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h2. MDC logging
*Available as of Camel 2.7*

In Camel 2.7 we migrated to use [slf4j|http://www.slf4j.org/] as the logging framework in Camel. This allows us to support [MDC logging|http://www.slf4j.org/api/org/slf4j/MDC.html].
See more details about MDC logging in the [logback manual|http://logback.qos.ch/manual/mdc.html].

The log kit in use must support MDC, such as:
- [log4j|http://logging.apache.org/log4j/1.2/]
- [logback|http://logback.qos.ch/]
- [pax logging|http://wiki.ops4j.org/display/paxlogging/Pax+Logging]

See the log kit documentation how to configure and use MDC.

h3. Enabling in Camel

To enable MDC logging in Camel you can do it as follows:
{code}
CamelContext context = ...
context.setUseMDCLogging(true);
...
{code}

In XML you enable it using the {{useMDCLogging}} attribute as follows:
{code:xml}
<camelContext xmlns="http://camel.apache.org/schema/spring" useMDCLogging="true">
  ...
</camelContext>
{code}

h3. MDC information

Camel provides the following context information available for MDC:

h4. Camel 2.9.x or older
{div:class=confluenceTableSmall}
|| Key || Description ||
| {{exchangeId}} | The exchange id |
| {{messageId}} | *Camel 2.9.1:* The message id |
| {{correlationId}} | The correlation id of the exchange if it's correlated. For example a sub message from the [Splitter] EIP |
| {{transactionKey}} | The id of the transaction for transacted exchanges. Note the id is not unique, but its the id of the transaction template that marks the transaction boundary for the given transaction. Hence we decided to name the key {{transactionKey}} and not {{transactionID}} to point out this fact. |
| {{routeId}} | The id of the route, in which the exchange is currently being routed |
| {{breadcrumbId}} | *Camel 2.8:* An unique id used for tracking messages across transports. |
| {{camelContextId}} | *Camel 2.8.3/2.9:* the camel context id used for tracking the message from different camel context. |
{div}

h4. Camel 2.10 onwards
{div:class=confluenceTableSmall}
|| Key || Description ||
| {{camel.exchangeId}} | The exchange id |
| {{camel.messageId}} | The message id |
| {{camel.correlationId}} | The correlation id of the exchange if it's correlated. For example a sub message from the [Splitter] EIP |
| {{camel.transactionKey}} | The id of the transaction for transacted exchanges. Note the id is not unique, but its the id of the transaction template that marks the transaction boundary for the given transaction. Hence we decided to name the key {{transactionKey}} and not {{transactionID}} to point out this fact. |
| {{camel.routeId}} | The id of the route, in which the exchange is currently being routed |
| {{camel.breadcrumbId}} | An unique id used for tracking messages across transports. |
| {{camel.camelContextId}} | The camel context id used for tracking the message from different camel context. |
{div}

The keys are subject to change as we want to align and leverage MDC across other Apache products such as [ActiveMQ|http://activemq.apache.org], [ServiceMix|http://servicemix.apache.org/] and [Karaf|http://karaf.apache.org/]. 

h3. Example using log4j

If you use log4j you can configure MDC in the log4j.properties file as shown:
{code}
log4j.appender.out.layout.ConversionPattern=%d [%-15.15t] %-5p %-30.30c{1} - %-10.10X{camel.exchangeId} - %-10.10X{camel.routeId} - %m%n
{code}

Camel will log on startup if MDC is enabled or not
{code}
INFO  SpringCamelContext -            -            - MDC logging is enabled on CamelContext: camel-1
{code}

The follow snippet is from an unit test which shows MDC in use. Notice the exchange id and route id is displayed in their respective columns in the log file:
{code}
INFO  SpringCamelContext -            -            - Apache Camel  (CamelContext: camel-1) started in 1.228 seconds
INFO  foo                - 358739-0-2 - route-a    - Exchange[ExchangePattern:InOnly, BodyType:String, Body:Hello World]
INFO  bar                - 358739-0-2 - route-b    - Exchange[ExchangePattern:InOnly, BodyType:String, Body:Hello World]
INFO  MockEndpoint       -            -            - Asserting: Endpoint[mock://result] is satisfied
{code}

h3. Using breadcrumb
*Available as of Camel 2.8*
The {{breadcrumbId}} key for MDC logging is only available if {{useBreadcrumb}} option has been enabled on [CamelContext] (its default enabled). When enabled Camel will enrich the Camel [Message] by adding a header to it with the key {{breadcrumbId}} containing the id. Camel will use the messageId if no existing breadcrumbId was found in the message. By storing the breadcrumb as a header allow us to transport the breadcrumb across transports which supports message body and headers. For example [HTTP] and [JMS] and many other transports does that.

If you want to disable breadcrumb you do as follows in Java
{code}
CamelContext context = ...
context.setUseBreadcrumb(false);
...
{code}

And in XML DSL
{code:xml}
<camelContext xmlns="http://camel.apache.org/schema/spring" useBreadcrumb="false">
  ...
</camelContext>
{code}