Ack mode of individual pulsar

[li.peng...@zhaopin.com.cn]

In introduce of pulsar, i interest in ack mode of individual. 
I know that pulsar use bookkeeper to store messages, and entries in bookkeeper 
write only once.
so i want to know how pulsar server do message ack in individual mode. if ack 
failed, how long will i receive the message again.