Generating a message flow exception
- Assuming that the WebSphere Message Broker Toolkit has not yet been started, from the Start menu, choose Start => Programs => IBM WebSphere Message Broker Toolkit =>
IBM WebSphere Message Broker Toolkit 8.0 => WebSphere Message Broker Toolkit 8.0. You will be asked for the location of a workspace -- the rest of this tutorial assumes that you are
- Close the Welcome screen and from your empty workspace, launch the Quick Start wizard: Click Start by creating an application as shown below in the red box.
If you have chosen to use your existing workspace, you can also launch the Quick Start menu by clicking the New link at the top of the Broker Development view on the left:
- The Quick Start wizard requires an application name. Specify App_DotNet_Exceptions and click Finish:
- In the Application Development window, click New underneath the application you just created, and then on the pop-up New Artifact window, select Message Flow, as shown below:
- In the New Message Flow wizard that opens, specify a Message Flow name of
MyFlow, and then click Finish:
- The message flow will appear with an empty canvas. Create the contents of a simple message flow by dragging and dropping two nodes onto the flow canvas. Take an MQ Input node from the WebSphere MQ drawer, and a Throw node from the Construction drawer of the palette. Rename the MQ Input node
DOTNET.EXCEPTIONS.INand set the Basic property of Queue name to the same value. Wire the Out terminal of the MQ Input node to the In terminal of the Throw node. The resulting message flow should look like this:
The Basic properties panel of the MQInput node, where the queue name is configured, should look like this:
- Configure the Throw node's basic property of Message text to a value such as
This is a deliberate exception!:
- Save the message flow: Type Ctrl-S or select File => Save.
- To test the message flow, you need to define an input queue. Assuming that WebSphere Message Broker Explorer has not yet been started, from the Start menu, choose
Start => Programs => IBM WebSphere Message Broker 126.96.36.199 => IBM WebSphere Message Broker Explorer. This tutorial assumes your user ID has permissions to administer WebSphere MQ.
This tutorial also assumes that you have set up a runtime broker using the default configuration, which provides a message broker named
MB8BROKERassociated with a queue manager named
MB8QMGR. For more information on the default configuration, see Creating the Default Configuration in the Message Broker information center. Right-click the Queues folder underneath the queue manager
MB8QMGRand select New => Local Queue:
- In the New Local Queue wizard, specify the Name as
DOTNET.EXCEPTIONS.INand click Finish:
- For the purpose of the later examples, create another two queues using the same New Local Queue wizard. Name the queues
DOTNET.EXCEPTIONS.OUT. The Message Broker Explorer view of the queue manager should then look like this:
- Now return to the WebSphere Message Broker Toolkit and the message flow
MyFlowthat you constructed earlier. This simple example demonstrates what message flows do when they encounter an exception. The Throw node is a simple way for a flow developer to generate an exception. When an input message reaches the Throw node, an exception is created and the propagation flows backwards through the flow until the exception is caught. In this simple flow, the Catch terminal of the MQInput node has not been wired, so when the rollback occurs, the message is backed out to the input queue's backout queue if one exists, or placed on the queue manager's dead letter queue if one is defined. If neither of these options is possible, then the message is discarded if non-persistent, or left back on the input queue if persistent, and the exception is recorded in the Windows Application Log. To demonstrate this behaviour, right-click the MQInput node and select Test:
- Click OK to confirm that you are happy to deploy the message flow within an application:
- A test client file will be created named App_DotNet_Exceptions.mbtest. Type any input message (the text is not important) into the main box, then click Send Message in the bottom right corner:
- The Select Deployment Location wizard opens. Keep the default settings and click Finish, as shown below:
- After a few seconds, the message flow will deploy and the input message will be sent, exercising the flow:
- Open the Windows Event Viewer and in the Application Log, you should see information messages showing the deployment that you just executed. Check the three most recent entries.
The first error (BIP2628) shows the exception being caught at the MQInput after the flow rolled back. The information event (BIP3001) explains what caused the rollback
(the deliberate exception thrown by the Throw node). The last error message (BIP2648) indicates that the backout threshold has been reached, and therefore the message will be discarded:
This section of the tutorial showed a simple exception being deliberately thrown from a message flow. In the next section, you will catch this exception using the Catch terminal of the MQInput node, and then in later sections you will explore the .NETCompute node's ability to generate and catch exceptions.