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In Mule, selecting a value by name from a payload and calling the DataWeave lookup function are different operations. Use a DataWeave selector such as payload.userName to read a known field, or payload[(nameExpression)] when the field name is dynamic. The lookup(flowName, payload, timeoutMillis) function instead invokes a named flow and returns that flow’s payload.
Select a value by a known field name
DataWeave objects contain key/value pairs. To read a field whose key is known, use the single-value selector with the key after the object expression:
%dw 2.0
output application/json
---
payload.userNameHere, userName is the literal key being selected from the payload object. This expression returns that field’s value; it does not call a Mule flow.
Select a value when the key name is dynamic
If the name of the field is stored in a variable or another payload field, use a dynamic selector. Put the expression that evaluates to the key name inside parentheses and square brackets:
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%dw 2.0
output application/json
---
payload[(payload.ref)]If payload.ref contains the string name, this selects the value at payload.name. In general, the form is payload[(nameExpression)].
Check whether a key exists
To test for a key’s presence rather than retrieve its value, use the presence selector syntax keyName?. It returns a Boolean indicating whether the selected object key is present. A presence test and a value selection answer different questions: the first checks existence; the second reads the field’s value.
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What DataWeave’s lookup function does
MuleSoft documents lookup as a function for executing a flow within a Mule app and retrieving its resulting payload. Its signature is lookup(flowName: String, payload: Any, timeoutMillis: Number = 2000). The function is documented for Mule Runtime 4.1.4 and later.
This is not a general-purpose object-key lookup. The first argument identifies a flow, and the payload argument is passed to that flow. The called flow receives neither the original message’s attributes nor its variables, and the function returns only the flow’s resulting payload. The function does not support subflows.
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When to use flow-ref instead
For the usual case of invoking another flow and using its result in a DataWeave script, MuleSoft recommends a flow-ref component with the target attribute. The target captures the flow’s result in a variable that the script can reference.
Do not rely on lookup to perform side effects. MuleSoft cautions that the functional DataWeave engine may invoke lookups in parallel with other lookups, or may not invoke them at all. Use it only when obtaining the called flow’s returned payload is the intended operation.
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Choose the right operation
| Need | Operation | What it uses or returns |
|---|---|---|
| Read a payload field with a known key | payload.userName |
Selects the value for the literal key userName. |
| Read a payload field when its key is held in data | payload[(nameExpression)] |
Evaluates the expression to get the key, then selects that field’s value. |
| Check whether a payload key is present | keyName? |
Returns a Boolean presence result. |
| Run a named flow and retrieve its payload | lookup(flowName, payload, timeoutMillis) |
Invokes a flow and returns its resulting payload; it does not pass attributes or variables. |
| Invoke a flow and use its result in DataWeave | flow-ref with target |
Captures the result in a variable for the script to reference. |
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