mandatory和immediate是AMQP協議中basic.pulish方法中的兩個標誌位，它們都有當訊息傳遞過程中不可達目的地時將訊息返回給生產者的功能。具體區別在於：

當mandatory標誌位設定為true時，如果exchange根據自身型別和訊息routeKey無法找到一個符合條件的queue，那麼會呼叫basic.return方法將訊息返還給生產者；當mandatory設為false時，出現上述情形broker會直接將訊息扔掉。

當immediate標誌位設定為true時，如果exchange在將訊息route到queue(s)時發現對應的queue上沒有消費者，那麼這條訊息不會放入佇列中。當與訊息routeKey關聯的所有queue(一個或多個)都沒有消費者時，該訊息會通過basic.return方法返還給生產者。

概括來說，mandatory標誌告訴伺服器至少將該訊息route到一個佇列中，否則將訊息返還給生產者；immediate標誌告訴伺服器如果該訊息關聯的queue上有消費者，則馬上將訊息投遞給它，如果所有queue都沒有消費者，直接把訊息返還給生產者，不用將訊息入佇列等待消費者了。

用前一篇中的程式amqp_sendstring來測試這兩個標誌位情況如下

設定了mandatory，路由不到佇列的情況：

    die_on_error(amqp_basic_publish(conn,
                                    1,
                                    amqp_cstring_bytes(exchange),
                                    amqp_cstring_bytes(routingkey),
                                    1,   //mandatory標誌位,訊息不能到達佇列則返回basic.return
                                    0,   //immediate標誌位,訊息不能到達消費者返回basic.return
                                    &props,
                                    amqp_cstring_bytes(messagebody)),
                 "Publishing");

./amqp_sendstring localhost 5672 amq.direct jfy test
method.id=003C0032,method.name=AMQP_BASIC_RETURN_METHOD
Return.reply_code=312
Return.reply_text=NO_ROUTE
Return.message=test

返回"NO_ROUTE"
 

    die_on_error(amqp_basic_publish(conn,
                                    1,
                                    amqp_cstring_bytes(exchange),
                                    amqp_cstring_bytes(routingkey),
                                    0,   //mandatory標誌位,訊息不能到達佇列則返回basic.return
                                    1,   //immediate標誌位,訊息不能到達消費者返回basic.return
                                    &props,
                                    amqp_cstring_bytes(messagebody)),
                 "Publishing");

./amqp_sendstring localhost 5672 amq.direct jfy test
method.id=003C0032,method.name=AMQP_BASIC_RETURN_METHOD
Return.reply_code=313
Return.reply_text=NO_CONSUMERS
Return.message=test

返回"NO_CONSUMERS"
 

注意：在RabbitMQ3.0以後的版本里，去掉了immediate引數支援，傳送帶immediate標記的publish會返回如下錯誤：

“{amqp_error,not_implemented,"immediate=true",'basic.publish'}”

為什麼移除immediate標記，參見如下版本變化描述：

Removal of "immediate" flag
What changed? We removed support for the rarely-used "immediate" flag on AMQP's basic.publish.
Why on earth did you do that? Support for "immediate" made many parts of the codebase more complex, particularly around mirrored queues. It also stood in the way of our being able to deliver substantial performance improvements in mirrored queues.
What do I need to do? If you just want to be able to publish messages that will be dropped if they are not consumed immediately, you can publish to a queue with a TTL of 0.
If you also need your publisher to be able to determine that this has happened, you can also use the DLX feature to route such messages to another queue, from which the publisher can consume them.

這段解釋的大概意思就是：immediate標記會影響映象佇列效能，增加程式碼複雜性，並建議採用"設定訊息TTL"和"DLX"等方式替代。