= Message Redelivery and Undelivered Messages
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Messages can be delivered unsuccessfully (e.g. if the transacted session used to consume them is rolled back).
Such a message goes back to its queue ready to be redelivered.
However, this means it is possible for a message to be delivered again and again without success thus remaining in the queue indefinitely, clogging the system.

There are 2 ways to deal with these undelivered messages:

* Delayed redelivery.
+
It is possible to delay messages redelivery.
This gives the client some time to recover from any transient failures and to prevent overloading its network or CPU resources.

* Dead Letter Address.
+
It is also possible to configure a dead letter address so that after a specified number of unsuccessful deliveries, messages are removed from their queue and sent to the dead letter address.
These messages will not be delivered again from this queue.

Both options can be combined for maximum flexibility.

== Delayed Redelivery

Delaying redelivery can often be useful in cases where clients regularly fail or rollback.
Without a delayed redelivery, the system can get into a "thrashing" state, with delivery being attempted, the client rolling back, and delivery being re-attempted ad infinitum in quick succession, consuming valuable CPU and network resources.

#Persist Redelivery

Two Journal update records are stored every time a redelivery happens.
One for the number of deliveries that happened, and one in case a scheduled redelivery is being used.

It is recommended to keep max-redelivery-records=1 in situations where you are operating with very short redelivery delays as you will be creating unecessary records on the journal.

=== Configuring Delayed Redelivery

Delayed redelivery is defined in the address-setting configuration:

[,xml]
----
<!-- delay redelivery of messages for 5s -->
<address-setting match="exampleQueue">
   <!-- default is 1.0 -->
   <redelivery-delay-multiplier>1.5</redelivery-delay-multiplier>
   <!-- default is 0 (no delay) -->
   <redelivery-delay>5000</redelivery-delay>
   <!-- default is 0.0) -->
   <redelivery-collision-avoidance-factor>0.15</redelivery-collision-avoidance-factor>
   <!-- default is redelivery-delay * 10 -->
   <max-redelivery-delay>50000</max-redelivery-delay>
</address-setting>
----

If a `redelivery-delay` is specified, the broker will wait this delay before redelivering the messages.

By default, there is no redelivery delay (``redelivery-delay``is set to 0).

Other subsequent messages will be delivery regularly, only the cancelled message will be sent asynchronously back to the queue after the delay.

You can specify a multiplier (the `redelivery-delay-multiplier`) that will take effect on top of the `redelivery-delay`.
Each time a message is redelivered the delay period will be equal to the previous delay * `redelivery-delay-multiplier`.
A `max-redelivery-delay` can be set to prevent the delay from becoming too large.
The `max-redelivery-delay` is defaulted to `redelivery-delay` * 10.

*Example:*

* redelivery-delay=5000, redelivery-delay-multiplier=2, max-redelivery-delay=15000, redelivery-collision-avoidance-factor=0.0

. Delivery Attempt 1.
(Unsuccessful)
. Wait Delay Period: 5000
. Delivery Attempt 2.
(Unsuccessful)
. Wait Delay Period: 10000                   // (5000  * 2) < max-delay-period.
Use 10000
. Delivery Attempt 3: (Unsuccessful)
. Wait Delay Period: 15000                   // (10000 * 2) > max-delay-period:  Use max-delay-delivery

Address wildcards can be used to configure redelivery delay for a set of addresses (see xref:wildcard-syntax.adoc#wildcard-syntax[Understanding the Wildcard Syntax]), so you don't have to specify redelivery delay individually for each address.

The `redelivery-delay` can be also be modified by configuring the `redelivery-collision-avoidance-factor`.
This factor will be made either positive or negative at random to control whether the ultimate value will increase or decrease the `redelivery-delay`.
Then it's multiplied by a random number between 0.0 and 1.0.
This result is then multiplied by the `redelivery-delay` and then added to the `redelivery-delay` to arrive at the final value.

The algorithm may sound complicated but the bottom line is quite simple: the larger `redelivery-collision-avoidance-factor` you choose the larger the variance of the `redelivery-delay` will be.
The `redelivery-collision-avoidance-factor` must be between 0.0 and 1.0.

*Example:*

* redelivery-delay=1000, redelivery-delay-multiplier=1, max-redelivery-delay=15000, redelivery-collision-avoidance-factor=0.5, (bold values chosen using `java.util.Random`)

. Delivery Attempt 1.
(Unsuccessful)
. Wait Delay Period: 875                     // 1000 + (1000 * ((0.5 * *-1*) * *.25*)
. Delivery Attempt 2.
(Unsuccessful)
. Wait Delay Period: 1375                    // 1000 + (1000 * ((0.5 * *1*) * *.75*)
. Delivery Attempt 3: (Unsuccessful)
. Wait Delay Period: 975                     // 1000 + (1000 * ((0.5 * *-1*) * *.05*)

This feature can be particularly useful in environments where there are multiple consumers on the same queue all interacting transactionally with the same external system (e.g. a database).
If there is overlapping data in messages which are consumed concurrently then one transaction can succeed while all the rest fail.
If those failed messages are redelivered at the same time then this process where one consumer succeeds and the rest fail will continue.
By randomly padding the redelivery-delay by a small, configurable amount these redelivery "collisions" can be avoided.

