【原创】RabbitMQ 之 no_ack 分析

      熟悉 RabbitMQ 的人肯定知道  no_ack 属性是在调用 Basic.Consume 方法时可以设置的一个重要参数。本文主要针对 no_ack 设置的两种情况，通过抓包分析的形式讲解下实际应用中的异同，并总结一下相关的处理经验。

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      no_ack 的用途：确保 message 被 consumer “成功”处理了。这里“成功”的意思是，（在设置了 no_ack=false 的情况下）只要 consumer 手动应答了 Basic.Ack ，就算其“成功”处理了。 

情况一：no_ack=true （此时为自动应答） 
      在这种情况下，consumer 会在接收到 Basic.Deliver + Content-Header + Content-Body 之后，立即回复 Ack 。而这个 Ack 是 TCP 协议中的 Ack 。此 Ack 的回复不关心 consumer 是否对接收到的数据进行了处理，当然也不关心处理数据所需要的耗时。 

图1：（Producer+Consumer） 
 

图2：（Consumer）
 

图3：（Producer） 
 

情况二：no_ack=false （此时为手动应答） 
      在这种情况下，要求 consumer 在处理完接收到的 Basic.Deliver + Content-Header + Content-Body 之后才回复 Ack 。而这个 Ack 是 AMQP 协议中的 Basic.Ack 。此 Ack 的回复是和业务处理相关的，所以具体的回复时间应该要取决于业务处理的耗时。

图4：（Producer+Consumer） 
 

图5：（Consumer） 
 

图6：（Producer） 
 

总结： 

• Basic.Ack 发回给 RabbitMQ 以告知，可以将相应 message 从 RabbitMQ 的消息缓存中移除。
• Basic.Ack 未被 consumer 发回给 RabbitMQ 前出现了异常，RabbitMQ 发现与该 consumer 对应的连接被断开，之后将该 message 以轮询方式发送给其他 consumer （假设存在多个 consumer 订阅同一个 queue）。
• 在 no_ack=true 的情况下，RabbitMQ 认为 message 一旦被 deliver 出去了，就已被确认了，所以会立即将缓存中的 message 删除。所以在 consumer 异常时会导致消息丢失。
• 来自 consumer 侧的 Basic.Ack 与 发送给 Producer 侧的 Basic.Ack 没有直接关系。

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最后贴上自己改造的、基于 libevent 实现的 rabbitmq-c 的测试打印。 

情况一： 

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[warn] evsignal_init: socketpair: No error
drive_machine: [conn_init]  ---  in TCP 3-way connecting!
drive_machine: [conn_connecting]  ---  connection timeout 1 time on socket(6040)
drive_machine: [conn_connected]  ---  connected on socket(6040)

6040: conn_state change   connected ==> snd_protocol_header
  --> Send Protocol.Header!
6040: conn_state change   snd_protocol_header ==> rcv_connection_start_method
  <-- Recv Connection.Start Method frame!
6040: conn_state change   rcv_connection_start_method ==> snd_connection_start_rsp_method
  --> Send Connection.Start-Ok Method frame!
6040: conn_state change   snd_connection_start_rsp_method ==> rcv_connection_tune_method
  <-- Recv Connection.Tune Method frame!
6040: conn_state change   rcv_connection_tune_method ==> snd_connection_tune_rsp_method
  --> Send Connection.Tune-Ok Method frame!
6040: conn_state change   snd_connection_tune_rsp_method ==> snd_connection_open_method
  --> Send Connection.Open Method frame!
6040: conn_state change   snd_connection_open_method ==> rcv_connection_open_rsp_method
  <-- Recv Connection.Open-Ok Method frame!
6040: conn_state change   rcv_connection_open_rsp_method ==> snd_channel_open_method
  --> Send Channel.Open Method frame!
6040: conn_state change   snd_channel_open_method ==> rcv_channel_open_rsp_method
  <-- Recv Channel.Open-Ok Method frame!
6040: conn_state change   rcv_channel_open_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Queue Declaring!
6040: conn_state change   idle ==> snd_queue_declare_method
  --> Send Queue.Declare Method frame!
6040: conn_state change   snd_queue_declare_method ==> rcv_queue_declare_rsp_method
  <-- Recv Queue.Declare-Ok Method frame!
6040: conn_state change   rcv_queue_declare_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Queue Binding!
6040: conn_state change   idle ==> snd_queue_bind_method
  --> Send Queue.Bind Method frame!
6040: conn_state change   snd_queue_bind_method ==> rcv_queue_bind_rsp_method
  <-- Recv Queue.Bind-Ok Method frame!
6040: conn_state change   rcv_queue_bind_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Basic QoS!
6040: conn_state change   idle ==> snd_basic_qos_method
  --> Send Basic.Qos Method frame!
6040: conn_state change   snd_basic_qos_method ==> rcv_basic_qos_rsp_method
  <-- Recv Basic.Qos-Ok Method frame!
6040: conn_state change   rcv_basic_qos_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Basic Consuming!
6040: conn_state change   idle ==> snd_basic_consume_method
  --> Send Basic.Consume Method frame!
6040: conn_state change   snd_basic_consume_method ==> rcv_basic_consume_rsp_method
  <-- Recv Basic.Consume-Ok Method frame!
6040: conn_state change   rcv_basic_consume_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Start waiting to recv!
6040: conn_state change   idle ==> rcv_basic_deliver_method
drive_machine: wait for Basic.Deliver method another 10 seconds!!
drive_machine: wait for Basic.Deliver method another 10 seconds!!

