[CONF] Apache Camel: Tracer Example (page edited)

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[CONF] Apache Camel: Tracer Example (page edited)

Dhiraj Bokde (Confluence)

Tracer Example has been edited by Claus Ibsen (Jan 19, 2009).

(View changes)


Tracer Example


This example demonstrates the Tracer. Tracer is a tracing feature build in camel core to log snapshots of Exchanges while they are routed. This allows you to see:

  • how a given Exchange was routed
  • a snapshot of the content of the Exchange at any given node it passed in the route

When used Camel will by default log the snapshot at INFO level. This example demonstrates how to persist trace snapshots using JPA into a database. This allows you to store this information and query them from a SQL prompt, giving you full power to analyze the data.


This requires Camel 2.0, the camel-jpa component and configuration of the target database.

Data Model

Camel uses the org.apache.camel.processor.interceptor.JpaTraceEventMessage JPA @Entity as data model.
This class has been enhanced with JPA annotations.

The class has the following properties in the JPA model:

Property Type Description
Id Long Primary key that is generated by the database
Timestamp Date Timestamp when the snapshot was generated. Is the system time of the JMV in which Camel is running
FromEndpoint String the URI of the starting consumer the Exchange was created (usually a from in the route).
PreviousNode String id of the previous step in the route. Is null if there wasn't a previous node such as the start.
ToNode String id of the next step in the route
ExchangeId String unique id of the Exchange
ShortExchangeId String id of the Exchange without the machine name
ExchangePattern String The Exchange Pattern such as InOnly or InOut
Properties String The Exchange properties dumped as a String
Headers String The Exchange IN headers dumped as a String
Body String The Exchange IN body dumped as a String
BodyType String The Exchange IN body java type such as String, org.w3c.Document, com.mycompany.MyOrder etc.
OutHeaders String The Exchange OUT (if any) headers dumped as a String
OutBody String The Exchange OUT body (if any) dumped as a String
OutBodyType String The Exchange OUT body (if any) java type such as String, org.w3c.Document, com.mycompany.MyOrder etc.
CausedByException String The Exchange exception (if any) dumped as a String including stacktrace

The table name for persisting trace events is: CAMEL_MESSAGETRACED

Configuration of the database

The Tracer uses standard JPA configuration for setting the database. In the META-INF/persistence.xml file we setup the service unit and the database configuration as:

<persistence xmlns="http://java.sun.com/xml/ns/persistence"

    <persistence-unit name="tracer" transaction-type="RESOURCE_LOCAL">

            <property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/>
            <property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver"/>
            <property name="hibernate.connection.username" value="sa"/>
            <property name="hibernate.connection.password" value=""/>
            <property name="hibernate.connection.url" value="jdbc:hsqldb:mem:camel_tracer"/>
            <property name="hibernate.hbm2ddl.auto" value="create"/>

            <!-- debugging flags -->
                  <property name="hibernate.show_sql" value="true"/>
                  <property name="hibernate.format_sql" value="true"/>


What is important is to add the JpaTraceEventMessage as a class in the persistence.xml file to register our data model:


In this example we use Hibernate JPA and a HSQLDB as database.

Running the example

The README.txt states how to run the example from either ANT or Maven.

Here we show running with Maven:

mvn camel:run

When the application starts it start:

  • in the console
  • a GUI for browsing the SQL database

Select the console where the application should prompt you to enter some words. Try entering: Camel
The application should respond with a text quote.

You can also enter multiple quotes separate with space, and the response should be the best quote based on the list of words given.
See the file src/main/resources/META-INF/spring/camel-context.xml to give you an idea how it works.

You can enter: Camel Beer and it should be smart enough to find a quote for the beer

Seeing the trace events

When the program was started a GUI application was started as well. Its a SQL prompt for the database. So try entering:

select * from camel_messagetraced

And it should return the list of trace events in the SQL.

We enter this sql: select id, shortExchangeId, previousNode, toNode, body from camel_messagetraced and get the output as the picture below:


The diagram below illustrates the route diagram generated using Visualisation.

We receive an Exchange from the in stream, then its split using the splitWords method. Then the quote method is invoked before its aggregated and finally sent to the stream out to be printed in the console.

If we look at the 6 rows from the traced SQL and with the route diagram in mind we can get a better understand how the Exchange was routed.

1. The Exchange does not have a previousNode so its the first step where its consumed from the input stream and that its going to the splitter.
2. The exchange id has changed and this is the output of the splitter as it creates a new Exchange. We can also see this one has one word in the body. This Exchange is being routed to the quote bean next.
3. This is the 2nd output from the splitter containing the 2nd word. This Exchange is being routed to the quote bean next.
4. This is the Beer Exchange where we can see the output from the quote server and that its being routed to the aggregator.
5. This is the Camel Exchange where we can see the output from the quote server and that its being routed to the aggregator.
6. This is the result of the aggregator where the Exchange ending with id 0-2 "was the winner" and is being routed as the aggregated result to the stream out.

See also