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Introduction to Mule 4: The Mule Message

Mule 4 includes a simplified Mule message model in which each Mule event has a message and variables associated with it. A Mule message is composed of a payload and its attributes (metadata, such as file size). Variables hold arbitrary user information such as operation results, auxiliary values, and so on. This simplified message model makes it easier to work with data in a consistent way across connectors without overwriting information.

Inbound Properties Are Now Attributes

In Mule 3, inbound properties stored additional information about a payload obtained through a Message Source, such as the query parameters coming through an HTTP listener. Attributes in Mule 4 replace inbound properties and have these advantages:

  • They are strongly typed, so you can easily see what data is available.

  • They can easily be stored in variables that you can access throughout your flow. Here’s an HTTP listener example of a typical Mule Message in Mule 4:

Mule Message structure in Mule 4

Attributes can be easily accessed through expressions, just as inbound properties were in Mule 3, for example:


Outbound Properties

In Mule 3, connectors and transports that need to send additional data, such as headers, must explicitly specify Outbound properties. For example, with the HTTP listener, you might want to specify the outgoing status code response or headers. In Mule 4, you can set each of those separately using an individual DataWeave expression for each one of them, without introducing any side effects in the main flow:

<http:request path="issues" config-ref="http" method="GET">

The example above performs an HTTP request in which individual DataWeave scripts generate headers and query parameters without the need to set message properties and without generating any side effects on the message.

The same concept can be applied for message sources that emits responses, such an HTTP listener:

<http:listener config-ref="api-httpListenerConfig" path="/api/*" doc:name="Listener">
    <http:response statusCode="#[vars.httpStatus default 200]">
        <http:headers>#[vars.outboundHeaders default {}]</http:headers>
    <http:error-response statusCode="#[vars.httpStatus default 500]">
        <http:headers>#[vars.outboundHeaders default {}]</http:headers>

Session Properties

Because transport barriers don’t exist anymore, Session properties are no longer needed and have been removed in Mule 4. You should store data in variables.


Inbound and outbound attachments have been removed. The affected connectors now handle the attachment concept on their own:

  • HTTP now relies on DataWeave’s multipart format support.

  • Email and Web Service Consumer allow attachments to be explicitly added using DataWeave.

Message Collections

One of the benefits of the new Mule Message structure is when dealing with collections. In Mule 3, components that returned multiple payloads, used a special structure called the MuleMessageCollection. In Mule 4, any component that needs to deal with multiple messages can simply set the payload of the message to a List of Mule Messages. You can then iterate over these messages using DataWeave, For Each, or other components.

For example, the <file:list> operation retrieves a List of Messages. Each message has attributes for each file, which makes it easy to make decisions based on file size, whether it’s a directory, and so on.