Messages that cannot be delivered successfully to a consumer (for instance, because the client is using a transacted session and rolls-back the transaction) can be made available on the queue again and then subsequently be redelivered, depending on the precise session acknowledgement mode and messaging model used by the application. This is normally desirable behaviour that contributes to the ability of a system to withstand unexpected errors. However, it leaves open the possibility for a message to be repeatedly redelivered (potentially indefinitely), consuming system resources and preventing the delivery of other messages. Such undeliverable messages are sometimes known as poison messages.
For an example, consider a stock ticker application that has been designed to consume prices contained within JMS TextMessages. What if inadvertently a BytesMessage is placed onto the queue? As the ticker application does not expect the BytesMessage, its processing might fail and cause it to roll-back the transaction, however the default behavior of the Broker would mean that the BytesMessage would be delivered over and over again, preventing the delivery of other legitimate messages, until an operator intervenes and removes the erroneous message from the queue.
Qpid has maximum delivery count and dead-letter queue (DLQ) features which can be used in concert to construct a system that automatically handles such a condition. These features are described in the following sections.
Maximum delivery count is a property of a queue. If a consumer application is unable to process a message more than the specified number of times, then the broker will either route the message to a dead-letter queue (if one has been defined), or will discard the message.
In order for a maximum delivery count to be enforced, the consuming client must call Session#rollback() (or Session#recover() if the session is not transacted). It is during the Broker's processing of Session#rollback() (or Session#recover()) that if a message has been seen at least the maximum number of times then it will move the message to the DLQ or discard the message.
If the consuming client fails in another manner, for instance, closes the connection, the message will not be re-routed and consumer application will see the same poison message again once it reconnects.
If the consuming application is using AMQP 0-9-1, 0-9, or 0-8 protocols, it is necessary to
set the client system property
qpid.reject.behaviour or connection or binding
rejectbehaviour to the value
It is possible to determine the number of times a message has been sent to a consumer via the Management interfaces, but is not possible to determine this information from a message client. Specifically, the optional JMS message header JMSXDeliveryCount is not supported.
Maximum Delivery Count can be specified when a new queue is created or using the the queue declare property x-qpid-maximum-delivery-count
A Dead Letter Queue (DLQ) acts as an destination for messages that have somehow exceeded the normal bounds of processing and is utilised to prevent disruption to flow of other messages. When a DLQ is enabled for a given queue if a consuming client indicates it no longer wishes the receive the message (typically by exceeding a Maximum Delivery Count) then the message is moved onto the DLQ and removed from the original queue.
The DLQ feature causes generation of a Dead Letter Exchange and a Dead Letter Queue. These are named convention QueueName_DLE and QueueName_DLQ.
DLQs can be enabled when a new queue is created or using the queue declare property x-qpid-dlq-enabled.
Applications making use of DLQs should make provision for the frequent examination of messages arriving on DLQs so that both corrective actions can be taken to resolve the underlying cause and organise for their timely removal from the DLQ. Messages on DLQs consume system resources in the same manner as messages on normal queues so excessive queue depths should not be permitted to develop.
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