[[deadLetterChannel-eip]]
== Dead Letter Channel

Camel supports the
http://www.enterpriseintegrationpatterns.com/DeadLetterChannel.html[Dead
Letter Channel] from the link:enterprise-integration-patterns.html[EIP
patterns] using the
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/processor/DeadLetterChannel.html[DeadLetterChannel]
processor which is an link:error-handler.html[Error Handler].

image:http://www.enterpriseintegrationpatterns.com/img/DeadLetterChannelSolution.gif[image]

The Dead Letter Channel lets you control behaviors including redelivery,
whether to propagate the thrown Exception to the caller (the *handled*
option), and where the (failed) Exchange should now be routed to.

The Dead Letter Channel is also by default configured to not be verbose
in the logs, so when a message is handled and moved to the dead letter
endpoint, then there is nothing logged. If you want some level of
logging you can use the various options on the redelivery policy / dead
letter channel to configure this. For example if you want the message
history then set logExhaustedMessageHistory=true (and logHandled=true
for Camel 2.15.x or older).

When the DeadLetterChannel moves a message to the dead letter endpoint,
any new Exception thrown is by default handled by the dead letter
channel as well. This ensures that the DeadLetterChannel will always
succeed. From *Camel 2.15* onwards this behavior can be changed by
setting the option deadLetterHandleNewException=false. Then if a new
Exception is thrown, then the dead letter channel will fail and
propagate back that new Exception (which is the behavior of the default
error handler). When a new Exception occurs then the dead letter channel
logs this at WARN level. This can be turned off by setting
logNewException=false.

TIP: *Difference between Dead Letter Channel and Default Error
Handler* The Default Error Handler does very little: it ends the Exchange
immediately and propagates the thrown Exception back to the caller.

[[deadLetterChannel-Redelivery]]
=== Redelivery

It is common for a temporary outage or database deadlock to cause a
message to fail to process; but the chances are if its tried a few more
times with some time delay then it will complete fine. So we typically
wish to use some kind of redelivery policy to decide how many times to
try redeliver a message and how long to wait before redelivery attempts.

The
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/processor/RedeliveryPolicy.html[RedeliveryPolicy]
defines how the message is to be redelivered. You can customize things
like

* how many times a message is attempted to be redelivered before it is
considered a failure and sent to the dead letter channel
* the initial redelivery timeout
* whether or not exponential backoff is used (i.e. the time between
retries increases using a backoff multiplier)
* whether to use collision avoidance to add some randomness to the
timings
* delay pattern (see below for details)
* *Camel 2.11:* whether to allow redelivery during stopping/shutdown

Once all attempts at redelivering the message fails then the message is
forwarded to the dead letter queue.

[[deadLetterChannel-AboutmovingExchangetodeadletterqueueandusinghandled]]
=== About moving Exchange to dead letter queue and using handled

*Handled* on <<deadLetterChannel-eip,Dead Letter Channel>>

When all attempts of redelivery have failed the
link:exchange.html[Exchange] is moved to the dead letter queue (the dead
letter endpoint). The exchange is then complete and from the client
point of view it was processed. As such the
<<deadLetterChannel-eip,Dead Letter Channel>> have handled the
link:exchange.html[Exchange].

For instance configuring the dead letter channel as:

[source,java]
----
errorHandler(deadLetterChannel("jms:queue:dead")
    .maximumRedeliveries(3).redeliveryDelay(5000));
----

And in XML:

[source,xml]
----
<route errorHandlerRef="myDeadLetterErrorHandler">
   ...
</route>

<bean id="myDeadLetterErrorHandler" class="org.apache.camel.builder.DeadLetterChannelBuilder">
    <property name="deadLetterUri" value="jms:queue:dead"/>
    <property name="redeliveryPolicy" ref="myRedeliveryPolicyConfig"/>
</bean>

<bean id="myRedeliveryPolicyConfig" class="org.apache.camel.processor.RedeliveryPolicy">
    <property name="maximumRedeliveries" value="3"/>
    <property name="redeliveryDelay" value="5000"/>
</bean>
----

The <<deadLetterChannel-eip,Dead Letter Channel>> above will clear
the caused exception (`setException(null)`), by moving the caused
exception to a property on the link:exchange.html[Exchange], with the
key `Exchange.EXCEPTION_CAUGHT`. Then the link:exchange.html[Exchange]
is moved to the `"jms:queue:dead"` destination and the client will not
notice the failure.

[[deadLetterChannel-AboutmovingExchangetodeadletterqueueandusingtheoriginalmessage]]
=== Moving Exchange to dead letter queue and using the original message

The option *useOriginalMessage* is used for routing the original input
message instead of the current message that potentially is modified
during routing.

