Troubleshooting a Failing DataWeave Script

When you troubleshoot a failing script, one challenge to reproducing an error is having the same inputs the script had when it executed, especially in production environments where inputs can change unexpectedly. Therefore, it’s important to capture the inputs going into a script debugging or using loggers. Often, the failures occur because the input coming from another component upstream is not valid. You can find here a listing of the most common DataWeave errors and how to overcome them.

Dump Input Context and the Script into a Folder

In Mule 4.2.1, DataWeave introduced an experimental feature that enables you to dump the input context and the failing script into a folder so that you can track the failing script along with the data that makes the script fail. This tool is particularly useful for checking that received input data is valid because incorrect scripts often fail when an upstream component generates invalid data.

To use this feature:

  1. Set the following system property to enable the dump feature:


  2. [Optional] Specify the path in which to generate the dump files:


    The default directory is set in the java.io.tmpdir property.

DataWeave Exceptions

The following are some common DataWeave exceptions that you can find in your dump file, along with some details to troubleshoot these errors.

Incorrect Arguments

When a function is called with the incorrect kind of argument, it throws the exception, org.mule.weave.v2.exception.UnsupportedTypeCoercionException. Causes of this exception include:

Function Does Not Accept Null Argument

The most common cause of this exception occurs when one of the arguments is Null and the function does not accept Null as an argument. This issue results in an error message similar to the following:

You called the function '++' with these arguments:
  1: Null (null)
  2: String (" A text")

But it expects one of these combinations:
  (Array, Array)
  (Date, Time)
  (Date, LocalTime)
  (Date, TimeZone)
  (LocalDateTime, TimeZone)
  (LocalTime, Date)
  (LocalTime, TimeZone)
  (Object, Object)
  (String, String)
  (Time, Date)
  (TimeZone, LocalDateTime)
  (TimeZone, Date)
  (TimeZone, LocalTime)

1| payload.message ++ " A text"
  at ++ (line: 1, column: 1)
  at main (line: 1, column: 17)

One way to resolve this issue uses the default operator. For example, using payload.message default "" ++ " A text" appends empty text when the message is null.

MIME Type Is Not Set

When the MIME type is not set, you receive an error message similar to the following:

You called the function 'Value Selector' with these arguments:

  1: String ("{ \"message\": 123}")
  2: Name ("message")

But it expects one of these combinations:
  (Array, Name)
  (Array, String)
  (Date, Name)
  (DateTime, Name)
  (LocalDateTime, Name)
  (LocalTime, Name)
  (Object, Name)
  (Object, String)
  (Period, Name)
  (Time, Name)

1| payload.message
  at main (line: 1, column: 1)

When no MIME type is set on the payload, the MIME type defaults to application/java, and the content is handled as a String instead of a JSON object.

Reader Properties Not Working In a Mule Application

In a Mule application, the input directive to a DataWeave script does not work. Unlike Mule runtime, a standalone DataWeave runtime, such as the one in the DataWeave Playground+, can process a valid MIME type set through the input directive in the same DataWeave script. To input reader properties to a script in a Mule application, configure the outputMimeType attribute for the data source to produce the same results. See Using Reader and Writer Properties for further details.

Stack Overflow

When a function recurses too deeply, an error like the following one is thrown:

Stack Overflow. Max stack is 256

You can configure the maximum stack size by using the property com.mulesoft.dw.stacksize.

No space left on device

To handle large payloads, DataWeave generates data that is handled in memory unless the payload exceeds a configurable limit. If the payload exceeds the limit, the data is stored on disk as output, input, and buffer files in a temporary directory. See DataWeave Memory Management for further details.

When the streams that reference the files are closed, the files are released. Closure normally occurs when flows complete their execution, so many buffer files in the temporary folder can remain in use during long-running and concurrent executions.

To avoid the exception No space left on device, try to provide more resources to run the application, or determine whether you can reduce the application’s resource consumption.

Sometimes bugs prevent the release of the files even after the flow has completed. If the latest Mule release does not fix this issue, please report the issue to the MuleSoft support team.

Closed Streams

Input data that reaches DataWeave is usually a stream. Mule handles the stream and adds auto-closing and repeatable capabilities to it by generating “cursors” over the stream. Sometimes a stream closes prematurely, before it reaches the DataWeave script, and it can be difficult to understand what closed it. In these circumstances, the script fails with an error similar to Cannot open a new cursor on a closed stream.

