255 lines
11 KiB
Markdown
255 lines
11 KiB
Markdown
# Message Redelivery and Undelivered Messages
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Messages can be delivered unsuccessfully (e.g. if the transacted session
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used to consume them is rolled back). Such a message goes back to its
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queue ready to be redelivered. However, this means it is possible for a
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message to be delivered again and again without success thus remaining
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in the queue indefinitely, clogging the system.
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There are 2 ways to deal with these undelivered messages:
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- Delayed redelivery.
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It is possible to delay messages redelivery. This gives the client some
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time to recover from any transient failures and to prevent overloading
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its network or CPU resources.
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- Dead Letter Address.
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It is also possible to configure a dead letter address so that after
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a specified number of unsuccessful deliveries, messages are removed
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from their queue and sent to the dead letter address. These messages
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will not be delivered again from this queue.
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Both options can be combined for maximum flexibility.
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## Delayed Redelivery
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Delaying redelivery can often be useful in cases where clients regularly
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fail or rollback. Without a delayed redelivery, the system can get into a
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"thrashing" state, with delivery being attempted, the client rolling back,
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and delivery being re-attempted ad infinitum in quick succession,
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consuming valuable CPU and network resources.
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### Configuring Delayed Redelivery
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Delayed redelivery is defined in the address-setting configuration:
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```xml
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<!-- delay redelivery of messages for 5s -->
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<address-setting match="exampleQueue">
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<!-- default is 1.0 -->
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<redelivery-delay-multiplier>1.5</redelivery-delay-multiplier>
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<!-- default is 0 (no delay) -->
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<redelivery-delay>5000</redelivery-delay>
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<!-- default is 0.0) -->
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<redelivery-collision-avoidance-factor>0.15</redelivery-collision-avoidance-factor>
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<!-- default is redelivery-delay * 10 -->
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<max-redelivery-delay>50000</max-redelivery-delay>
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</address-setting>
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```
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If a `redelivery-delay` is specified, Apache ActiveMQ Artemis will wait this delay
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before redelivering the messages.
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By default, there is no redelivery delay (`redelivery-delay`is set to
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0).
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Other subsequent messages will be delivery regularly, only the cancelled
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message will be sent asynchronously back to the queue after the delay.
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You can specify a multiplier (the `redelivery-delay-multiplier`) that will
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take effect on top of the `redelivery-delay`. Each time a message is redelivered
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the delay period will be equal to the previous delay * `redelivery-delay-multiplier`.
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A `max-redelivery-delay` can be set to prevent the delay from becoming too large.
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The `max-redelivery-delay` is defaulted to `redelivery-delay` \* 10.
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**Example:**
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- redelivery-delay=5000, redelivery-delay-multiplier=2, max-redelivery-delay=15000,
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redelivery-collision-avoidance-factor=0.0
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1. Delivery Attempt 1. (Unsuccessful)
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2. Wait Delay Period: 5000
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3. Delivery Attempt 2. (Unsuccessful)
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4. Wait Delay Period: 10000 // (5000 * 2) < max-delay-period. Use 10000
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5. Delivery Attempt 3: (Unsuccessful)
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6. Wait Delay Period: 15000 // (10000 * 2) > max-delay-period: Use max-delay-delivery
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Address wildcards can be used to configure redelivery delay for a set of
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addresses (see [Understanding the Wildcard Syntax](wildcard-syntax.md)), so you don't have to specify redelivery delay
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individually for each address.
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The `redelivery-delay` can be also be modified by configuring the
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`redelivery-collision-avoidance-factor`. This factor will be made either
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positive or negative at random to control whether the ultimate value will
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increase or decrease the `redelivery-delay`. Then it's multiplied by a random
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number between 0.0 and 1.0. This result is then multiplied by the
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`redelivery-delay` and then added to the `redelivery-delay` to arrive at the
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final value.
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The algorithm may sound complicated but the bottom line is quite simple: the
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larger `redelivery-collision-avoidance-factor` you choose the larger the variance
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of the `redelivery-delay` will be. The `redelivery-collision-avoidance-factor`
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must be between 0.0 and 1.0.
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**Example:**
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- redelivery-delay=1000, redelivery-delay-multiplier=1, max-redelivery-delay=15000,
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redelivery-collision-avoidance-factor=0.5, (bold values chosen using
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`java.util.Random`)
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1. Delivery Attempt 1. (Unsuccessful)
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2. Wait Delay Period: 875 // 1000 + (1000 * ((0.5 \* __-1__) * __.25__)
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3. Delivery Attempt 2. (Unsuccessful)
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4. Wait Delay Period: 1375 // 1000 + (1000 * ((0.5 \* __1__) * __.75__)
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5. Delivery Attempt 3: (Unsuccessful)
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6. Wait Delay Period: 975 // 1000 + (1000 * ((0.5 \* __-1__) * __.05__)
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This feature can be particularly useful in environments where there are
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multiple consumers on the same queue all interacting transactionally
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with the same external system (e.g. a database). If there is overlapping
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data in messages which are consumed concurrently then one transaction can
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succeed while all the rest fail. If those failed messages are redelivered
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at the same time then this process where one consumer succeeds and the
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rest fail will continue. By randomly padding the redelivery-delay by a
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small, configurable amount these redelivery "collisions" can be avoided.
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### Example
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See [the examples chapter](examples.md) for an example which shows how
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delayed redelivery is configured and used with JMS.
