Dead Letter Channel
Camel supports the Dead Letter Channel from the EIP patterns using the DeadLetterChannel processor which is an Error Handler.
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The DefaultErrorHandler
does very little: it ends the Exchange immediately and propagates the thrown Exception back to the caller.
The DeadLetterChannel
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 DeadLetterChannel
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: 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
.
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 RedeliveryPolicy defines how the message is to be redelivered. You can customize things like
- how many The number of times a message is attempted to be redelivered before it is considered a failure and sent to the dead letter channel.
- the The initial redelivery timeout.
- whether Whether or not exponential backoff is used (, i.e., the time between retries increases using a backoff multiplier).
- whether 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.
About Moving Exchange to Dead Letter Queue and Using handled()
handled()
on Dead Letter Channel
When all attempts of redelivery have failed the 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 Dead Letter Channel have handled the Exchange.
For instance configuring the dead letter channel as:
Using the Fluent Builders
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Using the Spring XML Extensions
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The Dead Letter Channel above will clear the caused exception setException(null)
, by moving the caused exception to a property on the Exchange, with the key Exchange.EXCEPTION_CAUGHT
. Then the Exchange is moved to the jms:queue:dead
destination and the client will not notice the failure.
About 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:
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The route listen for JMS messages and validates, transforms and handle it. During this the 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 Dead Letter Channel with the useOriginalMessage
option. But when we move the 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:
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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.
OnRedelivery
When Dead Letter Channel is doing redeliver its possible to configure a 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.
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We also support for per onException to set an onRedeliver
. That means you can do special on redelivery for different exceptions, as opposed to onRedelivery
set on Dead Letter Channel can be viewed as a global scope.
Redelivery Default Values
Redelivery is disabled by default
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.
The default redeliver policy will use the following values:
maximumRedeliveries=
60
- initialRedeliveryDelay
redeliverDelay=1000L
(1 second) - And the backoff and collision avoidance is turned off.
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maximumRedeliveryDelay = 60 * 1000L
(60 seconds)backOffMultiplier
anduseExponentialBackOff
are ignored.retriesExhaustedLogLevel=LoggingLevel.ERROR
retryAttemptedLogLevel=LoggingLevel.DEBUG
- Stack traces are logged for exhausted messages, from Camel 2.2.
- Handled exceptions are not logged, from Camel 2.3.
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 istrue
, thenlogExhaustedMessageHistory
must also betrue
.
The maximum redeliver delay ensures that a delay is never longer than the value, default 1 minute. This can happen when useExponentialBackOff=true
.
The maximumRedeliveries
is the number of re-delivery 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=-1
(or < -1
) will then always redelivery (unlimited).
Setting the maximumRedeliveries=0
will disable re-delivery.
Camel will log delivery failures at the DEBUG
logging level by default. You can change this by specifying retriesExhaustedLogLevel
and/or retryAttemptedLogLevel
. See ExceptionBuilderWithRetryLoggingLevelSetTest for an example.
You can turn logging of stack traces on/off. If turned off Camel will still log the redelivery attempt. It's just much less verbose.
Redeliver Delay Pattern
Delay pattern is used as a single option to set a range pattern for delays. When a delay pattern is in use the following options no longer 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 limitdelay
= delay in milliseconds
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 three groups:
5:1000
10:5000
20:20000
Resulting in these delays between redelivery attempts:
- Redelivery attempt number
1..4 = 0ms
(as the first group start with 5) - Redelivery attempt number
5..9 = 1000ms
(the first group) - Redelivery attempt number
10..19 = 5000ms
(the second group) - Redelivery attempt number
20.. = 20000ms
(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 e.g., have a starting delay: delayPattern=1:1000;5:5000
- Redelivery attempt number
1..4 = 1000ms
(the first group) - Redelivery attempt number
5.. = 5000ms
(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.
