xujianhai666 commented on a change in pull request #5293: [ISSUE #4675][docs] 
Add *Delayed Message Delivery* 
URL: https://github.com/apache/pulsar/pull/5293#discussion_r332912201
 
 

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 File path: site2/docs/concepts-messaging.md
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 @@ -411,4 +411,32 @@ Message deduplication makes Pulsar an ideal messaging 
system to be used in conju
 
 > More in-depth information can be found in [this 
 > post](https://streaml.io/blog/pulsar-effectively-once/) on the [Streamlio 
 > blog](https://streaml.io/blog)
 
+## Delayed Message Delivery
+Delayed Message Delivery enables you to consume new message later, instead of 
immediately as normal. In this mechanism, messages is stored in memory after 
publish to broker, after specific delayed time passed, the message will be 
delivered to consumer.  
 
+The diagram below illustrates concept:
+
+![Delayed Message Delivery](assets/message_delay.png)
+
+broker save message without any check. when consumer consume message, if 
message is delayed type, message will add to DelayedDeliveryTracker. 
subscription will check and get timeout messages from DelayedDeliveryTracker.
+
+### broker 
+Delayed Message Delivery is enabled by default on broker conf as below
+```
+# Whether to enable the delayed delivery for messages.
+# If disabled, messages will be immediately delivered and there will
+# be no tracking overhead.
+delayedDeliveryEnabled=true
+
+# Control the tick time for when retrying on delayed delivery,
 
 Review comment:
   @Anonymitaet done

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