Mule messages contain a header and payload. The header includes properties and variables that provide metadata about the message. Properties are immutable inbound properties or mutable outbound properties that provide information to downstream flows. Variables are user-defined and come in three scopes: flow, session, and record. While properties are set by systems and pass with the message, variables are temporary pieces of information for the processing application.

1. Mule Message Structure
Properties and Variables
By Srilatha Kante

2. Agenda
• Introduction to Mule Message
• Message Headers
• Properties
• Variables
• Flow and Flow References

3. Mule Message
• The Mule message is the data that passes
through an application via one or more flows.
It consists of two main parts:
– the message header, which contains metadata
about the message
– the message payload, which contains your
business-specific data.
• A Mule message is, itself, embedded within
a Mule message object.

4. Mule Message Structure

5. Properties
• The metadata contained in the message
header consists of properties which provide
useful information about the message.
• Properties has a name and a value.
• Inbound Properties
• Outbound Properties

6. Properties
• Want to interact with server properties
– #[server.date]
• Current Application properties
– #[app.workDir]

7. Inbound Properties
• Inbound properties are immutable, are
automatically generated by the message
source and cannot be set or manipulated by
the user
– For HTTP – http.query.string

8. Outbound Properties
• Outbound properties are mutable; they are
set during the course of a flow and can
become inbound properties when the
message passes from the outbound endpoint
of one flow to the inbound endpoint of a
different flow via a transport.
• They contain metadata similar to that of an
inbound property, but an outbound property
is applied after the message enters the flow.

9. Outbound Properties
• Outbound properties can be set automatically
by Mule or a user can set them by manually
inserting one or more transformer elements in
the flow.
• Note that if the message is passed to a new
flow via a flow-ref rather than a connector, the
outbound properties remain outbound
properties rather than being converted to
inbound properties

10. Variables
• Variables are user-defined metadata about a
message. Variables have three scopes:
– Flow variables apply only to the flow in which
they exist.
– Session variables apply across all flows within the
same application.
– Record variables apply to only to records
processed as part of a batch.

11. Variables vs Properties
• Variables are temporary pieces of information
about a message that are meant to be used by
the application that is processing it, rather than
passed along with the message to its destination.
• Variables are more likely to be set by humans
• Properties are more likely to be set and invoked
by systems.
• Variables also have name and value
• Properties have two main
scopes: inbound and outbound.