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Mule 4 Programming Model

Mule 4 changes the programming paradigm used in previous versions. In Mule 3, the main data component of the flow is the message. All transport provided by the runtime is mainly on the content of message payload. For instance, an http:outbound-endpoint is only able to send an HTTP body as the message’s payload content in Mule 3. This behavior led users to add other components before the endpoint simply to properly configure the payload with the proper content.

Mule 3
<set-payload value="{ 'name' : 'pepe' }"/>
<http:request config-ref="HTTP_Request_Configuration" path="/customer" method="POST"/>

Mule 4 simplifies these use cases by allowing for the customization of all input parameters of the operations (http:request).

Mule 4
<http:request method="GET" url="" doc:name="Request" config-ref="HTTP_Request_configuration">
    <http:body ><![CDATA[#[%dw 2.0
            output application/json
            {'name' : 'pepe'}

As the example above shows, you can now define the body to be sent by the http:request operation within the body parameter. This not only reduces the number of components that you need to add to your flow, but it also makes it more readable and prevents affecting the data context just for the purpose of the content required by the http:request operation.

When creating a flow in a Mule app, you might need to store data in a variable so that any component in the flow can use it. Non-void operations (such as the Read operation to the File connector) can store the message data that they return in a variable. Once defined, variables created with the Target (target) parameter are available for use within the flow, and you can access them like you access any other variable.

You often define variables through these parameters:

  • Target (target): Name of the variable in which to store message data. Names can only include numbers, characters, and underscores. For example, hyphens are not allowed in the name.

  • Target Value (targetValue): Value of the data to store in the target variable. By default, the value is the message payload (payload). The field accepts any value that a variable accepts: any supported data type, DataWeave expressions, the keywords payload, attributes, and message, but not the keyword vars.

It is important to understand that setting a target variable changes the typical course of the message through the flow. Normally, an operation outputs a different message than it receives as input. However, when you set a variable from an operation, the operation outputs to the next component in the flow the same message that it received as input. For example, assume you have a flow consisting of component A, followed by component B with a target value of myMessage, and then component C. In this case, component C will receive the same message that B received from A.

Here, a Read operation stores the payload of readme.txt in myVar:

<file:read path="readme.txt" target="myVar" />

Note that the target variable myVar stores the payload by default. You only need to specify a Target Value (targetValue) if you want to store something other than the full payload.

Here, the entire message is stored in myVar:

<file:read path="readme.txt" target="myVar" targetValue="#[message]" />

This way the variable myVar will not only contain the content of the file but also all the related metadata.

  • Expression to access the file content: #[myVar.payload]

  • Expression to access the file metadata: #[myVar.attributes]

Here, only the file size is stored in myVar:

<file:read path="readme.txt" target="size" targetValue="#[attributes.size]" />

The scenarios describe some cases where you might use target variables.

Scenario: Setting a Target Variable from within an Operation

Assume that you want to log the entire message (payload and attributes) returned by a Database connector operation, such as Insert. Instead of using a Set Variable component after the operation, you can instead capture the message data by adding a target variable directly to the operation:

<db:insert config-ref="dbConfig" target="result" targetValue="#[message]">
    <db:sql>INSERT INTO PLANET(POSITION, NAME, DESCRIPTION) VALUES (777, 'Pluto', :description)</db:sql>
        <db:parameter-type key="description" type="CLOB"/>
        #[{{'description' : payload}}]
  • Target Variable for the Insert operation: result

  • Target Value: message in Design Center, #[message] in Anypoint Studio.

Then you might retrieve the result from the Logger like this:

<logger level="INFO" doc:name="Logger" doc:id="8dca355c-a85c-44db-8c53-5b9c188a2431" message="Payload is: #[vars.result.payload] and attributes are: #[vars.result.attributes]"/>
  • Message: Payload is: #[vars.result.payload] and attributes are: #[vars.result.attributes]

Assume that you want to log the name of the first entry in a PLANET database. You can define a target variable like this one through the Select operation of the Database connector:

<db:select config-ref="dbConfig" target="planetName" targetValue="#[payload[0].name]">
    <db:parameterized-query>select * from PLANET order by ID</db:parameterized-query>
  • Target Variable for the Select operation: planetName

  • Target Value: payload[0].name in Design Center, #[payload[0].name] in Anypoint Studio.

Then you might retrieve the result from the Logger like this:

<logger level="INFO" doc:name="Logger" doc:id="8dca355c-a85c-44db-8c53-5b9c188a2431" message="#['Planet name is: ' ++ vars.planetName]"/>
  • Message: 'Planet name is: ' ++ vars.planetName

Scenario: Using the Stored Value as Input to an Operation

Assume that you want to access all planets discovered after a given planet was discovered. You might create a target variable called discoveryDate within a select operation to capture the a planet’s discovery date.

<db:select config-ref="dbConfig" target="discoveryDate" targetValue="#[payload[0].discoveryDate]">
    <db:sql>select discoveryDate from PLANET where name = :name</db:sql>
        #[{'name' : 'pluto'}]
  • Target Variable for the Retrieve operation: discoveryDate

Then you can use the Input Parameter of another operation in your flow (such as the Select operation to the Database connector) to make the variable available for use in your query, for example:

<db:select config-ref="dbConfig" target="discoveryDate" targetValue="#[payload[0].discoveryDate]">
    <db:sql>select * from PLANET where discoveryDate > :discoveryDate</db:sql>
        #[{'discoveryDate' : vars.discoveryDate}]
  • Input Parameter definition for the Select operation:

    • Key: discoveryDate

    • Value: vars.discoveryDate in Design Center, #[vars.discoveryDate] in Anypoint Studio.

Scenario: Bypassing the Normal Message Flow

Assume that you want to insert a number of records into a database that are located in the messages’s payload, then pass those same records on for further processing by the next component in your flow. Though you want to use the Bulk Insert operation to the Database connector to insert the records, the operation returns a success message which would replace the current payload thus making the records inaccessible. So, to pass on the records to the next component instead of replacing the payload with the bulk insert result, you can store the success message in a target variable, for example:

  • Target Variable: bulkInsertResult

Then the next operation in your flow can process the records located in the payload.