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Mule Message Data and Expression Language

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Mule Message Data and Expression Language

We take a look at Mule Express Language (MEL), what it can be used to do, as well as how to use the data generated by Mule Message.

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Accessing information about Mule messages using Datasense :

  • DataSense is Anypoint Studio’s ability to proactively discover metadata from internal and external resources.

  • Keeps you from having to manually discover information about the data.

  • Facilitates transformations by providing DataWeave expected input or output.

  • We saw this with Transform Message component.

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Other ways to view message information :

  • Add a Logger component to a flow and view its output in the Anypoint Studio console.

  • Use the Anypoint Studio Visual Debugger which is the most comprehensive way. We also have a Mule Expression Evaluator.

  • Use autocomplete when writing expressions in the Anypoint Studio Visual Editor.

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Reviewing the structure of Mule messages:

  • Mule message is a core part of mule applications.

  • Message processors may or may not modify Mule messages.

  • We can enrich Mule messages through Message Enricher which is a part of Mule's advanced training.

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Message Properties :

  • Inbound properties

    • Set from the message source

    • Read-only access

    • Persist throughout the flow

  • Outbound properties

    • Added by

    • Read/write access

    • Can set, remove, copy 

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Inbound message properties :

  • These message properties are read only.

  • Query parameters, URI parameters, Method name, and Content-type are examples of inbound properties.

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Outbound message properties:

  • We can use the Set Property transformer to set outbound properties.

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Payload representation:

  • This is a core part of Mule Message.

  • We can use Set Payload or the Ddataweave Transformer to modify payloads accordingly.

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Setting message properties:

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Debugging applications with the Mule Debugger:

  • Locate the port used by the Mule Debugger.

  • Add a breakpoint, debug an application, and step through the code.

  • Use the Mule Debugger to view message properties.

  • Pass query parameters to a request and locate them in the Debugger.

  • You can add breakpoints to processors and step through the application.

  • By default, Debugger listens for incoming TCP connections on localhost port 6666.

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Mule Expression Language (MEL):

  • Use MEL to access and evaluate the data in the payload, properties, and variables of a Mule message.

  • MEL is a lightweight, Mule-specific expression language.

  • Accessible and usable from within virtually every message processor in Mule.

  • Is used to modify the way the processors act upon the message such as routing or filtering.

  • Makes use of Mule-specific context objects.

  • Case-sensitive.

  • Easy to use with autocomplete everywhere.

Basic MEL Syntax:

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Context Objects:

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Accessing message data:

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Accessing message payload data:

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Accessing relational map data :

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Accessing relational map data :

  • Evaluation: ==, !=, >, <, >=, <=, contains, is #[message.inboundProperties.'http.query.params'.lastname != null]

  • The literal empty tests the emptiness of a value.

  • XPath: #[xpath('expression')]

  • RegEx: #[regex('expression')]

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