This document summarizes the key components of a Mule message - header, payload, properties, and variables - and how they may change when a message passes through a message processor like a transformer. It provides examples of a message before and after being processed by a DataMapper transformer, demonstrating how the payload, properties, and variables can be modified. It also provides an example of how to set a variable on a message using the set-variable processor.

1. Message State

2. • To better understand how Mule message processors
act upon messages, it is useful to examine the state of
a message before and after it is processed. When a
transformer converts the content of a message payload
from one data structure to another, or from one data
format to another
• For example, JSON to Java Object, Map to CSV, or Java
Object to XML – you may wonder exactly how Mule
has changed, removed or added to the contents of a
message. Reviewing its "before and after" state should
help you to be better able to work with the message
further in the flow.

3. • Message Header
• Message Payload
• Properties
• Variables
• After DataMapper
• Message Header
• Message Payload
• Properties
• Variables
• More Examples
• Setting a Variable on a Message
• Setting a Property on a Message
• Setting a Payload on a Message

4. Message Header
• The Message (see image below) contains data
from the header of the message (i.e. metadata).
In this example, you can see the message's
identifiers and determine if there are
attachments, which would be arrays if they
existed.
• Note that the Message Processor name indicates
a Value of DataMapper; the Message Processor
item indicates the next message processor in the
flow that the message will encounter.

5. Message Payload
• The payload (see image below), not
surprisingly, contains the payload of the
message, or rather, the data that the Mule
application processes. Before it encounters
the DataMapper, the payload contains a CSV
file – currentFile – with type java.io.File.

6. Properties
• The Visual Debugger also displays any inbound
and outbound properties on the message as it
enters the DataMapper. The Inbound
Properties on the message include metadata
about the payload, including its filename,
timstamp and the endpoint through which it
entered the application,
MULE_ORIGINATING_ENDPOINT. Inbound
properties are read-only and cannot be added,
removed or copied by message processors in the
application.

7. Variables
• The Visual Debugger displays any variables or
session variables included in the message
object as it encounters the DataMapper. The
File endpoint in this flow set two Variables on
the message to indicate where DataMapper
should move the file after processing, and the
frequency with which the endpoint polls the
input folder for new data.

8. Setting a Variable on a Message
• The Variable transformer in a flow sets the
payload of the message as a minPrice variable on
the message. Recall that the Message Processor
item indicates the next message processor in the
flow that the message will encounter.
• <flow>
• <set-variable doc:name="Variable"
value="#[payload]" variableName="minPrice"/>
• </flow>