.core.service.Directives;
import de.deepamehta.core.service.event.PostCreateTopicListener;
import de.deepamehta.core.service.event.PostUpdateTopicListener;

import java.util.logging.Logger;



public class MyCoolPlugin extends PluginActivator implements PostCreateTopicListener, PostUpdateTopicListener {

    private Logger log = Logger.getLogger(getClass().getName());

    @Override
    public void postCreateTopic(Topic topic) {
        log.info("### Topic created: " + topic);
    }

    @Override
    public void postUpdateTopic(Topic topic, TopicModel newModel, TopicModel oldModel) {
        log.info("### Topic updated: " + topic + "\nOld topic: " + oldModel);
    }
}

This example plugin listens to 2 DeepaMehta Core events: POST_CREATE_TOPIC and POST_UPDATE_TOPIC.

These particular events are fired after the DeepaMehta Core has created resp. updated a topic. The DeepaMehta Core passes the created/updated topic to the respective listener method. In case of "update" the previous topic content (oldModel) is also passed to enable the plugin to investigate what exactly has changed.

The example plugin just logs the created resp. updated topic. In case of "update" the previous topic content is logged as well.

A list of all DeepaMehta Core events? is available in the reference section.

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).

The service interface

For a plugin to provide a service you must define a service interface. The service interface contains all the method signatures that make up the service. When other plugins consume your plugin's service they do so via the service interface.

To be recogbized the service interface must by convention end its name on ...Service. The service interface must be declared public and is a regular Java interface.

A DeepaMehta plugin can define one service interface at most. More than one service interface is not supported.

As an example see the Topicmaps plugin (part of the DeepaMehta Standard Distribution):

dm4-topicmaps/
    src/
        main/
            java/
                de/
                    deepamehta/
                        topicmaps/
                            TopicmapsService.java

The service interface of the Topicmaps plugin is named TopicmapsService. The plugin package is de.deepamehta.topicmaps.

The Topicmaps service interface looks like this:

package de.deepamehta.topicmaps.service;

import de.deepamehta.topicmaps.TopicmapRenderer;
import de.deepamehta.topicmaps.model.ClusterCoords;
import de.deepamehta.topicmaps.model.Topicmap;

import de.deepamehta.core.Topic;


public interface TopicmapsService {

    Topic createTopicmap(String name,             String topicmapRendererUri);
    Topic createTopicmap(String name, String uri, String topicmapRendererUri);

    // ---

    Topicmap getTopicmap(long topicmapId);

    // ---

    void addTopicToTopicmap(long topicmapId, long topicId, int x, int y);

    void addAssociationToTopicmap(long topicmapId, long assocId);

    void moveTopic(long topicmapId, long topicId, int x, int y);

    void setTopicVisibility(long topicmapId, long topicId, boolean visibility);

    void removeAssociationFromTopicmap(long topicmapId, long assocId);

    void moveCluster(long topicmapId, ClusterCoords coords);

    void setTopicmapTranslation(long topicmapId, int trans_x, int trans_y);

    // ---

    void registerTopicmapRenderer(TopicmapRenderer renderer);
}

You see the Topicmaps service consist of methods to create topicmaps, retrieve topicmaps, and manipulate topicmaps.

Implementing the service

After defining the plugin's service interface you must implement the actual service methods. Implementation takes place in the plugin main file.

The plugin main class must declare that it implements the plugin's service interface. (So you need to import the service interface.) Each service method implementation must be public. Annotate each service method implementation with @Override.

As an example see the implementation of the Topicmaps service:

package de.deepamehta.topicmaps;

import de.deepamehta.topicmaps.model.Topicmap;
import de.deepamehta.topicmaps.TopicmapsService;

import de.deepamehta.core.Topic;
import de.deepamehta.core.osgi.PluginActivator;



public class TopicmapsPlugin extends PluginActivator implements TopicmapsService {

    // *** TopicmapsService Implementation ***

    @Override
    public Topic createTopicmap(String name, String topicmapRendererUri) {
        ...
    }

    @Override
    public Topic createTopicmap(String name, String uri, String topicmapRendererUri) {
        ...
    }

    // ---

    @Override
    public Topicmap getTopicmap(long topicmapId) {
        ...
    }

    // ---

    @Override
    public void addTopicToTopicmap(long topicmapId, long topicId, int x, int y) {
        ...
    }

    ...