Redelivery header
When a message is redelivered the DeadLetterChannel will append a customizable header to the message to indicate how many times its been redelivered. The default value is org.apache.camel.redeliveryCount.
The header org.apache.camel.Redelivered contains a boolean if the message is redelivered or not.
Configuring via the DSL
Before Camel 2.6: The header is CamelRedeliveryCounter
, which is also defined on the Exchange.REDELIVERY_COUNTER
.
From Camel 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
. If this header is absent, normal redelivery rules apply.
Which Endpoint Failed
Available as of Camel 2.1
When Camel routes messages it will decorate the Exchange with a property that contains the last endpoint Camel send the Exchange to:
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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 Exchange at a given 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:
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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 usable if the Camel route is a bit dynamic such as the dynamic Recipient List so you know which endpoints failed.
Note: this information is retained on the Exchange even if the message is subsequently processed successfully by a given endpoint only to fail, for example, in local Bean processing instead. So, beware that this is a hint that helps pinpoint errors.
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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
.
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:
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Then configure the error handler to use the processor as follows:
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Configuring this from XML DSL is as follows:
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The onPrepare
is also available using the default error handler.
Which Route Failed
Available as of Camel 2.10.4/2.11
When Camel error handler handles an error such as Dead Letter Channel or using Exception Clause with handled=true
, then Camel will decorate the Exchange with the route id where the error occurred.
Example:
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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.
Control if Redelivery is Allowed During Stopping/Shutdown
Available as of Camel 2.11
Before Camel 2.10, Camel would perform redelivery while stopping a route, or shutting down Camel. This has improved a bit in Camel 2.10: Camel will no longer perform redelivery attempts when shutting down aggressively, e.g., during Graceful Shutdown and timeout hit.
From Camel 2.11: 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 disallowed then a RejectedExcutionException
is set on the Exchange and the processing of the Exchange stops. This means any consumer will see the Exchange as failed due the RejectedExcutionException
. The default value is true
for backward compatibility.
For example, the following snippet shows how to do this with Java DSL and XML DSL:
Samples
The following example shows how to configure the Dead Letter Channel configuration using the DSL
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You can also configure the RedeliveryPolicy as this example shows
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ExceptionPolicyStrategy (@since Camel 1.4)
ExceptionPolicyStrategy is a strategy for resolving which rule (ExceptionType) should handle the given thrown exception.
DeadLetterChannel supports pluggable strategies for resolving how exceptions should be handled. It is common to how different strategies for different types of exceptions. For instance network and IO related exceptions is more prone for network outages so the redeliver policy could have a higher attempts, timeouts etc. Where as a NullPointerException is typically a programming error so these kind of exception is severe and should not be redelivered. Camel uses a default strategy DefaultExceptionPolicyStrategy that applies the following rules:
- The exception type must be configured with an Exception that is an instance of the thrown exception
- If the exception type has exactly the thrown exception then its selected
- Otherwise the type that has an exception that is super of the thrown exception is selected (recurring up the exception hierarchy)
The example below illustrates a common exception handling configuration in Camel:
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How Can I Modify the Exchange Before Redelivery?
We support directly in Dead Letter Channel to set a Processor that is executed before each redelivery attempt. When Dead Letter Channel is doing redeliver its possible to configure a 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 Dead Letter Channel to use our processor MyRedeliveryProcessor
to be executed before each redelivery.MyRedeliveryProcessor
where we alter the message.
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Here we have configured the handling of exceptions into three categories:
- NullPointerException (for special handling of these hard to track down bugs)
- IOException (for IO and network related issues we can attempt many times)
- Exception (fallback exception handling for all other kind of exceptions)
Camel will with the default strategy try to select the best suited category from above for any thrown exception.
So if a java.net.ScoketException is thrown then the IOException category will handle it. If a NumberFormatException or CamelExchangeException is thrown it is handled by the general purpose Exception category.
Camel supports pluggable exception policy strategies. See Error Handler for such details.
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
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