If this error occurs in latest Mule update, please report it to the MuleSoft support team. It is not possible to work around the issue, but you can use the com.mulesoft.dw.track.cursor.close system property to determine which component closed the stream prematurely. With the property set, the error shows the stack trace from the moment that the stream was closed, which points to the component that triggered the closure.

Output Mismatch When Undefined

Unlike transformations, DataWeave expressions do not require you to define an output format because DataWeave can infer the output based on the expression and the variables you use. Occasionally, the inference process results in a mismatch between the inferred type and the expected type. To resolve this issue, you must make the output explicit. Common examples of this situation occur when:

Extract Data from XML

When extracting a String, for example, from an XML payload with the expression payload.order.product.model, DataWeave infers an XML output based on the payload format. In such cases, an error similar to the following one occurs:

"Trying to output non-whitespace characters outside main element tree (in prolog or epilog), while writing Xml at ." evaluating expression: "payload.order.product.model".

For such an error, you must make the output format explicit, for example: output text/plain --- payload.order.product.model.

Handle Multipart Entries

A common inference error occurs with multipart data, which has a very specific structure. Consider a multipart payload and the expression dw::core::Objects::keySet(payload.parts). Without an explicit output format, DataWeave must infer, based on the payload type, that you intend to output multipart content. In this case, an error similar to the following is thrown:

"Expecting type is {
  preamble?: String,
  parts: {
    _*: {
      headers: Object,
      content: Any
} but got Array, while writing MultiPart.
  at main (Unknown)" evaluating expression: "dw::core::Objects::keySet(payload.parts)".

To resolve this issue, you must define an output format, for example: output application/json --- dw::core::Objects::keySet(payload.parts)

Manipulate Text Data

You can use text data to create a more complex object. However, if you do not define the output format for the input text data, DataWeave infers that it must use the plain text writer for the output. The expression payload splitBy ' ', for example, will fail with an error similar to:

"Text plain writer is unable to write Array.
Cannot coerce Array (org.mule.weave.v2.model.values.ArrayValue$ArraySeqArrayValue@1331b353) to String
  at main (Unknown), while writing TextPlain.
  at main (Unknown)" evaluating expression: "payload splitBy ' '".

Making the output explicit solves the issue: output application/java --- payload splitBy ' '

Encoding Issues

Encoding issues can occur because of a mismatch between encodings used to read and write a file or when the encoding to write some text does not support some of the characters. Common examples include the following:

Incorrect Encoding for the Reader

In Mule, when you use components or connector operations that provide a MIME Type configuration (such as Set Payload, File Read, HTTP Listener), check that the encoding you set corresponds to the encoding used to write the payload. DataWeave reads the encoding that you set in the MIME Type configuration.

If you do not set the MIME type, DataWeave uses the default encoding provided by Mule through the mule.encoding system property.

Encoding Used by the Writer Does Not Support Some Characters

Check that your writer encoding supports the characters that you are trying to write.

For example, writing the text "~―-$¢£㈱①" with the encoding sjis outputs "???$????" because many of the input characters are not supported in that encoding (unlike UTF-8, for example).

%dw 2.0
output application/json encoding="sjis"

DataWeave Warnings

The following are some common DataWeave warnings that appear in your dump file, along with some details to address these warnings.

End of Input Was Reached

This warning appears because DataWeave treats the Unicode non-character U+FFFF as a special character that indicates the end of input.

To avoid this warning, replace the special character in the raw payload and then proceed as usual.

read(payload.^raw replace "\uFFFF" with "NonChar")

Low Performance Issues

application/dw Format

Using output application/dw format can impact the performance of transformations. This format is intended to help you debug the results of DataWeave transformations. It is significantly slower than other formats, so avoid using this format in production applications.

multipart/form-data Content Parsing

For multipart/form-data inputs, accessing content of a large part can cause performance degradation, especially for high-memory formats like application/xlsx, because the interpreter attempts to parse the content of the part for further querying and analysis.

You can use the ^raw selector to avoid parsing binary contents of parts.

The following example uses the ^raw selector on specific operations over each part.


Content-Disposition: form-data; name="file1"; filename="a.json"
Content-Type: application/json

"title": "Java 8 in Action",
"author": "Mario Fusco",
"year": 2014
Content-Disposition: form-data; name="file2"; filename="a.xml"
Content-Type: application/xml

    <title> Available for download! </title>
    <content> Really large content </content>


%dw 2.0
input payload multipart boundary='myboundary'
output application/json
payload.parts mapObject ((value, key, index) ->
    (key): {
	fileName: payload.parts[index].headers.'Content-Disposition'.filename,
    rawContent: payload.parts[index].content.^raw

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