For instance if you have this route:

[source,java]
-----
from("jms:queue:order:input")
   .to("bean:validateOrder")
   .to("bean:transformOrder")
   .to("bean:handleOrder");
-----

The route listen for JMS messages and validates, transforms and handle
it. During this the link:exchange.html[Exchange] payload is
transformed/modified. So in case something goes wrong and we want to
move the message to another JMS destination, then we can configure our
<<deadLetterChannel-eip,Dead Letter Channel>> with the
*useOriginalMessage* option. But when we move the
link:exchange.html[Exchange] to this destination we do not know in which
state the message is in. Did the error happen in before the
transformOrder or after? So to be sure we want to move the original
input message we received from `jms:queue:order:input`. So we can do
this by enabling the *useOriginalMessage* option as shown below:

[source,java]
----
// will use original body
errorHandler(deadLetterChannel("jms:queue:dead")
   .useOriginalMessage().maximumRedeliveries(5).redeliveryDelay(5000);
----

Then the messages routed to the `jms:queue:dead` is the original input.
If we want to manually retry we can move the JMS message from the failed
to the input queue, with no problem as the message is the same as the
original we received.

[[deadLetterChannel-OnRedelivery]]
=== OnRedelivery

When <<deadLetterChannel-eip,Dead Letter Channel>> is doing
redeliver its possible to configure a link:processor.html[Processor]
that is executed just *before* every redelivery attempt. This can be
used for the situations where you need to alter the message before its
redelivered. See below for sample.

TIP: *onException and onRedeliver*
We also support for per link:exception-clause.html[*onException*] to set
a *onRedeliver*. That means you can do special on redelivery for
different exceptions, as opposed to onRedelivery set on
<<deadLetterChannel-eip,Dead Letter Channel>> can be viewed as a
global scope.


[[deadLetterChannel-Redeliverydefaultvalues]]
=== Redelivery default values

Redelivery is disabled by default.

The default redeliver policy will use the following values:

* maximumRedeliveries=0
* redeliveryDelay=1000L (1 second)
* maximumRedeliveryDelay = 60 * 1000L (60 seconds)
* And the exponential backoff and collision avoidance is turned off.
* The retriesExhaustedLogLevel are set to LoggingLevel.ERROR
* The retryAttemptedLogLevel are set to LoggingLevel.DEBUG
* Stack traces is logged for exhausted messages from Camel 2.2 onwards.
* Handled exceptions is not logged from Camel 2.3 onwards
* logExhaustedMessageHistory is true for default error handler, and
false for dead letter channel.
* logExhaustedMessageBody *Camel 2.17:* is disabled by default to avoid
logging sensitive message body/header details. If this option is true,
then logExhaustedMessageHistory must also be true.

The maximum redeliver delay ensures that a delay is never longer than
the value, default 1 minute. This can happen if you turn on the
exponential backoff.

The maximum redeliveries is the number of redelivery attempts. By
default Camel will try to process the exchange 1 + 5 times. 1 time for
the normal attempt and then 5 attempts as redeliveries.
Setting the maximumRedeliveries to a negative value such as -1 will
then always redelivery (unlimited). Setting the maximumRedeliveries to 0
will disable any redelivery attempt.

Camel will log delivery failures at the DEBUG logging level by default.
You can change this by specifying retriesExhaustedLogLevel and/or
retryAttemptedLogLevel.

You can turn logging of stack traces on/off. If turned off Camel will
still log the redelivery attempt. Its just much less verbose.

[[deadLetterChannel-RedeliverDelayPattern]]
=== Redeliver Delay Pattern

Delay pattern is used as a single option to set a range pattern for
delays. If used then the following options does not apply: (delay,
backOffMultiplier, useExponentialBackOff, useCollisionAvoidance,
maximumRedeliveryDelay).

The idea is to set groups of ranges using the following syntax:
`limit:delay;limit 2:delay 2;limit 3:delay 3;...;limit N:delay N`

Each group has two values separated with colon

* limit = upper limit
* delay = delay in millis 
And the groups is again separated with semi colon. 
The rule of thumb is that the next groups should have a higher limit
than the previous group.

Lets clarify this with an example: 
 
`delayPattern=5:1000;10:5000;20:20000`

That gives us 3 groups:

* 5:1000
* 10:5000
* 20:20000

Resulting in these delays for redelivery attempt:

* Redelivery attempt number 1..4 = 0 millis (as the first group start
with 5)
* Redelivery attempt number 5..9 = 1000 millis (the first group)
* Redelivery attempt number 10..19 = 5000 millis (the second group)
* Redelivery attempt number 20.. = 20000 millis (the last group)

Note: The first redelivery attempt is 1, so the first group should start
with 1 or higher.

