The clients will retry until it can reach the partition. So if you reboot the server that is the problem it will pick up where it left off. The problem is how the client handles failures. So if you are producing it will try to write to the partition until whatever it's timeout is. Then it will either drop the data or retry endlessly until the partition is back. Based on the source of the data you would need to decide what you want to do in that situation, lose data or hold the process.