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Use Dynamic Writer Properties

The following example shows a Mule flow that transforms comma-separated CSV data into pipe-separated (|) CSV data. Instead of setting a literal value in the script, the example selects the pipe value from a Mule variable, which is part of a Mule event that travels through data-processing components in the flow. The example uses the DataWeave writer function in the body of the script to change the comma-separated input to pipe-separated output.

The Scheduler component (<scheduler/>) in the example generates a Mule event each time it triggers the flow.

The Set Variable component (<set-variable/>) creates a Mule variable named delimiter with the value '|'. In Studio, the value is created as an fx expression (value="#['|']"), rather than a simple string (value="'|'").

The Transform Message component (<ee:transform-message/>) contains a DataWeave script that reads comma-separated CSV input and uses the Mule variable to write pipe-separated CSV output.

The Logger component (<logger/>), which is set to payload, writes the pipe-separated output as INFO in its LoggerMessageProcessor message: macaroni | rigatoni | ravioli | spaghetti.

Mule Flow:
<flow name="ex-use-mule-var-as-prop-config-val" >
  <scheduler doc:name="Scheduler" >
    <scheduling-strategy >
      <fixed-frequency frequency="45" timeUnit="SECONDS"/>
  <set-variable value="#['|']" doc:name="Set Variable" variableName="delimiter" />
  <ee:transform doc:name="Transform Message" >
      <ee:set-payload><![CDATA[%dw 2.0
var myVar = read("macaroni , rigatoni , ravioli , spaghetti", "application/csv" , {"header":false, separator:','})
output application/csv with binary
write(myVar, "application/csv", {"header":false, "separator":vars.delimiter})]]></ee:set-payload>
  <logger level="INFO" doc:name="Logger" message="#[payload]"/>

To provide comma-separated input for the example, the script in the Transform Message component uses a read function as a value to a DataWeave variable, myVar:

var myVar = read("macaroni , rigatoni , ravioli , spaghetti", "application/csv" , {"header":false, separator:','})

To write pipe-separated output, the DataWeave write function in the body of the script changes the comma-separated input to pipe-separated output. To select the pipe value from the Mule variable (delimiter), the function passes the writer property configuration "separator":vars.delimiter as an argument:

write(myVar, "application/csv", {"header":false, "separator":vars.delimiter})

The script uses the following DataWeave output directive in the script’s header: output application/csv with binary. It is necessary to append the with binary setting (introduced in Mule 4.3.0) to invoke the binary writer because the write function returned CSV output. Without the setting, DataWeave would attempt to write CSV again and produce the following issue: CSV Structure should be an Array<Object> or Object but got String, while writing CSV.

Note that if you want to see pipe-separated content from the Tranform Message component’s Preview screen in the Studio UI, you must create sample data for it:

  1. Double-click the Transform Message component from the Message Flow area of the Studio app.

  2. Right-click delimiter: String in the Context tab, which is located on the bottom left of the configuration UI for the component.

  3. Select Edit Sample Data to open a vars-delimiter tab.

  4. Replace any content in the vars-delimiter with the following sample data for the delimiter value: "|". You must put the delimiter in quotation marks.

For further guidance with sample data, see Previewing the Output of a Transformation.