On Mon, Apr 13, 2015 at 10:10 PM, bit1...@163.com <bit1...@163.com> wrote:
> Hi, Kafka experts: > > I got serveral questions about auto.offset.reset. This configuration > parameter governs how consumer read the message from Kafka when there is > no initial offset in ZooKeeper or if an offset is out of range. > > Q1. "no initial offset in zookeeper " means that there isn't any consumer > to consume the message yet(The offset is set once the consumer starts to > consume)? > Yes, or if you consumed messages, but auto offset commit is disabled and you haven't explicitly committed any offsets. > Q2: What does "offset is out of range" mean? Can you eleborate one > scenario when "offset is out of range" could happen? > Kafka uses a retention policy for topics to expire data and clean it up. If some messages expire and your consumer hasn't run in awhile, the last committed offset may no longer exist. > > auto.offset.reset has two values:smallest and largest. > Assume one scenario: A producer has produced 10 messages to kafka, and > there is no consumer yet to consume it. > Q3: If auto.offset.reset is set to "smallest", does it mean that the > consumer will read the message from the offset 0?(0 is smallest here) > Yes. > Q4: If auto.offset.reset is set to "largest", does it mean that the > consumer will not read any message but wait until new messages come? > Also correct. This is why in the quickstart you need to use the --from-beginning flag on the console consumer. Since the consumer is executed after the console producer it wouldn't see any messages unless it set auto.offset.reset to smallest, which is what --from-beginning does. > > > > bit1...@163.com > -- Thanks, Ewen