You see, the plugin main class TopicmapsPlugin implements the plugin's service interface TopicmapsService.

Consuming a service

Your plugin can consume the services provided by other plugins. To do so your plugin must get hold of the service object of the other plugin. Through the service object your plugin can call all the service methods declared in the other's plugin service interface.

To tell the DeepaMehta Core which plugin service your plugin wants to consume you need to declare an instance variable in your plugin like using the @Inject notation:

    @Inject
    private AccessControlService acService;

Make sure to add your interest in building on the respective plugin service as dependencies to your pom.xml file. In the case of using the AccessControlService? we would need to add the following:

    <dependencies>
        <dependency>
            <groupId>de.deepamehta</groupId>
            <artifactId>deepamehta-accesscontrol</artifactId>
            <version>4.8</version>
        </dependency>
    </dependencies>

Behind the scenes the DeepaMehta Core handles a plugin service as an OSGi service. Because of the dynamic nature of an OSGi environment DeepaMehta plugin services can arrive and go away at any time. Your plugin must deal with that. However, you as a plugin developer must not care about DeepaMehta's OSGi foundation. The DeepaMehta Core hides the details from you and provides an easy-to-use API for consuming plugin services.

To deal with other plugin services coming and going your plugin can override 2 hooks: serviceArrived and serviceGone. These 2 hooks are called by the DeepaMehta Core as soon as a desired plugin becomes available resp. goes away.

The single argument of the 2 serviceArrived and serviceGone hooks is the respective service object, declared generically just as PluginService. (Remember, PluginService is the common base interface for all plugin services.) So casting is required. In serviceArrived you typically store the service object in a private instance variable. In serviceGone you typically set the instance variable to null in order to release the service object.

As an example, see how the Workspaces plugin (part of the DeepaMehta Standard Distribution) consumes the Facets service:

package de.deepamehta.workspaces;

import de.deepamehta.facets.FacetsService;

import de.deepamehta.core.osgi.PluginActivator;
import de.deepamehta.core.service.PluginService;
import de.deepamehta.core.service.annotation.ConsumesService;



public class WorkspacesPlugin extends PluginActivator {

    @Inject
    private FacetsService facetsService;

    // *** Hook Implementations ***

    @Override
    public void serviceArrived(PluginService service) {
        if (service instanceof FacetsService) {
            // do something when the facet service comes around
        }
    }

    @Override
    public void serviceGone(PluginService service) {
        // do something when a service goes away
    }

You see the Workspaces plugin consumes a plugin service: the Facets service. The PluginService object passed to the 2 hooks needs not being further investigated.

In this way your plugin could also consume more than one service.

Providing a RESTful web service

Until here your plugin service is accessible from within the OSGi environment only. You can make the service accessible from outside the OSGi environment as well by promoting it to a RESTful web service. Your plugin service is then accessible from external applications via HTTP. (External application here means both, the client-side portion of a DeepaMehta plugin, or an arbitrary 3rd-party application).

To provide a RESTful web service you must provide a generic plugin service first (as described above in Providing a service) and then make it RESTful by using JAX-RS annotations. With JAX-RS annotations you basically control how HTTP requests will be mapped to your service methods.

To make your plugin service RESTful you must:

• Annotate the plugin main class with @Path to anchor the plugin service in URI space.

• Annotate the plugin main class with @Consumes and @Produces to declare the supported HTTP request and response media types. You can use these annotations also at a particular service method to override the class-level defaults.

• Annotate each service method with one of @GET, @POST, @PUT, or @DELETE to declare the HTTP method that will invoke that service method.

• Annotate each service method with @Path to declare the URI template that will invoke that service method. The URI template can contain parameters, notated with curly braces {...}.

• Annotate service method parameters with @PathParam to map URI template parameters to service method parameters.