You can start a group with limit 1 to eg have a starting delay:
`delayPattern=1:1000;5:5000`

* Redelivery attempt number 1..4 = 1000 millis (the first group)
* Redelivery attempt number 5.. = 5000 millis (the last group)

There is no requirement that the next delay should be higher than the
previous. You can use any delay value you like. For example with
`delayPattern=1:5000;3:1000` we start with 5 sec delay and then later
reduce that to 1 second.

[[deadLetterChannel-Redeliveryheader]]
=== Redelivery header

When a message is redelivered the
http://camel.apache.org/maven/camel-core/apidocs/org/apache/camel/processor/DeadLetterChannel.html[DeadLetterChannel]
will append a customizable header to the message to indicate how many
times its been redelivered.  
Before Camel 2.6: The header is *CamelRedeliveryCounter*, which is also
defined on the `Exchange.REDELIVERY_COUNTER`. 
Starting with 2.6: The header *CamelRedeliveryMaxCounter*, which is
also defined on the `Exchange.REDELIVERY_MAX_COUNTER`, contains the
maximum redelivery setting. This header is absent if you use
`retryWhile` or have unlimited maximum redelivery configured.

And a boolean flag whether it is being redelivered or not (first
attempt) 
The header *CamelRedelivered* contains a boolean if the message is
redelivered or not, which is also defined on the `Exchange.REDELIVERED`.

Dynamically calculated delay from the exchange 
In Camel 2.9 and 2.8.2: The header is *CamelRedeliveryDelay*, which is
also defined on the `Exchange.REDELIVERY_DELAY`. 
Is this header is absent, normal redelivery rules apply.

[[deadLetterChannel-Whichendpointfailed]]
=== Which endpoint failed

*Available as of Camel 2.1*

When Camel routes messages it will decorate the
link:exchange.html[Exchange] with a property that contains the *last*
endpoint Camel send the link:exchange.html[Exchange] to:

[source,java]
----
String lastEndpointUri = exchange.getProperty(Exchange.TO_ENDPOINT, String.class);
----

The `Exchange.TO_ENDPOINT` have the constant value `CamelToEndpoint`.

This information is updated when Camel sends a message to any endpoint.
So if it exists its the *last* endpoint which Camel send the Exchange
to.

When for example processing the link:exchange.html[Exchange] at a given
<<Endpoint-Endpoints,Endpoint>> and the message is to be moved into the
dead letter queue, then Camel also decorates the Exchange with another
property that contains that *last* endpoint:

[source,java]
----
String failedEndpointUri = exchange.getProperty(Exchange.FAILURE_ENDPOINT, String.class);
----

The `Exchange.FAILURE_ENDPOINT` have the constant value
`CamelFailureEndpoint`.

This allows for example you to fetch this information in your dead
letter queue and use that for error reporting. +
 This is useable if the Camel route is a bit dynamic such as the dynamic
link:recipient-list.html[Recipient List] so you know which endpoints
failed.

These information is kept on the Exchange even if the message
was successfully processed by a given endpoint, and then later fails for
example in a local <<bean-component,Bean>> processing instead. So beware
that this is a hint that helps pinpoint errors.

[source,java]
----
from("activemq:queue:foo")
    .to("http://someserver/somepath")
    .beanRef("foo");
----

Now suppose the route above and a failure happens in the `foo` bean.
Then the `Exchange.TO_ENDPOINT` and `Exchange.FAILURE_ENDPOINT` will
still contain the value of `http://someserver/somepath`.

[[deadLetterChannel-OnPrepareFailure]]
=== OnPrepareFailure

*Available as of Camel 2.16*

Before the exchange is sent to the dead letter queue, you can use
onPrepare to allow a custom `Processor` to prepare the exchange, such as
adding information why the Exchange failed. For example the following
processor adds a header with the exception message

[source,java]
----
public static class MyPrepareProcessor implements Processor {
    @Override
    public void process(Exchange exchange) throws Exception {
        Exception cause = exchange.getProperty(Exchange.EXCEPTION_CAUGHT, Exception.class);
        exchange.getIn().setHeader("FailedBecause", cause.getMessage());
    }
}
----

Then configure the error handler to use the processor as follows:

[source,java]
----
errorHandler(deadLetterChannel("jms:dead").onPrepareFailure(new MyPrepareProcessor()));
----

Configuring this from XML DSL is as shown:

[source,xml]
----
<bean id="myPrepare"
      class="org.apache.camel.processor.DeadLetterChannelOnPrepareTest.MyPrepareProcessor"/>


<errorHandler id="dlc" type="DeadLetterChannel" deadLetterUri="jms:dead" onPrepareFailureRef="myPrepare"/>
----

The onPrepare is also available using the default error handler.