=== Example

See xref:examples.adoc#examples[the examples chapter] for an example which shows how delayed redelivery is configured and used with JMS.

== Dead Letter Addresses

To prevent a client infinitely receiving the same undelivered message (regardless of what is causing the unsuccessful deliveries), messaging systems define _dead letter addresses_: after a specified unsuccessful delivery attempts, the message is removed from its queue and sent to a dead letter address.

Any such messages can then be diverted to queue(s) where they can later be perused by the system administrator for action to be taken.

Addresses can be assigned a dead letter address.
Once the messages have been unsuccessfully delivered for a given number of attempts, they are removed from their queue and sent to the relevant dead letter address.
These _dead letter_ messages can later be consumed from the dead letter address for further inspection.

=== Configuring Dead Letter Addresses

Dead letter address is defined in the address-setting configuration:

[,xml]
----
<!-- undelivered messages in exampleQueue will be sent to the dead letter address
deadLetterQueue after 3 unsuccessful delivery attempts -->
<address-setting match="exampleQueue">
   <dead-letter-address>deadLetterAddress</dead-letter-address>
   <max-delivery-attempts>3</max-delivery-attempts>
</address-setting>
----

If a `dead-letter-address` is not specified, messages will be removed after `max-delivery-attempts` unsuccessful attempts.

By default, messages are redelivered 10 times at the maximum.
Set `max-delivery-attempts` to -1 for infinite redeliveries.

A `dead letter address` can be set globally for a set of matching addresses and you can set `max-delivery-attempts` to -1 for a specific address setting to allow infinite redeliveries only for this address.

Address wildcards can be used to configure dead letter settings for a set of addresses (see xref:wildcard-syntax.adoc#wildcard-syntax[Understanding the Wildcard Syntax]).

=== Dead Letter Properties

Dead letter messages get xref:copied-message-properties.adoc#properties-for-copied-messages[special properties].

=== Automatically Creating Dead Letter Resources

It's common to segregate undelivered messages by their original address.
For example, a message sent to the `stocks` address that couldn't be delivered for some reason might be ultimately routed to the `DLQ.stocks` queue, and likewise a message sent to the `orders` address that couldn't be delivered might be routed to the `DLQ.orders` queue.

Using this pattern can make it easy to track and administrate undelivered messages.
However, it can pose a challenge in environments which predominantly use auto-created addresses and queues.
Typically administrators in those environments don't want to manually create an `address-setting` to configure the `dead-letter-address` much less the actual `address` and `queue` to hold the undelivered messages.

The solution to this problem is to set the `auto-create-dead-letter-resources` `address-setting` to `true` (it's `false` by default) so that the broker will create the `address` and `queue` to deal with the undelivered messages automatically.
The `address` created will be the one defined by the `dead-letter-address`.
A `MULTICAST` `queue` will be created on that `address`.
It will be named by the `address` to which the message was previously sent, and it will have a filter defined using the property `_AMQ_ORIG_ADDRESS` so that it  will only receive messages sent to the relevant `address`.
The `queue` name  can be configured with a prefix and suffix.
See the relevant settings in the  table below:

|===
| `address-setting` | default

| `dead-letter-queue-prefix`
| `DLQ.`

| `dead-letter-queue-suffix`
| (empty string)
|===

Here is an example configuration:

[,xml]
----
<address-setting match="#">
   <dead-letter-address>DLA</dead-letter-address>
   <max-delivery-attempts>3</max-delivery-attempts>
   <auto-create-dead-letter-resources>true</auto-create-dead-letter-resources>
   <dead-letter-queue-prefix></dead-letter-queue-prefix> <!-- override the default -->
   <dead-letter-queue-suffix>.DLQ</dead-letter-queue-suffix>
</address-setting>
----

The queue holding the undeliverable messages can be accessed directly either by using the queue's name by itself (e.g. when using the Core client) or by using the fully qualified queue name (e.g. when using a JMS client) just like any other queue.
Also, note that the queue is auto-created which means it will be auto-deleted as per the relevant `address-settings`.

=== Example

See: Dead Letter section of the xref:examples.adoc#examples[Examples] for an example that shows how dead letter resources can be statically configured and used with JMS.

== Delivery Count Persistence

In normal use, the broker does not update delivery count _persistently_ until a message is rolled back (i.e. the delivery count is not updated _before_ the message is delivered to the consumer).
In most messaging use cases, the messages are consumed, acknowledged and forgotten as soon as they are consumed.
In these cases, updating the delivery count persistently before delivering the message would add an extra persistent step _for each message delivered_, implying a significant performance penalty.

However, if the delivery count is not updated persistently before the message delivery happens, in the event of a server crash, messages might have been delivered but that will not have been reflected in the delivery count.
During the recovery phase, the server will not have knowledge of that and will deliver the message with `redelivered` set to `false` while it should be `true`.

As this behavior breaks strict JMS semantics, the broker can persist the delivery count before message delivery, but this feature is disabled by default due to performance implications.

To enable it, set `persist-delivery-count-before-delivery` to `true` in `broker.xml`:

[,xml]
----
<persist-delivery-count-before-delivery>true</persist-delivery-count-before-delivery>
----
