attributes validates to null after using Scatter-Gather - attributes

Question

I have a flow that reads a file, distributes to content to 2 subflows using Scatter-Gather and moves the file upon completion to Output/XML or to Failure/XML on failure.
In both scenarios the expression: attributes.fileName fails because attributes is null. How can I set attributes to be persistent?
<flow name="validateFileContent" doc:id="58c241c9-7227-41a8-a67f-5cf4f52bb4bf" >
<file:listener doc:name="Input changed" doc:id="2e6228a7-e153-489a-b6f6-468f67e76475" config-ref="File_Config" directory="Input/XML" watermarkMode="MODIFIED_TIMESTAMP">
<scheduling-strategy >
<fixed-frequency frequency="5" timeUnit="SECONDS" />
</scheduling-strategy>
</file:listener>
<scatter-gather doc:name="Distribute workload" doc:id="15151949-c61d-4629-9fd4-0a4e16d62eec" >
<route >
<flow-ref doc:name="CategoryToType" doc:id="e5d39afa-3b2a-45ef-81c0-afce77a76aef" name="validateCategoryToType" />
</route>
<route >
<flow-ref doc:name="CategoryToSector" doc:id="d3790ba2-b1ac-4511-b748-91adf255344a" name="validateCategoryToSector"/>
</route>
</scatter-gather>
<file:move doc:name="MoveUponSuccess" doc:id="ee659c03-7af5-4a0e-93b0-edb69aefd3ae" sourcePath='#["Input/XML/" ++ attributes.fileName]' targetPath='#["Output/XML/" ++ now() as String{format: "yyyyMMdd_HHmmss"} ++ attributes.fileName]' config-ref="File_Config"/>
<error-handler>
<on-error-propagate enableNotifications="true" logException="true" doc:name="On Error Propagate" doc:id="16001c58-7609-42a6-9bfa-1cd756998f27" >
<logger level="INFO" doc:name="Logger" doc:id="8ec2af18-3e45-4d15-a373-5e9af74723d7" message="#[error]"/>
<file:move doc:name="MoveUponFailure" doc:id="53653598-1d44-4d9a-903c-e82fa61be842" config-ref="File_Config" sourcePath='#["Input/XML/" ++ attributes.fileName]' targetPath='#["Failure/XML/" ++ now() as String{format: "yyyyMMdd_HHmmss"} ++ attributes.fileName]' />
</on-error-propagate>
</error-handler>
</flow>
I tried setting Metadata for input / output attribute, which failed. I could imagine putting it into a var, but I think that there has to be a better solution.
Stacktrace:
Message : "You called the function '++' with these
arguments: 1: String ("Input/XML/") 2: Null (null)
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| "Input/XML/" ++ attributes.fileName
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Trace: at ++ (line: 1, column:
1) at main (line: 1, column: 14)" evaluating expression:
""Input/XML/" ++ attributes.fileName". Error type :
MULE:EXPRESSION

Answer 1

No, you must save the attribute to a variable if you want to preserve it. There are no guarantees something else in the flows will not override it. For example using any connectors.
Also, metadata is for design time, not execution time.