Sample 4: Specifying a Fault Sequence with a Regular Mediation Sequence

Introduction

This sample demonstrates how to specify a fault sequence with a regular mediation sequence. Here a message is sent from the sample client to the back-end service through the ESB via 3 mediation options which are a success scenario, a failure scenario without error handling and a failure scenario with proper error handling.

Prerequisites

For a list of prerequisites, see Prerequisites to Start the ESB Samples.

Building the sample

The XML configuration for this sample is as follows:

<definitions xmlns="http://ws.apache.org/ns/synapse">
    <!-- the default fault handling sequence used by Synapse - named 'fault' -->
    <sequence name="fault">
        <log level="custom">
            <property name="text" value="An unexpected error occured"/>
            <property name="message" expression="get-property('ERROR_MESSAGE')"/>
        </log>
        <drop/>
    </sequence>
    <sequence name="sunErrorHandler">
        <log level="custom">
            <property name="text" value="An unexpected error occured for stock SUN"/>
            <property name="message" expression="get-property('ERROR_MESSAGE')"/>
        </log>
        <drop/>
    </sequence>
    <!-- default message handling sequence used by Synapse - named 'main' -->
    <sequence name="main">
        <in>
            <switch source="//m0:getQuote/m0:request/m0:symbol" xmlns:m0="http://services.samples">
                <case regex="IBM">
                    <send>
                        <endpoint><address uri="http://localhost:9000/services/SimpleStockQuoteService"/></endpoint>
                    </send>
                </case>
                <case regex="MSFT">
                    <send>
                        <endpoint key="bogus"/>
                    </send>
                </case>
                <case regex="SUN">
                    <sequence key="sunSequence"/>
                </case>
            </switch>
            <drop/>
        </in>
        <out>
            <send/>
        </out>
    </sequence>
    <sequence name="sunSequence" onError="sunErrorHandler">
        <send>
            <endpoint key="sunPort"/>
        </send>
    </sequence>
</definitions>

This configuration file synapse_sample_4.xml is available in the <ESB_HOME>/repository/samples directory.

To build the sample

Start the ESB with the sample 4 configuration. For instructions on starting a sample ESB configuration, see Starting the ESB with a sample configuration.

The operation log keeps running until the server starts, which usually takes several seconds. Wait until the server has fully booted up and displays a message similar to "WSO2 Carbon started in n seconds."
Start the Axis2 server. For instructions on starting the Axis2 server, see Starting the Axis2 server.
Deploy the back-end service SimpleStockQuoteService. For instructions on deploying sample back-end services, see Deploying sample back-end services.
Now you have a running ESB instance and a back-end service deployed. In the next section, we will send a message to the back-end service through the ESB using a sample client.

Executing the sample

The sample client used here is the Stock Quote Client, which can operate in several modes. For further details on this sample client and its operation modes, see Stock Quote Client.

To execute the sample client

Run each of the following commands from the <ESB_HOME>/samples/axis2Client directory to trigger sample messages to the back-end service.

ant stockquote -Daddurl=http://localhost:9000/services/SimpleStockQuoteService -Dtrpurl=http://localhost:8280/ -Dsymbol=IBM
 
ant stockquote -Daddurl=http://localhost:9000/services/SimpleStockQuoteService -Dtrpurl=http://localhost:8280/ -Dsymbol=MSFT
 
ant stockquote -Daddurl=http://localhost:9000/services/SimpleStockQuoteService -Dtrpurl=http://localhost:8280/ -Dsymbol=SUN

Analyzing the output

When the IBM stock quote is requested, the configuration routes it to the defined inline endpoint , which then routes the message to the SimpleStockQuoteService on the local Axis2 instance. Therefore, a valid response message is shown at the client.

When the MSFT stock quote is requested, the ESB is instructed to route the message to the endpoint defined as the bogus endpoint, which does not exist. This triggers a fault scenario.

synapse_sample_4.xml - MSFT Quote

<case regex="MSFT"> 
   <send> 
      <endpoint key="bogus"/> 
   </send> 
</case>

The ESB executes the specified error handler sequence closest to the point where the error was encountered. In this case, the currently-executing sequence is main and it does not specify an onError attribute. Whenever ESB cannot find an error handler, it looks for a sequence named fault. As a result, the fault sequence starts executing and it writes a generic error message to the log.

[HttpServerWorker-1] DEBUG SendMediator - Send mediator :: mediate()
[HttpServerWorker-1] ERROR IndirectEndpoint - Reference to non-existent endpoint for key : bogus
[HttpServerWorker-1] DEBUG MediatorFaultHandler - MediatorFaultHandler :: handleFault
[HttpServerWorker-1] DEBUG SequenceMediator - Sequence mediator <fault> :: mediate()
[HttpServerWorker-1] DEBUG LogMediator - Log mediator :: mediate()
[HttpServerWorker-1] INFO  LogMediator - text = An unexpected error occured, message = Reference to non-existent endpoint for key : bogus

When the SUN stock quote is requested, it invokes the sunSequence custom sequence which specifies sunErrorHandler as the error handler as you can see in the synapse_sample_4.xml file.

<case regex="SUN">
 <sequence key="sunSequence"/>
</case>

When the send fails, you will see a proper error handler invocation and the custom error message that is printed as follows.

[HttpServerWorker-1] DEBUG SequenceMediator - Sequence mediator <sunSequence> :: mediate()
[HttpServerWorker-1] DEBUG SequenceMediator - Setting the onError handler for the sequence
[HttpServerWorker-1] DEBUG AbstractListMediator - Implicit Sequence <SequenceMediator> :: mediate()
[HttpServerWorker-1] DEBUG SendMediator - Send mediator :: mediate()
[HttpServerWorker-1] ERROR IndirectEndpoint - Reference to non-existent endpoint for key : sunPort
[HttpServerWorker-1] DEBUG MediatorFaultHandler - MediatorFaultHandler :: handleFault
[HttpServerWorker-1] DEBUG SequenceMediator - Sequence mediator <sunErrorHandler> :: mediate()
[HttpServerWorker-1] DEBUG AbstractListMediator - Implicit Sequence <SequenceMediator> :: mediate()
[HttpServerWorker-1] DEBUG LogMediator - Log mediator :: mediate()
[HttpServerWorker-1] INFO  LogMediator - text = An unexpected error occured for stock SUN, message = Reference to non-existent endpoint for key : sunPort