WebSphere Message Brokers
File: ac04780_
Writer: Terry Cowling

Reference topic

This build: July 31, 2007 21:17:38

Filter node

This topic describes the Filter node.

This topic contains the following sections:

Purpose

Use the Filter node to route a message according to message content. Define the route by coding a filter expression in ESQL. You can include elements of the input message or message properties in the filter expression, and you can use data that is held in an external database to complete the expression. The output terminal to which the message is routed depends on whether the expression evaluates to true, false, or unknown.

Connect the terminals that cover all situations that could result from the filter; if the node propagates the message to a terminal that is not connected, the message is discarded even if it is transactional.

The Filter node accepts ESQL statements in the same way as the Compute and Database nodes. The last statement that is executed must be a RETURN <expression> statement, whose expression evaluates to a Boolean value. This Boolean value determines the terminal to which the message is routed. In many cases, the routing algorithm is a simple comparison of message field values. The comparison is described by the expression and the RETURN statement is the only statement. If you code RETURN without an expression (RETURN;) or with a NULL expression, the node propagates the message to the Unknown terminal.

If your message flow requires more complex routing options, use the RouteToLabel and Label nodes.

The Filter node is contained in the Routing drawer of the palette, and is represented in the workbench by the following icon:

Filter node icon

Using this node in a message flow

Look at the following samples for examples of how to use this node:

You can view samples only when you use the information center that is integrated with the Message Brokers Toolkit.

Consider a situation in which you have produced an online test with ten multiple choice questions. Each message coming in has a candidate name and address followed by a series of answers. Each answer is checked, and if it is correct, the field SCORE is incremented by one. When all the answers have been checked, the field SCORE is tested to see if it is greater than five. If it is, the Filter node propagates the message to the flow that handles successful candidate input; otherwise, the message is filtered into the rejection process, and a rejection message is created.

Configuring the Filter node

When you have put an instance of the Filter node into a message flow, you can configure it; see Configuring a message flow node. The properties of the node are displayed in the Properties view.

All mandatory properties for which you must enter a value (those that do not have a default value defined) are marked with an asterisk.

  1. Optional: On the Description tab, enter a short description, a long description, or both. You can also rename the node on this tab.
  2. On the Basic tab:
    1. Specify in Data Source the name by which the appropriate database is known on the system on which this message flow is to execute. The broker connects to this database with user ID and password information that you have specified on the mqsicreatebroker, mqsichangebroker, or mqsisetdbparms command.

      z/OS platform On z/OS systems, the broker uses the broker started task ID, or the user ID and password that were specified on the mqsisetdbparms command JCL, BIPSDBP, in the customization data set <hlq>.SBIPPROC.

    2. Select the Transaction setting from the drop-down menu. The values are:
      • Automatic (the default). The message flow, of which the Filter node is a part, is committed if it is successful. That is, the actions that you define in the ESQL module are performed and the message continues through the message flow. If the message flow fails, it is rolled back. Therefore, if you choose Automatic, the ability to commit or rollback the action of the Filter node on the database depends on the success or failure of the entire message flow.
      • Commit. To commit any uncommitted actions that are performed in this message flow on the database that is connected to this node, irrespective of the success or failure of the message flow as a whole, select Commit. The changes to the database are committed even if the message flow itself fails.
    3. In Filter Expression, identify the module within an ESQL file that contains the ESQL statements that the node executes. The ESQL file, which by default has the name <message_flow_name>.esql, contains ESQL for every node in the message flow that requires it. Each portion of code that is related to a specific node is known as a module. If you want the module name to include one or more spaces, enclose it in double quotes in the Filter Expression property.

      Code ESQL statements to customize the behavior of the Filter node in an ESQL file that is associated with the message flow in which you have included this instance of the Filter node.

      If an ESQL file does not already exist for this message flow, double-click the Filter node, or right-click the node and click Open ESQL to create and open a new ESQL file in the ESQL editor view.

      If the file exists already, click Browse beside the Filter Expression property to display the Module Selection dialog box, which lists the available Filter node modules defined in the ESQL files that can be accessed by this message flow (ESQL files can be defined in other, dependent, projects). Select the appropriate module and click OK; if no suitable modules are available, the list is empty.

