Dead Lettering

What is a dead letter exchange

Messages can be “dead-lettered” which means they can be re-published to an exchange (Dead Letter eXchange) when:

A message is “negatively acknoledged” or NACK’ed

The message expires on its TTL

A message is dropped because the queue exceeded a length limit

The message is returned more times to a quorum queue than the delivery limit

Messages are NOT dead lettered when an entire queue expires

How to configure

You specify the DLX via polices linked to a queue

dead-letter-exchange: The name of the DLX to use

dead-letter-routing-key: The routing key to use when dead-lettering messages. IF NOT SPECIFIED, then the routing key originally used will be used instead.

You can also specify the these values via arguments in application code, and these will take presidence over polices.

Please consult the docs on how to do these. LINK

Routing Dead-Lettered Messages

Deal-letter loop

It is possible to loop rabbit messages, via passing them to the dead letter exchange and then back to the original exchange with the same routing key. If this happens rabbit will kill the message if no rejection was ever detected.

Safety

It is possible for messages to become lost.

Publisher Confirms is turned off by default. Thus using DLX is not always safe. Messages are removed from the original queue after the letter has arrived at the target DLX queue.

Dead-Lettered Effects on Messages

Dead-lettering a message will modify its headers:

Exchange name is replaced to the one of the DLX

Routing Key is replaced to the DL routing key
- If this occurs then the CC will also be removed

BCC header will be removed as per some scenario specified in the docs

Within the header a {Queue, Reason} pair specifies the history of the letter.

A x-death header holds an array of objects specifying information about previous dead lettering events. The most recent element in the array is the most recent dead-lettering event.

Table specifying all the fields

AMQP 1.0 key	AMQP 1.0 value type	AMQP 0.9.1 key	AMQP 0.9.1 value type	Description
queue	string	queue	longstr	The name of the queue this message was dead lettered from.
reason	symbol	reason	longstr	Why this message was dead lettered (described below).
count	ulong	count	long	How many times this message was dead lettered from this queue for this reason.
first-time	timestamp			When this message was dead lettered the first time from this queue for this reason.
last-time	timestamp			When this message was dead lettered the last time from this queue for this reason.
		time	timestamp	When this message was dead lettered the first time from this queue for this reason.
exchange	string	exchange	longstr	The exchange this message was published to before this message got dead lettered for the first time from this queue for this reason.
routing-keys	array of string	routing-keys	array of longstr	The routing keys (including `CC` but excluding `BCC`) of this message before it got dead lettered for the first time from this queue for this reason.
ttl	uint			AMQP 1.0 header's `ttl` (time to live in milliseconds) before this message got dead lettered for the first time from this queue for this reason.
		original-expiration	longstr	The original `expiration` property of this message before it got dead lettered for the first time from this queue for this reason.