[[ErrorhandlinginCamel-ErrorhandlinginCamel]]
=== Error handling in Camel

Error handling in Camel can roughly be separated into two distinct
types:

* non transactional
* transactional

Where non transactional is the most common type that is enabled
out-of-the-box and handled by Camel itself. The transaction type is
handled by a backing system such as a J2EE application server.

[TIP]
====
**Using try ... catch ... finally**

Related to error handling is the link:try-catch-finally.adoc[Try Catch
Finally] feature in Camel.
====

[[ErrorhandlinginCamel-Whendoesanerrorhappen]]
==== When does an error happen

An error happens when

* any uncaught exception is thrown during routing and processing of
messages within Camel

TIP: So think of this as a big exception interceptor that catches all
exceptions and handles what to do.

[[ErrorhandlinginCamel-Nontransactional]]
==== Non transactional

By default Camel uses the non transaction type and orchestrates the
error handling during processing and routing.

As there isn't a single error handling configuration that suites all
uses cases, you should consider altering the default configurations to
better suit you needs.

[[ErrorhandlinginCamel-Camel1.xdefaulterrorhandler]]
==== Camel 1.x default error handler

In Camel 1.x a global <<deadLetterChannel-eip,Dead Letter Channel>>
is setup as the link:error-handler.adoc[Error Handler] by default. It's
configured as:

* redeliver up to 6 times
* pause 1 second between each redelivery attempt
* if all redelivery attempts failed then move exchange into the dead
letter queue
* the default dead letter queue is a logger that logs the exchange at
ERROR level :star:

[WARNING]
====
**Dead Letter Queue (*)**

A dead letter queue is like a black hole, it will consume the
link:exchange.adoc[Exchange] and the link:exchange.adoc[Exchange]
routing is ended with no indication that it failed. +
This works great in the <<jms-component,JMS>> Messaging world where we
don't want a bad message to cause endless retries and causing the system
to exhaust. The message is said to be poison and thus we want to move it
to a dead letter queue so the system can continue to operate and work
with the next message.

This default does not go well with other transports using in a
request/reply messaging style. If the link:exchange.adoc[Exchange]
failed then the original caller will still want a response after the
failure.

So the bottom line is that you *must* configure and setup the error
handling strategies that suits your business needs.
====

[[ErrorhandlinginCamel-Camel2.0onwardsdefaulterrorhandler]]
==== Camel 2.0 onwards default error handler

In Camel 2.0 onwards a global
link:defaulterrorhandler.adoc[DefaultErrorHandler] is set up as the
link:error-handler.adoc[Error Handler] by default. It's configured as:

* no redeliveries
* no dead letter queue
* if the exchange failed an exception is thrown and propagated back to
the original caller wrapped in a `RuntimeCamelException`.

[[ErrorhandlinginCamel-Scopes]]
==== Scopes

Camel supports 2 scopes that is determined by the DSL in use:

[width="100%",cols="25%,25%,25%,25%",options="header",]
|=======================================================================
|DSL |Scope 1 |Scope 2 |Note
|XML DSL |CamelContext |route |Scope 2 takes precedence over scope 1

|Java/Scala DSL |RouteBuilder |route |Scope 2 takes precedence over
scope 1
|=======================================================================

When using XML DSL then scope 1 applies for all routes. Where as when
using Java DSL then route 1 only applies for the given RouteBuilder
instance. So if you have multiple RouteBuilder's then each route builder
has its own scope 1.

[TIP]
====
If you want to share scope among RouteBuilder's you can use class
inheritance and create a base class, and then extend this class for your
RouteBuilder's and invoke the super.configure() method.
====

Mind that there was a bug in Camel that affected the scopes when using
multiple RouteBuilder classes. See more details at
https://issues.apache.org/jira/browse/CAMEL-5456[CAMEL-5456].

[[ErrorhandlinginCamel-Howdoestheerrorhandlerwork]]
==== How does the <<deadLetterChannel-eip,Dead Letter Channel>> error handler work

When Camel is started it will inspect the routes and weave in the error
handling into the routing. With up to 3 supported scopes, the error
handling can be quite complex. And on top of that you have inherited
error handling and you can even configure
link:exception-clause.adoc[Exception Clause]s to handle specific
exception types differently. So yes it's advanced but very powerful when
you get the grip of it.

