Writing Custom Handlers

This section introduces handlers and explains how to write a custom handler using an example.

Introducing Handlers

When an API is created and deployed, the API is added to the API Gateway Memory. It has a set of handlers, each of which is executed on the APIs in the same order they appear in the configuration. You can find the default handlers in any API's Synapse definition as shown below if you enable Synapse artifacts file storage.

<handlers>
    <handler class="org.wso2.carbon.apimgt.gateway.handlers.common.APIMgtLatencyStatsHandler">
        <property name="apiUUID" value="<apiUUID>"/>
    </handler>
    <handler class="org.wso2.carbon.apimgt.gateway.handlers.security.CORSRequestHandler">
        <property name="apiImplementationType" value="ENDPOINT"/>
        <property name="AuthorizationHeader" value="Authorization"/>
    </handler>
    <handler class="org.wso2.carbon.apimgt.gateway.handlers.common.APIStatusHandler"/>
    <handler class="org.wso2.carbon.apimgt.gateway.handlers.security.APIAuthenticationHandler">
        <property name="RemoveOAuthHeadersFromOutMessage" value="true"/>
        <property name="APILevelPolicy" value=""/>
        <property name="AuthorizationHeader" value="Authorization"/>
        <property name="keyManagers" value="all"/>
        <property name="CertificateInformation" value="{}"/>
        <property name="APISecurity" value="oauth2,oauth_basic_auth_api_key_mandatory"/>
        <property name="apiUUID" value="<apiUUID>"/>
    </handler>
    <handler class="org.wso2.carbon.apimgt.gateway.handlers.throttling.ThrottleHandler"/>
    <handler class="org.wso2.carbon.apimgt.gateway.handlers.analytics.APIMgtGoogleAnalyticsTrackingHandler">
        <property name="configKey" value="ga-config-key"/>
    </handler>
    <handler class="org.wso2.carbon.apimgt.gateway.handlers.ext.APIManagerExtensionHandler"/>
</handlers>

Let's see what each handler does:

  • APIMgtLatencyStatsHandler: Publishes request and response latencies, if analytics is enabled.
  • CORSRequestHandler: Sets the CORS headers to the request and executes the CORS sequence mediation logic. This handler is thereby responsible for returning the CORS headers from the gateway or routing the requests to the backend and letting the backend send the CORS headers.
  • APIStatusHandler: Handles the status of APIs. For instance this handler mediates blocked APIs from blocked sequence.
  • APIAuthenticationHandler: Validates the OAuth2 bearer token used to invoke the API. It also determines whether the token is of type Production or Sandbox and sets MessageContext variables as appropriate.
  • ThrottleHandler: Throttles requests based on the throttling policy specified by the policyKey property. Throttling is applied both at the application level as well as subscription level.
  • APIMgtGoogleAnalyticsTrackingHandler: Publishes events to Google Analytics. This handler only comes into effect If Google analytics tracking is enabled. See Integrating with Google Analytics for more information.
  • APIManagerExtensionHandler : Triggers extension sequences. By default, the extension handler is listed at last in the handler chain, and therefore is executed last. You cannot change the order in which the handlers are executed, except the extension handler. To configure the API Gateway to execute extension handler first, log in to management console (https://localhost:9443/carbon) and in the main tab, expand the resources section and browse for _system/config/apimgt/applicationdata/tenant-conf.json. Edit the field ExtensionHandlerPosition and provide the value top. This is useful when you want to execute your own extensions before our default handlers in situations like doing additional security checks such as signature verification on access tokens before executing the default security handler. See Adding Mediation Extensions.

Using APILogMessageHandler

Message logging is handled by APIManagerExtensionHandler. APILogMessageHandler is a sample handler that comes with MWARE ESB that can be used for logging.

Info

Why are logs removed from APIManagerExtensionHandler?

The primary purpose of ExtensionHandler is handling extensions to mediation and not for logging messages. When the logs are also included in ExtensionHandler, there's a limitation to improve the ExtensionHandler for developing features because it breaks the logs.

For example, When the ExtensionHandler moves to the top of the handlers set, most of the logs print null values since the handler runs before the APIAuthenticationHandler. Therefore, the logs are removed from the extension handler and APILogMessageHandler introduced as a sample.

Note

To achieve logging requirements, this handler is not the only approach and with custom sequences also it is possible to log messages using the Log Mediator.

In order to enable logging by invoking APILogMessageHandler, follow the steps below.

To enable Message Logging into APIS created from publisher automatically :

  1. Open the <APIM_HOME>/repository/resources/api_templates/velocity_template.xml file and copy the following handler before </Handlers>.

    <handler class="org.wso2.carbon.apimgt.gateway.handlers.logging.APILogMessageHandler"/>

    Note

    In a fully distributed setup, this configuration should be done in the Traffic manager Node.

  2. Copy the following code into the <APIM_HOME>/repository/conf/log4j2.properties file to enable printing DEBUG logs.

    logger.log-msg-handler.name = org.wso2.carbon.apimgt.gateway.handlers.logging.APILogMessageHandler
logger.log-msg-handler.level = DEBUG
    Append the log-msg-handler logger name to loggers configuration which is a comma-separated list of all active loggers. The sample configuration is given below.

    loggers = log-msg-handler, trace-messages, org-apache-coyote,com-hazelcast

    Note

    The logger name log-msg-handler can be replaced by any logger-name.