As an example let's see how the Topicmaps plugin (part of the DeepaMehta Standard Distribution) annotates its main class and service methods:

package de.deepamehta.topicmaps;

import de.deepamehta.topicmaps.model.Topicmap;
import de.deepamehta.topicmaps.TopicmapsService;

import de.deepamehta.core.Topic;
import de.deepamehta.core.osgi.PluginActivator;

import javax.ws.rs.GET;
import javax.ws.rs.PUT;
import javax.ws.rs.POST;
import javax.ws.rs.DELETE;
import javax.ws.rs.HeaderParam;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import javax.ws.rs.Consumes;



@Path("/topicmap")
@Consumes("application/json")
@Produces("application/json")
public class TopicmapsPlugin extends PluginActivator implements TopicmapsService {

    // *** TopicmapsService Implementation ***

    @POST
    @Path("/{name}/{topicmap_renderer_uri}")
    @Override
    public Topic createTopicmap(@PathParam("name") String name,
                                @PathParam("topicmap_renderer_uri") String topicmapRendererUri) {
        ...
    }

    @GET
    @Path("/{id}")
    @Override
    public Topicmap getTopicmap(@PathParam("id") long topicmapId) {
        ...
    }

    @POST
    @Path("/{id}/topic/{topic_id}/{x}/{y}")
    @Override
    public void addTopicToTopicmap(@PathParam("id") long topicmapId, @PathParam("topic_id") long topicId,
                                   @PathParam("x") int x, @PathParam("y") int y) {
        ...
    }

    ...

JAX-RS: Java API for RESTful Web Services
​http://jsr311.java.net/nonav/releases/1.1/spec/spec.html

Extract values from a HTTP request

This section describes in more detail how DeepaMehta (resp. the underlying JAX-RS implementation to be precise) extracts the service method argument values from the various parts of a HTTP request. As seen in the example above this is controlled by annotating the service method arguments. Besides @PathParam you can use further annotations:

Annotation	Semantics
@PathParam	Extracts the value of a URI template parameter
@QueryParam	Extracts the value of a URI query parameter
@HeaderParam	Extracts the value of a header

A value extracted from a HTTP request is inherently a string. So the JAX-RS implementation must know how to actually construct a Java object (resp. a primitive value) from it. That's why the type of a service method argument that is annotated with one of these annotations must satisfy one of these criteria:

1. The type is a primitive type like int, long, float, double, boolean, char.

2. The type has a constructor that accepts a single String argument.

3. The type has a static method named valueOf that takes a single String argument and returns an instance of the type.

Enum types are special as they already have a static valueOf method. If this one does not fit your need add a fromString method to your enum type that has the same characteristics as the valueOf method mentioned above.

4. The type is List<T>, Set<T>, or SortedSet<T>, where T satisfies criterion 2 or 3.

So, when you use a self-defined class (including enum classes) along with @PathParam, @QueryParam, or @HeaderParam make sure your class satisfies criterion 2 or 3.

As an example lets revisit the getTopicmap method from the previous section:

    @GET
    @Path("/{id}")
    @Override
    public Topicmap getTopicmap(@PathParam("id") long topicmapId) {
        ...
    }

Now you know how exactly the JAX-RS implementation extracts the topicmapId parameter value from the HTTP request:

The topicmapId value is extracted from the request's URI path and then converted to a long. Here criterion 1 is satisfied and the conversion is straight-forward.

Parsing the HTTP request body

Until here we talked about how to extract values from the HTTP request's path, the request's query string, or the request headers. This section describes how to feed the HTTP request body into your service methods. Feeding here refers to a) parsing the body's byte stream, b) constructing a Java object from it, and passing that Java object to a particular service method.

JAX-RS can't know how to construct arbitrary application objects from a sole byte stream. That's why JAX-RS comprises a extension facility called Provider Classes. A provider class is responsible to read the request body, parse it, and construct an particular application object from it. It is the duty of the application developer to implement the required provider classes for the application objects.

A service method that want to receive the constructed application object must have a dedicated parameter called (in JAX-RS speak) the Entity Parameter. The entity parameter stands for the entity that is represented in the request body. Unlike the other service method parameters the entity parameter has no annotation. A service method can have one entity parameter at most (a HTTP request has one body).

To feed the HTTP request body into a service method you must:

• Add an entity parameter to the service method. That is a parameter without any annotation.

• Implement a provider class for the type of the entity parameter, resp. make sure such a provider class already exists (as part of the DeepaMehta Core or one of the installed DeepaMehta plugins).