To keep things simple we first look at the basic concept how Camel
orchestrates the redelivery attempt. At any given node in the route
graph Camel intercepts the current Exchange being routed and wraps it
with the link:error-handler.adoc[Error Handler]. This ensures that the
link:error-handler.adoc[Error Handler] can kick in, just as the AOP
around concept. If the exchange can be routed without any problems then
it's forwarded to the next node in the route graph, *But* if there was
an exception thrown, then the link:error-handler.adoc[Error Handler]
kicks in and decides what to do.

An example illustrating this:

[source,java]
----
errorHandler(deadLetterChannel("jms:queue:dead"));

from("seda:newOrder")
   .to("bean:validateOrder")
   .to("bean:storeOrder")
   .to("bean:confirmOrder");
----

In this route we have 3 nodes (the dots) where the
link:error-handler.adoc[Error Handler] is watching us (The AOP around
stuff). So when an order arrives on the seda queue we consume it and
send it to the validateOrder bean. In case the validation bean processed
ok, we move on to the next node. In case the storeOrder bean failed and
throws an exception it's caught by the
<<deadLetterChannel-eip,Dead Letter Channel>> that decides what to
do next. Either it does a:

* redeliver
* or move it to dead letter queue

It will continue to do redeliveries based on the policy configured. By
default <<deadLetterChannel-eip,Dead Letter Channel>> will attempt
at most 6 redeliveries with 1 second delay. So if the storeOrder bean
did succeed at the 3rd attempt the routing will continue to the next
node the confirmOrder bean. In case all redeliveries failed the Exchange
is regarded as failed and is moved to the dead letter queue and the
processing of this exchange stops. By default the dead letter queue is
just a ERROR logger.

[NOTE]
====
This applies to all kind of link:component.adoc[Components] in Camel.
The sample above only uses <<bean-component,Bean>> but it's the same for
<<file-component,File>>, <<mail-component,Mail>>,
<<velocity-component,Velocity>> or whatever component you use.
====

[[ErrorhandlinginCamel-Transactional]]
==== Transactional

Camel leverages Spring transactions. Usually you can only use this with
a limited number of transport types such as JMS or JDBC based, that yet
again requires a transaction manager such as a Spring transaction, a
J2EE server or a Message Broker.

[[ErrorhandlinginCamel-Howdoesitwork]]
==== How does it work

*Camel 1.x*

Camel does the same weaving as for the non-transactional type. The
difference is that for transactional exchanges the
link:error-handler.adoc[Error Handler] does *not* kick in. You can say
the AOP around does not apply. Camel relies solely on the backing system
to orchestrate the error handling. And as such the when the backing
system does redeliver it will start all over again. For instance if the
exchange was started by a JMS consumer then it's started again as the
JMS message is rolled back on the JMS queue and Camel will re consume
the JMS message again.

*Camel 2.0*

In Camel 2.0 we have empowered the
link:transactionerrorhandler.adoc[TransactionErrorHandler] to build on
top of the same base that
link:defaulterrorhandler.adoc[DefaultErrorHandler] does. This allows you
to use Camel redelivery with transactional routes as well. The Spring
transaction manager is still in charge and have the last say. But you
can use Camel to do some local redelivery, for instance to upload a file
to a FTP server, in which Camel can do local redelivery. So this gives
you the power from both worlds. In case Camel cannot redeliver the
exchange will be failed and rolled back. By default the
link:transactionerrorhandler.adoc[TransactionErrorHandler] does *not*
attempt any local redeliveries. You have to configure it to do so, for
instance to set a maximum redelivers to a number > 0.

See <<transactionalClient-eip,Transactional Client>>
for more.

[[ErrorhandlinginCamel-Seealso]]
==== See also

* link:error-handler.adoc[Error Handler]
* <<deadLetterChannel-eip,Dead Letter Channel>>
* link:exception-clause.adoc[Exception Clause]
* <<transactionalClient-eip,Transactional Client>>
* link:transactionerrorhandler.adoc[TransactionErrorHandler]
* link:defaulterrorhandler.adoc[DefaultErrorHandler]
* link:try-catch-finally.adoc[Try Catch Finally]
* <<loadBalance-eip,Failover Load Balancer>>