  3. Restart ESB.

Writing a custom handler

Note

The outcomes of using Class Mediator vs Custom Handler are very similar. However, when using a custom handler you need to maintain a customized velocity template file that needs to be manually merged when you upgrade your product to a newer version. Therefore, it is recommended to use custom Handlers when you wish to specify the exact order of execution of JARs as this can not be done with Mediators.

Custom Handler is a way of extending ESB which the product offer to change the API flow through the API Gateway. What is happening in custom handler can be decided by the code you are writing to extend the product. It can be adding extra security, logging database invocation or something else. This custom handler must extend the org.apache.synapse.rest.AbstractHandler class and implement handleRequest() and handleResponse() methods.

Let's see how you can write a custom handler and apply it to the ESB. In this example, we extend the authentication handler. Make sure your custom handler name is not the same as the name of an existing handler.

MWARE ESB provides the OAuth2 bearer token as its default authentication mechanism. A sample implementation is here. Similarly, you can extend the ESB to support any custom authentication mechanism by writing your own authentication handler class.

Given below is an example implementation. Please find the complete project archive here. You can download, unzip and build the project using maven and Java 7 or 8.

package org.wso2.carbon.test;
import org.apache.synapse.MessageContext;
import org.apache.synapse.core.axis2.Axis2MessageContext;
import org.apache.synapse.rest.AbstractHandler;
import org.wso2.carbon.apimgt.gateway.handlers.security.APISecurityException;

import java.util.Map;

public class CustomAPIAuthenticationHandler extends AbstractHandler {

    public boolean handleRequest(MessageContext messageContext) {
        try {
            if (authenticate(messageContext)) {
                return true;
            }
        } catch (APISecurityException e) {
            e.printStackTrace();
        }
        return false;
    }

    public boolean handleResponse(MessageContext messageContext) {
        return true;  
    }

    public boolean authenticate(MessageContext synCtx) throws APISecurityException {
        Map headers = getTransportHeaders(synCtx);
        String authHeader = getAuthorizationHeader(headers);
        if (authHeader.startsWith("userName")) {
            return true;
        }
        return false;
    }

    private String getAuthorizationHeader(Map headers) {
        return (String) headers.get("Authorization");
    }

    private Map getTransportHeaders(MessageContext messageContext) {
        return (Map) ((Axis2MessageContext) messageContext).getAxis2MessageContext().
        getProperty(org.apache.axis2.context.MessageContext.TRANSPORT_HEADERS);
    }
}
Make sure to update the pom file for the above project you created (or downloaded) with below dependency.

<dependencies>
    <dependency>
        <groupId>org.apache.synapse</groupId>
        <artifactId>synapse-core</artifactId>
        <version>4.0.0-wso2v20</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>org.apache.synapse</groupId>
        <artifactId>synapse-commons</artifactId>
        <version>4.0.0-wso2v20</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>org.wso2.carbon.apimgt</groupId>
        <artifactId>org.wso2.carbon.apimgt.gateway</artifactId>
        <version>9.28.112</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

Engaging the custom handler

  1. Build the custom authenticator code created/downloaded previously.
  2. If the jar you created is non-OSGI, you should place the jar in the <APIM_HOME>/repository/components/lib directory otherwise you should place the jar in the <APIM_HOME>/repository/components/dropins driectory.

  3. Engage the custom handler using the API template as explained below: You can engage a custom handler to all APIs at once or only to selected APIs.

    To engage a custom handler to all APIs, you need to add the custom handler with its logic in the <APIM_HOME>/repository/resources/api_templates/velocity_template.xml file. For example, the following code segment adds the custom authentication handler that you wrote earlier to the velocity_template.xml file while making sure that it skips the default APIAuthenticationHandler implementation:

    <handlers xmlns="http://ws.apache.org/ns/synapse">
<handler class="org.wso2.carbon.test.CustomAPIAuthenticationHandler" />
    #foreach($handler in $handlers)
        #if(!($handler.className == "org.wso2.carbon.apimgt.gateway.handlers.security.APIAuthenticationHandler"))
            <handler xmlns="http://ws.apache.org/ns/synapse" class="$handler.className">
                #if($handler.hasProperties())
                    #set ($map = $handler.getProperties() )
                    #foreach($property in $map.entrySet())
                        <property name="$!property.key" value="$!property.value"/>
                    #end
                #end
            </handler>
        #end
    #end
</handlers>

    To engage a custom handler only to selected APIs, you can add a condition. Given below is an example of adding only the CustomAPIAuthenticationHandler to the sample PizzaShackAPI.

    <handlers xmlns="http://ws.apache.org/ns/synapse">
    #if($apiName == 'admin--PizzaShackAPI')
        <handler class="org.wso2.carbon.sample.auth.CustomAPIAuthenticationHandler"/>
    #end
    #foreach($handler in $handlers)
        #if($apiName != 'admin--PizzaShackAPI' || !($handler.className == "org.wso2.carbon.apimgt.gateway.handlers.security.APIAuthenticationHandler"))
            <handler xmlns="http://ws.apache.org/ns/synapse" class="$handler.className">
                #if($handler.hasProperties())
                    #set ($map = $handler.getProperties() )
                    #foreach($property in $map.entrySet())
                        <property name="$!property.key" value="$!property.value"/>
                    #end
                #end
            </handler>
        #end
    #end
</handlers>

  4. Restart the ESB server.

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