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## Dead Letter Addresses
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To prevent a client infinitely receiving the same undelivered message
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(regardless of what is causing the unsuccessful deliveries), messaging
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systems define *dead letter addresses*: after a specified unsuccessful
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delivery attempts, the message is removed from its queue and sent
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to a dead letter address.
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Any such messages can then be diverted to queue(s) where they can later
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be perused by the system administrator for action to be taken.
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Apache ActiveMQ Artemis's addresses can be assigned a dead letter address. Once the
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messages have been unsuccessfully delivered for a given number of
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attempts, they are removed from their queue and sent to the relevant
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dead letter address. These *dead letter* messages can later be consumed
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from the dead letter address for further inspection.
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### Configuring Dead Letter Addresses
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Dead letter address is defined in the address-setting configuration:
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```xml
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<!-- undelivered messages in exampleQueue will be sent to the dead letter address
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deadLetterQueue after 3 unsuccessful delivery attempts -->
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<address-setting match="exampleQueue">
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<dead-letter-address>deadLetterAddress</dead-letter-address>
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<max-delivery-attempts>3</max-delivery-attempts>
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</address-setting>
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```
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If a `dead-letter-address` is not specified, messages will be removed after
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`max-delivery-attempts` unsuccessful attempts.
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By default, messages are redelivered 10 times at the maximum. Set
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`max-delivery-attempts` to -1 for infinite redeliveries.
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A `dead letter address` can be set globally for a set of matching
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addresses and you can set `max-delivery-attempts` to -1 for a specific
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address setting to allow infinite redeliveries only for this address.
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Address wildcards can be used to configure dead letter settings for a
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set of addresses (see [Understanding the Wildcard Syntax](wildcard-syntax.md)).
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### Dead Letter Properties
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Dead letter messages get [special properties](copied-message-properties.md).
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### Automatically Creating Dead Letter Resources
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It's common to segregate undelivered messages by their original address.
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For example, a message sent to the `stocks` address that couldn't be
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delivered for some reason might be ultimately routed to the `DLQ.stocks`
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queue, and likewise a message sent to the `orders` address that couldn't
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be delivered might be routed to the `DLQ.orders` queue.
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Using this pattern can make it easy to track and administrate
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undelivered messages. However, it can pose a challenge in environments
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which predominantly use auto-created addresses and queues. Typically
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administrators in those environments don't want to manually create
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an `address-setting` to configure the `dead-letter-address` much less
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the actual `address` and `queue` to hold the undelivered messages.
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The solution to this problem is to set the `auto-create-dead-letter-resources`
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`address-setting` to `true` (it's `false` by default) so that the broker will
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create the `address` and `queue` to deal with the undelivered messages
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automatically. The `address` created will be the one defined by the
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`dead-letter-address`. A `MULTICAST` `queue` will be created on that `address`.
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It will be named by the `address` to which the message was previously sent, and
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it will have a filter defined using the property `_AMQ_ORIG_ADDRESS` so that it
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will only receive messages sent to the relevant `address`. The `queue` name
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can be configured with a prefix and suffix. See the relevant settings in the
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table below:
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`address-setting`|default
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---|---
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`dead-letter-queue-prefix`|`DLQ.`
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`dead-letter-queue-suffix`|(empty string)
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Here is an example configuration:
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```xml
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<address-setting match="#">
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<dead-letter-address>DLA</dead-letter-address>
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<max-delivery-attempts>3</max-delivery-attempts>
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<auto-create-dead-letter-resources>true</auto-create-dead-letter-resources>
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<dead-letter-queue-prefix></dead-letter-queue-prefix> <!-- override the default -->
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<dead-letter-queue-suffix>.DLQ</dead-letter-queue-suffix>
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</address-setting>
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```
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The queue holding the undeliverable messages can be accessed directly
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either by using the queue's name by itself (e.g. when using the core
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client) or by using the fully qualified queue name (e.g. when using
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a JMS client) just like any other queue. Also, note that the queue is
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auto-created which means it will be auto-deleted as per the relevant
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`address-settings`.
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### Example
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See: Dead Letter section of the [Examples](examples.md) for an example
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that shows how dead letter resources can be statically configured and
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used with JMS.
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## Delivery Count Persistence
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In normal use, Apache ActiveMQ Artemis does not update delivery count *persistently*
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until a message is rolled back (i.e. the delivery count is not updated
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*before* the message is delivered to the consumer). In most messaging
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use cases, the messages are consumed, acknowledged and forgotten as soon
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as they are consumed. In these cases, updating the delivery count
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persistently before delivering the message would add an extra persistent
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step *for each message delivered*, implying a significant performance
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penalty.
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However, if the delivery count is not updated persistently before the
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message delivery happens, in the event of a server crash, messages might
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have been delivered but that will not have been reflected in the
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delivery count. During the recovery phase, the server will not have
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knowledge of that and will deliver the message with `redelivered` set to
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`false` while it should be `true`.
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As this behavior breaks strict JMS semantics, Apache ActiveMQ Artemis allows to persist
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delivery count before message delivery but this feature is disabled by default
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due to performance implications.
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To enable it, set `persist-delivery-count-before-delivery` to `true` in
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`broker.xml`:
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```xml
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<persist-delivery-count-before-delivery>true</persist-delivery-count-before-delivery>
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```
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