[[deadLetterChannel-Whichroutefailed]]
=== Which route failed

*Available as of Camel 2.10.4/2.11*

When Camel error handler handles an error such as
<<deadLetterChannel-eip,Dead Letter Channel>> or using
link:exception-clause.html[Exception Clause] with handled=true, then
Camel will decorate +
 the link:exchange.html[Exchange] with the route id where the error
occurred.

[source,java]
----
String failedRouteId = exchange.getProperty(Exchange.FAILURE_ROUTE_ID, String.class);
----

The `Exchange.FAILURE_ROUTE_ID` have the constant value
`CamelFailureRouteId`.

This allows for example you to fetch this information in your dead
letter queue and use that for error reporting.

[[deadLetterChannel-Controlifredeliveryisallowedduringstoppingshutdown]]
=== Control if redelivery is allowed during stopping/shutdown

*Available as of Camel 2.11*

Prior to Camel 2.10, Camel will perform redelivery while stopping a
route, or shutting down Camel. This has improved a bit in Camel 2.10
onwards, as Camel will not perform redelivery attempts when shutting
down aggressively (eg during link:graceful-shutdown.html[Graceful
Shutdown] and timeout hit). From Camel 2.11 onwards there is a new
option `allowRedeliveryWhileStopping` which you can use to control if
redelivery is allowed or not; notice that any in progress redelivery
will still be executed. This option can only disallow any redelivery to
be executed *after* the stopping of a route/shutdown of Camel has been
triggered. If a redelivery is dissallowed then a
`RejectedExecutionException` is set on the link:exchange.html[Exchange]
and the processing of the link:exchange.html[Exchange] stops. This means
any consumer will see the link:exchange.html[Exchange] as failed due the
`RejectedExecutionException`.

The default value is `true` to be backwards compatible as before. For
example the following sample shows how to do this with Java DSL and XML
DSL

And the sample sample with XML DSL

[[deadLetterChannel-Samples]]
=== Samples

The following example shows how to configure the Dead Letter Channel
configuration using the link:dsl.html[DSL]

You can also configure the
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/processor/RedeliveryPolicy.html[RedeliveryPolicy]
as this example shows

[[deadLetterChannel-HowcanImodifytheExchangebeforeredelivery]]
=== How can I modify the Exchange before redelivery?

We support directly in link:dead-letter-channel.html[Dead Letter
Channel] to set a link:processor.html[Processor] that is executed
*before* each redelivery attempt.

When <<deadLetterChannel-eip,Dead Letter Channel>> is doing
redeliver its possible to configure a link:processor.html[Processor]
that is executed just *before* every redelivery attempt. This can be
used for the situations where you need to alter the message before its
redelivered.

Here we configure the <<deadLetterChannel-eip,Dead Letter Channel>>
to use our processor `MyRedeliveryProcessor` to be executed before each
redelivery.

And this is the processor `MyRedeliveryProcessor` where we alter the
message.

[[deadLetterChannel-HowcanIlogwhatcausedtheDeadLetterChanneltobeinvoked]]
=== How can I log what caused the Dead Letter Channel to be invoked?

You often need to know what went wrong that caused the Dead Letter
Channel to be used and it does not offer logging for this purpose. So
the Dead Letter Channel's endpoint can be set to a endpoint of our own
(such as `direct:deadLetterChannel`). We write a route to accept this
Exchange and log the Exception, then forward on to where we want the
failed Exchange moved to (which might be a DLQ queue for instance). See
also http://stackoverflow.com/questions/13711462/logging-camel-exceptions-and-sending-to-the-dead-letter-channel[http://stackoverflow.com/questions/13711462/logging-camel-exceptions-and-sending-to-the-dead-letter-channel]

[[deadLetterChannel-UsingThisPattern]]
==== Using This Pattern

If you would like to use this EIP Pattern then please read the
<<GettingStarted-GettingStarted,Getting Started>>, you may also find the
<<Architecture-Architecture,Architecture>> useful particularly the description
of <<Endpoint-Endpoints,Endpoint>> and link:uris.html[URIs]. Then you could
try out some of the <<Examples-Examples,Examples>> first before trying
this pattern out.

* link:error-handler.html[Error Handler]
* link:exception-clause.html[Exception Clause]