      If the module that you specify does not exist, that module is created for you, and the editor displays it. If the file and the module exist already, the editor highlights the correct module.

      If a module skeleton is created for this node in a new or existing ESQL file, it consists of the following ESQL. The default module name is shown in this example:

      CREATE FILTER MODULE <flow_name>_Filter
             CREATE FUNCTION Main() RETURNS BOOLEAN
             BEGIN
                     RETURN TRUE; 
             END;
      END MODULE;

      If you create your own ESQL module, you must create this skeleton exactly. You can update the default name, but ensure that the name that you specify matches the name of the corresponding node property Filter Expression.

      To customize this node, add your own ESQL after the BEGIN statement, and before the RETURN statement. If the expression on the RETURN statement is not TRUE or FALSE, its value is resolved to determine the terminal to which the message is propagated. If the expression resolves to NULL, or you code RETURN;, or you omit the RETURN statement, the node propagates the message to the Unknown terminal.

      You can use all the ESQL statements including SET, WHILE, DECLARE, and IF in this module, but (unlike the Compute node) the Filter node propagates the message that it receives at its input terminal to its output terminal unchanged. Therefore, in the Filter node, like the Database node, you have only one message to which to refer.

      The ESQL correlation names that you use in a Filter node are different from those used for a Compute node. For more information about correlation names refer to the related links.

      You cannot modify any part of any message, so the assignment statement (the SET statement, not the SET clause of the INSERT statement) can assign values only to temporary variables. The scope of actions that you can take with an assignment statement is therefore limited.

    4. For database warning messages to be treated as errors, and to propagate the output message from the node to the Failure terminal, select Treat warnings as errors. The check box is cleared initially.

      When you select the check box, the node handles all positive return codes from the database as errors and generates exceptions in the same way as it does for the negative, or more serious, errors.

      If you do not select the check box, the node treats warnings as normal return codes and does not raise any exceptions. The most significant warning raised is not found, which can be handled safely as a normal return code in most circumstances.

    5. For the broker to generate an exception when a database error is detected, select Throw exception on database error. The check box is selected initially.

      If you clear the check box, you must include ESQL to check for any database error that might be returned after each database call that you make (you can use SQLCODE and SQLSTATE to do this). If an error has occurred, you must handle the error in the message flow to ensure the integrity of the broker and the database; the error is ignored if you do not handle it through your own processing because you have chosen not to invoke the default error handling by the broker. For example, you can include the ESQL THROW statement to throw an exception in this node, or you can use the Throw node to generate your own exception at a later point.

Terminals and properties

The Filter node terminals are described in the following table.

Terminal Description
In The input terminal that accepts a message for processing by the node
Failure The output terminal to which the message is routed if a failure is detected during the computation
Unknown The output terminal to which the message is routed if the specified filter expression evaluates to unknown or null
False The output terminal to which the message is routed if the specified filter expression evaluates to false
True The output terminal to which the message is routed if the specified filter expression evaluates to true

The following tables describe the node properties. The column headed M indicates whether the property is mandatory (marked with an asterisk if you must enter a value when no default value is defined); the column headed C indicates whether the property is configurable (you can change the value when you add the message flow to the bar file to deploy it).

The Filter node Description properties are described in the following table.

Property M C Default Description
Node name No No The node type The name of the node.
Short Description No No   A brief description of the node
Long Description No No   Text that describes the purpose of the node in the message flow

The Filter node Basic properties are described in the following table.

Property M C Default Description
Data Source No Yes   The ODBC data source name of the database in which reside the tables to which you refer in the ESQL that is associated with this node (identified by the Filter Expression property).
Transaction Yes No Automatic The transaction mode for the node. Valid values are Automatic and Commit.
Filter Expression Yes No Filter The name of the module within the ESQL resource (file) that contains the statements to execute against the message that is received in the node
Treat warnings as errors Yes No Cleared If you select the check box, database SQL warnings are treated as errors.
Throw exception on database error Yes No Selected If you select the check box, database errors cause the broker to throw an exception.
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Copyright IBM Corporation 1999, 2007Copyright IBM Corporation 1999, 2007. All Rights Reserved.
This build: July 31, 2007 21:17:38

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