codelipenghui commented on issue #4100: DeadLetterTopic is not working for 
FailOver subscription type
URL: https://github.com/apache/pulsar/issues/4100#issuecomment-489394769
 
 
   > May i know more details on why? 
   
   For `FailOver` and `Exclusive` subscription mode, pulsar do not track the 
ack status for each message. When message redelivery happens in  `FailOver` or 
`Exclusive` mode,  broker just rewind the cursor(Currently, no sign for ack 
timeout message).  So we originally supported this feature in `Shared` mode.
   
   This is possible to support this feature in  `FailOver` or `Exclusive` mode. 
Like in `Shared` mode, we need a `RedeliveryTracker` to track each ack timeout 
message, there are some differences between `Shared` mode, we need a ordered 
structure to maintain position. 
   
   > Also what are the alternatives suggested, does it mean cursor will not 
move until the message is processed and acked ?
   
   As far as I know there is no alternative currently, If message cannot be 
successfully acked, cursor will not move.
   
   By the way, can I know your usage scenario?

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