Generating multiple output messages

You can use the PROPAGATE statement to generate multiple output messages in the Compute node. The output messages that you generate can have the same or different content. You can also direct output messages to any of the four alternate output terminals of the Compute node, or to a Label node.

About this task

For example, to create three copies of the input message received by the Compute node, and send one to the standard Out terminal of the Compute node, one to the first alternate Out1 terminal of the Compute node, and one to the Label node ThirdCopy, code the following ESQL:

SET OutputRoot = InputRoot;
PROPAGATE;
SET OutputRoot = InputRoot;
PROPAGATE TO TERMINAL 'out1';
SET OutputRoot = InputRoot;
PROPAGATE TO LABEL 'ThirdCopy';

In the above example, the content of OutputRoot is reset before each PROPAGATE, because by default the node clears the output message buffer and reclaims the memory when the PROPAGATE statement completes. An alternative method is to instruct the node not to clear the output message on the first two PROPAGATE statements, so that the message is available for routing to the next destination. The code to achieve this result is:

SET OutputRoot = InputRoot;
PROPAGATE DELETE NONE;
PROPAGATE TO TERMINAL 'out1' DELETE NONE;
PROPAGATE TO LABEL 'ThirdCopy';

If you do not initialize the output buffer, an empty message is generated, and the message flow detects an error and throws an exception.

Also ensure that you copy all required message headers to the output message buffer for each output message that you propagate.

If you want to modify the output message content before propagating each message, code the appropriate ESQL to make the changes that you want before you code the PROPAGATE statement.

If you set up the contents of the last output message that you want to generate, and propagate it as the final action of the Compute node, you do not have to include the final PROPAGATE statement. The default action of the Compute node is to propagate the contents of the output buffer when it terminates. This is implemented by the RETURN TRUE statement, included as the final statement in the module skeleton.

For example, to generate three copies of the input message, and not perform any further action, include this code immediately before the RETURN TRUE statement:

SET OutputRoot = InputRoot;
PROPAGATE DELETE NONE;
PROPAGATE DELETE NONE;

Alternatively, you can modify the default behavior of the node by changing RETURN TRUE to RETURN FALSE:

SET OutputRoot = InputRoot;
PROPAGATE DELETE NONE;
PROPAGATE DELETE NONE;
PROPAGATE;
RETURN FALSE;

Three output messages are generated by the three PROPAGATE statements. The final RETURN FALSE statement causes the node to terminate but not propagate a final output message. Note that the final PROPAGATE statement does not include the DELETE NONE clause, because the node must release the memory at this stage.