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The ESB profile of WSO2 EI comes with an assortment of mediators to filter, transform, route and manipulate messages. Mediators provide an easy way of extending the ESB.  When you have a scenario that requires functionality not provided by the existing mediators, you can write your own custom mediators to implement your specific business requirements. Your custom mediators then must be plugged into the ESB profile. After adding them to the ESB profile, they function together with core mediators that come with the product. The custom mediators can be distributed in a packaged form that can be installed in another ESB profile instance.

Writing an ESB Mediator

There are two ways of writing an ESB mediator:

The easiest way to write a mediator is to extend your mediator class from the org.apache.synapse.mediators.AbstractMediator class. For example, you can see the following articles in the WSO2 library:

You can use the Class mediator and custom mediators for user-specific custom developments when there is no built-in mediator that already provides the required functionality. However, class and custom mediators incur a high maintenance overhead. Custom mediators in particular might introduce version migration complications when upgrading WSO2 EI. Therefore, avoid using them unless  the scenario is frequently re-used and heavily user-specific. For best results, use WSO2 EI Tooling to debug Class and custom mediators.

Building the mediator

After you write the mediator, you must build it and make it an OSGI bundle so that it will work with the ESB.

Basic approach

Create a regular JAR that links to the Synpase core JAR and place it in the <EI_HOME>/ lib directory. The platform will automatically make it an OSGI bundle and deploy it to the server.

Advanced approach

If you want to control the way your mediator is created as an OSGI bundle, you must write the POM files so that you can export and import the packages you need, as shown in the examples below.

Following is a POM file that creates the mediator using Class Mediator.

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>org.test</groupId>
    <artifactId>org.test</artifactId>
    <version>1.0.0</version>
    <packaging>bundle</packaging>
    <name>My Samples - Test mediator</name>
    <url>http://www.test.com</url>

    <repositories>
        <repository>
            <id>wso2-maven2-repository</id>
            <url>http://dist.wso2.org/maven2</url>
        </repository>
        <repository>
            <id>apache-Incubating-repo</id>
            <name>Maven Incubating Repository</name>
            <url>http://people.apache.org/repo/m2-incubating-repository</url>
        </repository>
        <repository>
            <id>apache-maven2-repo</id>
            <name>Apache Maven2 Repository</name>
            <url>http://repo1.maven.org/maven2/</url>
        </repository>
    </repositories>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.0</version>
                <configuration>
                    <source>1.5</source>
                    <target>1.5</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <version>1.4.0</version>
                <extensions>true</extensions>
                <configuration>
                    <instructions>
                        <Bundle-SymbolicName>org.test</Bundle-SymbolicName>
                        <Bundle-Name>org.test</Bundle-Name>
                        <Export-Package>
                            org.test.mediator.*,
                        </Export-Package>
                        <Import-Package>
                            *; resolution:=optional
                        </Import-Package>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>

    <dependencies>
        <dependency>
            <groupId>org.apache.synapse</groupId>
            <artifactId>synapse-core</artifactId>
            <version>2.1.1-wso2v5</version>
        </dependency>
    </dependencies>
</project>

The Maven bundle plug-in was used for creating the OSGI bundle here. Make sure you export the correct package that contains the mediator code. Otherwise, your mediator will not work. If you are adding third-party libraries to the class mediator, be sure to use the maven-shade-plugin to add the dependencies. See the sample shade plugin given below.

 Example maven-shade-plugin
<plugin>
        <artifactId>maven-shade-plugin</artifactId>
        <version></version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>shade</goal>
            </goals>
            <configuration>
              <artifactSet>
                <includes>
                  <include></include>
                  <include></include>
                </includes>
              </artifactSet>
            </configuration>
          </execution>
        </executions>
        <configuration>
          <finalName></finalName>
        </configuration>
</plugin>

Following is a POM file that creates the mediator with its own XML configuration using the Serialize and Factory classes.

