MDC Logging

Mapped Diagnostic Context (MDC) enriches logging and improves tracking by providing more context or information in the logs for the current Mule event.

By default, Mule logs two MDC entries: processor, which shows the location of the current event, and event, which shows the correlation ID of the event.

Mule Tracing module enables you to enhance your logs by adding, removing, and clearing variables from the logging context for a given Mule event. The logging context exists for the entire execution of the corresponding event.

Prerequisites

To use the MDC Logging operations, complete the following tasks:

Install the Mule Tracing module in your application.
Change the pattern layouts in the log4j2.xml file to MDC.

If you are using Anypoint Runtime Fabric or CloudHub 2.0 to deploy your Mule applications, MDC logging isn’t supported.

Install the Mule Tracing Module

Follow the next steps to install the Mule Tracing module in your application.

Open your Mule project in Anypoint Studio.
Go to the Mule Palette.
Select Search in Exchange, and search for the Mule Tracing module.
Select the module and click Add.
Click Finish.

Change the Pattern Layouts in the log4j2.xml File

This change instructs Mule to automatically add the MDC context, which includes the correlation ID and the processor path. You can modify the log4j2.xml file of your Mule application or the file of your Mule instance, which is located in /conf/log4j2.xml.

Follow these steps to change the pattern layouts to MDC:

Open the log4j2.xml file for editing.
Replace [processor: %X{processorPath}; event: %X{correlationId}] with [%MDC].

Example log4j2.xml File Configurations

Example: Default log4j2.xml file without MDC logging.

<PatternLayout pattern="%-5p %d [%t] [processor: %X{processorPath}; event: %X{correlationId}] %c: %m%n"/>

Example: Updated log4j2.xml file with MDC logging.

<PatternLayout pattern="%-5p %d [%t] [%MDC] %c: %m%n"/>

Available Operations

XML Element Description Accepted Attributes

set-logging-variable

Sets a logging variable and its value.

Attribute Name Description

variableName

The name of the variable to log.

value

The value to assign to the variable. Accepts DataWeave expressions.

remove-logging-variable

Removes a logging variable.

Attribute Name Description

variableName

The name of the variable to remove.

clear-logging-variables

Removes all the logging variables. This option does not remove the processor path or the correlation ID.

None

Configure MDC Logging in Your Application

To configure MDC logging, add any of the available operations to your Mule application flow and specify the corresponding attributes.

For example, to configure a logging variable, insert the <tracing:set-logging-variable> XML element into your application flow:

<flow name="exampleFlow">
    ...
    <tracing:set-logging-variable variableName="testVar" value="testValue" />
    ...
</flow>

After executing the flow, the output logs are:

INFO  2021-04-08 16:58:26,882 [[MuleRuntime].uber.15: [test-project-app].exampleFlow.CPU_LITE @18f679] [{correlationId=c85e16c0-98a4-11eb-bc34-cac765a2219b, processorPath=exampleFlow/processors/2, testVar=testValue}] org.mule.runtime.core.internal.processor.LoggerMessageProcessor: Example

This logging context affects any output of a Logger component and any internal logging that Mule runtime produces. Therefore, you can add more context to the event that Mule is processing if you need additional information for tracking down any potential issue, for example, when an unexpected error occurs.

Example

Consider the following application:

<flow name="logging-variables">
    <http:listener config-ref="HTTP_Listener_config" path="/order"/>
    <tracing:set-logging-variable variableName="customerId" value="#[payload.customerId]"/>
    <tracing:set-logging-variable variableName="requestPath" value='#["$(attributes.method):$(attributes.requestPath)"]'/>
    <logger level="INFO" message="#[output application/json --- payload]" />
</flow>

After sending the following request:

curl --location --request GET '0.0.0.0:8081/order' \
--header 'Content-Type: application/json' \
--data-raw '{
    "orderId": 548102842,
    "customerId": "ARG-12934",
    "items": [
        "CP-123",
        "CP-452"
    ]
}'

The output log is:

INFO  2021-04-09 11:14:38,409 [[MuleRuntime].uber.05: [tracing-module].tracing-moduleFlow.CPU_LITE @34a62707] [processor: tracing-moduleFlow/processors/2; event: eb2b2461-993d-11eb-8a64-4865ee1fd814] {correlationId=eb2b2461-993d-11eb-8a64-4865ee1fd814, customerId=ARG-12934, processorPath=tracing-moduleFlow/processors/2, requestPath=GET:/order} org.mule.runtime.core.internal.processor.LoggerMessageProcessor: {
    "orderId": 548102842,
    "customerId": "ARG-12934",
    "items": [
        "CP-123",
        "CP-452"
    ]
}