TransactionErrorHandler

Since Camel 2.0

This is the new default transaction error handler in Camel 2.0 onwards, used for transacted routes.

It uses the same base as the DefaultErrorHandler so it has the same feature set as this error handler.

By default any exception thrown during routing will be propagated back to the caller and the Exchange ends immediately. However you can use the Exception Clause to catch a given exception and lower the exception by marking it as handled. If so the exception will not be sent back to the caller and the Exchange continues to be routed.

Example

In this route below, any exception thrown in eg the validateOrder bean will be propagated back to the caller, and its the jetty endpoint. It will return a HTTP error message back to the client.

from("jetty:http://localhost/myservice/order").transacted().to("bean:validateOrder").to("jms:queue:order");

We can add a onException in case we want to catch certain exceptions and route them differently, for instance to catch a ValidationException and return a fixed response to the caller.

onException(ValidationException.class).handled(true).transform(body(constant("INVALID ORDER")));

from("jetty:http://localhost/myservice/order").transacted().to("bean:validateOrder").to("jms:queue:order");

When the ValidationException is thrown from the validate order bean it is intercepted by the TransactionErrorHandler and it let the onException(ValidationException.class handle it so the Exchange is routed to this route and since we use handled(true) then the original exception is lowered (= cleared) and we transform the message into a fixed response that are returned to jetty endpoint that returns it to the original caller.

Convention over configuration

When you configure a route to be transacted you just mark it as transacted as follows:

from("jms:queue:foo").transacted().to("bean:handleFoo");

And in Spring DSL:

<route>
   <from uri="jms:queue:foo"/>
   <transacted/>
   <to uri="bean:handleFoo"/>
</route>

What happens is that Camel will automatic lookup the right Spring transaction manager. It does using the following rules, in order:
1. If there is only 1 bean with the type org.apache.camel.spi.TransactedPolicy then its used.
2. If there is a bean with id PROPAGATION_REQUIRED and of type org.apache.camel.spi.TransactedPolicy then its used.
3. If there is only 1 bean with the type org.springframework.transaction.PlatformTransactionManager then its used.

However you can explicit configure what you want to use. For instance the transacted DSL accepts a String reference parameter to indicate the bean id of the org.apache.camel.spi.TransactedPolicy bean to use in case you have multiple beans. For instance a PROPAGATION_REQUIRES_NEW bean.

The transactionErrorHandler like the transacted also supprts convention over configuration and it will also automatic lookup the right Spring transaction manager. It does using the following rules, in order:
1. If there is only 1 bean with the type org.apache.camel.spi.TransactedPolicy then its used.
2. If there is a bean with id PROPAGATION_REQUIRED and of type org.apache.camel.spi.TransactedPolicy then its used.
3. If there is only 1 bean with the type org.springframework.transaction.support.TransactionTemplate then its used.
4. If there is only 1 bean with the type org.springframework.transaction.PlatformTransactionManager then its used.

However you can explicit configure what you want to use. For instance the transactionErrorHandler DSL accepts either a TransactedPolicy, a TransactionTemplate or a PlatformTransactionManager.

Using Camel to do redeliveries

As the TransactionErrorHandler also supports to let Camel do redeliveries you can use both worlds. Letting Camel do some redeliveries and at the end the backing transaction manager doing other redeliveries. In fact in the end the transaction manager have the final word. That means if Camel cannot process the exchange then its thrown back to the transaction manager that will perform the rollback, and redelivery.

Example with using Camel to do redeliveries

In the route below we have configured a transaction error handler. It will by default do local redeliveries up till 6 times. In Case Camel could not redeliver the message it will be thrown back to the transaction manager that will do a rollback, and a redelivery if it was configured to do so.

Notice that as we have all the powers from DefaultErrorHandler we can configure an onException where we state that in case of this particular exception, an IllegalArgumentException we will only do redeliveries up till 4 times. (Yes the code is based on an unit test).

And also notice that we mark the routes as transacted using transacted. This is always needed to instruct Camel that these routes are transacted.

If you do not provide any Spring TransactionTemplate to either the transactionErrorHandler, then Camel will automatic lookup in the Spring application context for a transaction manager to use. See more in the Convention over configuration section on this page.

And now the route:

// configure transacted error handler to use up till 4 redeliveries
// we have not passed in any spring TX manager. Camel will automatic
// find it in the spring application context. You only need to help
// Camel in case you have multiple TX managers
errorHandler(transactionErrorHandler().maximumRedeliveries(6));

// speical for this exception we only want to do it at most 4 times
onException(IllegalArgumentException.class).maximumRedeliveries(4);

from("direct:okay")
    // marks this route as transacted, and we dont pass in any parameters so we
    // will auto lookup and use the Policy defined in the spring XML file
    .transacted()
    .setBody(constant("Tiger in Action")).bean("bookService")
    .setBody(constant("Elephant in Action")).bean("bookService");

// marks this route as transacted that will use the single policy defined in the registry
from("direct:fail")
    // marks this route as transacted, and we dont pass in any parameters so we
    // will auto lookup and use the Policy defined in the spring XML file
    .transacted()
    .setBody(constant("Tiger in Action")).bean("bookService")
    .setBody(constant("Donkey in Action")).bean("bookService");
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