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>org.test</groupId>
    <artifactId>org.test</artifactId>
    <version>1.0.0</version>
    <packaging>bundle</packaging>
    <name>My Samples - Test mediator</name>
    <url>http://www.test.com</url>

    <repositories>
        <repository>
            <id>wso2-maven2-repository</id>
            <url>http://dist.wso2.org/maven2</url>
        </repository>
        <repository>
            <id>apache-Incubating-repo</id>
            <name>Maven Incubating Repository</name>
            <url>http://people.apache.org/repo/m2-incubating-repository</url>
        </repository>
        <repository>
            <id>apache-maven2-repo</id>
            <name>Apache Maven2 Repository</name>
            <url>http://repo1.maven.org/maven2/</url>
        </repository>
    </repositories>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.0</version>
                <configuration>
                    <source>1.5</source>
                    <target>1.5</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <version>1.4.0</version>
                <extensions>true</extensions>
                <configuration>
                    <instructions>
                        <Bundle-SymbolicName>org.test</Bundle-SymbolicName>
                        <Bundle-Name>org.test</Bundle-Name>
                        <Export-Package>
                            org.test.mediator.*,
                        </Export-Package>
                        <Import-Package>
                            *; resolution:=optional
                        </Import-Package>
                        <Fragment-Host>synapse-core</Fragment-Host>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>

    <dependencies>
        <dependency>
            <groupId>org.apache.synapse</groupId>
            <artifactId>synapse-core</artifactId>
            <version>2.1.1-wso2v5</version>
        </dependency>
    </dependencies>
</project>

In this case, it is necessary to make the mediator an OSGI fragment of the synapse-core bundler. To achieve this, use the <Fragment-Host>synapse-core</Fragment-Host>.

Create <PROJECT_HOME>/main/resources/META-INF.services/org.apache.synapse.config.xml.MediatorFactory and <PROJECT_HOME>/main/resources/META-INF.services/org.apache.synapse.config.xml.MediatorSerializer files with the following content as shown below, to add service provider definitions to your Maven project.

create the 2 text files

Content of the org.apache.synapse.config.xml.MediatorFactory file
org.wso2.carbon.mediator.cache.config.xml.CacheMediatorFactory 
Content of the org.apache.synapse.config.xml.MediatorSerializer file
org.wso2.carbon.mediator.cache.config.xml.CacheMediatorSerializer  

Deploying the custom mediator

For the above example, after you create the mediator, place the JAR file in the <EI_HOME>\dropins directory. However, there are three places inside the WSO2 EI distribution for placing the JAR file of a customer mediator. They are:

  • <EI_HOME>\extensions
  • <EI_HOME>\dropins
  • <EI_HOME>\lib

The recommended way is to copy the JAR files into the extensions directory. If you deploy a mediator through a Carbon Application (CAR file), it can only be accessed from the Sequences and Proxy Services within the same CAR file. You need to restart the server after copying the JAR file of the custom mediator into any of these directories.

Extensions directory

If you created a regular non-OSGI mediator, build it and copy the JAR file into this directory. The system will convert the JAR file of the mediator into an OSGI-JAR and deploy it into the server. This way is easy and simple.

However, in this method, you cannot use any OSGI features within the mediator implementation. For example, you cannot make certain packages private or import specific versions of packages. Also, in this method, other than Class mediators, you can use a mediator, which has its own XML configuration.

Dropins directory

 If your custom mediator is a Class mediator, it would be a normal bundle. Instead, if it is a mediator with an XML configuration, then it should be a fragment of the Synapse core, and thereby, you need to create an OSGI bundle for that. This requires basic knowledge about OSGI and the Maven bundle plug-in. If you created your custom mediator as an OSGI bundle, you can place its JAR file in the dropins directory. 

The benefit of creating the custom mediator as an OSGI bundle is that you can use the OSGI features in its implementation as preferred.

Lib directory

You can only copy JAR files of custom mediators, which are created using Class mediators into this directory. If you copy a regular JAR file into this directory, the system automatically converts it to an OSGI bundle.

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