Why QueueData is a list, because a topic may exist on different brokers. The meaning of the representation is that a topic has several queues in the broker.
In addition, when the broker starts, in ConsumerOffsetManage
private ConcurrentMap<String/* topic@group */, ConcurrentMap<Integer, Long>> offsetTable =
new ConcurrentHashMap<String, ConcurrentMap<Integer, Long>>(512);
Meaning explanation, topic@group has a clear meaning, the following map, In fact, it is the consumption of each queue in the broker.
commit For the case where the message is stored in writeBuffer, after commit, call channel.write(buffer) to write to the buffer. Subsequent flushes are flushed to disk.
So the question, when is writebuffer and when is writebuffer not used?