  <-- Recv Basic.Deliver Method frame!
6040: conn_state change   rcv_basic_deliver_method ==> rcv_basic_content_header
  <-- Recv Content.Header frame!
6040: conn_state change   rcv_basic_content_header ==> rcv_basic_content_body
  <-- Recv Content.Body frame!
Content Body is [Hello World betty].
@@@ CB: body len : [17]    body : [Hello World betty]
6040: conn_state change   rcv_basic_content_body ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Start waiting to recv!
6040: conn_state change   idle ==> rcv_basic_deliver_method
drive_machine: wait for Basic.Deliver method another 10 seconds!!
drive_machine: wait for Basic.Deliver method another 10 seconds!!

情况二： 

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[warn] evsignal_init: socketpair: No error
drive_machine: [conn_init]  ---  in TCP 3-way connecting!
drive_machine: [conn_connecting]  ---  connection timeout 1 time on socket(6040)
drive_machine: connected on socket(6040)
6040: conn_state change   connected ==> snd_protocol_header
  --> Send Protocol.Header!
6040: conn_state change   snd_protocol_header ==> rcv_connection_start_method
  <-- Recv Connection.Start Method frame!
6040: conn_state change   rcv_connection_start_method ==> snd_connection_start_rsp_method
  --> Send Connection.Start-Ok Method frame!
6040: conn_state change   snd_connection_start_rsp_method ==> rcv_connection_tune_method
  <-- Recv Connection.Tune Method frame!
6040: conn_state change   rcv_connection_tune_method ==> snd_connection_tune_rsp_method
  --> Send Connection.Tune-Ok Method frame!
6040: conn_state change   snd_connection_tune_rsp_method ==> snd_connection_open_method
  --> Send Connection.Open Method frame!
6040: conn_state change   snd_connection_open_method ==> rcv_connection_open_rsp_method
  <-- Recv Connection.Open-Ok Method frame!
6040: conn_state change   rcv_connection_open_rsp_method ==> snd_channel_open_method
  --> Send Channel.Open Method frame!
6040: conn_state change   snd_channel_open_method ==> rcv_channel_open_rsp_method
  <-- Recv Channel.Open-Ok Method frame!
6040: conn_state change   rcv_channel_open_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Queue Declaring!
6040: conn_state change   idle ==> snd_queue_declare_method
  --> Send Queue.Declare Method frame!
6040: conn_state change   snd_queue_declare_method ==> rcv_queue_declare_rsp_method
  <-- Recv Queue.Declare-Ok Method frame!
6040: conn_state change   rcv_queue_declare_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Queue Binding!
6040: conn_state change   idle ==> snd_queue_bind_method
  --> Send Queue.Bind Method frame!
6040: conn_state change   snd_queue_bind_method ==> rcv_queue_bind_rsp_method
  <-- Recv Queue.Bind-Ok Method frame!
6040: conn_state change   rcv_queue_bind_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Basic QoS!
6040: conn_state change   idle ==> snd_basic_qos_method
  --> Send Basic.Qos Method frame!
6040: conn_state change   snd_basic_qos_method ==> rcv_basic_qos_rsp_method
  <-- Recv Basic.Qos-Ok Method frame!
6040: conn_state change   rcv_basic_qos_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Basic Consuming!
6040: conn_state change   idle ==> snd_basic_consume_method
  --> Send Basic.Consume Method frame!
6040: conn_state change   snd_basic_consume_method ==> rcv_basic_consume_rsp_method
  <-- Recv Basic.Consume-Ok Method frame!
6040: conn_state change   rcv_basic_consume_rsp_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Start waiting to recv!
6040: conn_state change   idle ==> rcv_basic_deliver_method
drive_machine: wait for Basic.Deliver method another 10 seconds!!
drive_machine: wait for Basic.Deliver method another 10 seconds!!
drive_machine: wait for Basic.Deliver method another 10 seconds!!

  <-- Recv Basic.Deliver Method frame!
6040: conn_state change   rcv_basic_deliver_method ==> rcv_basic_content_header
  <-- Recv Content.Header frame!
6040: conn_state change   rcv_basic_content_header ==> rcv_basic_content_body
  <-- Recv Content.Body frame!
Content Body is [Hello World betty].
@@@ CB: body len : [17]    body : [Hello World betty]
6040: conn_state change   rcv_basic_content_body ==> snd_basic_ack_method
  --> Send Basic.Ack Method frame!
6040: conn_state change   snd_basic_ack_method ==> idle

drive_machine: [conn_idle]  ---  [CONSUMER]: Start waiting to recv!
6040: conn_state change   idle ==> rcv_basic_deliver_method
drive_machine: wait for Basic.Deliver method another 10 seconds!!
drive_machine: wait for Basic.Deliver method another 10 seconds!!