关于消息队列的使用场景：https://www.cnblogs.com/linjiqin/p/5720865.html
kafka:
Topic
Kafka将消息种子(Feed)分门别类 每一类的消息称之为话题(Topic).
Producer
发布消息的对象称之为话题生产者(Kafka topic producer)
Consumer
订阅消息并处理发布的消息的种子的对象称之为话题消费者(consumers)
Broker
已发布的消息保存在一组服务器中称之为Kafka集群。集群中的每一个服务器都是一个代理(Broker). 消费者可以订阅一个或多个话题并从Broker拉数据从而消费这些已发布的消息。
对于传统的message queue而言，一般会删除已经被消费的消息，而Kafka集群会保留所有的消息，无论其被消费与否。当然，因为磁盘限制，不可能永久保留所有数据（实际上也没必要），因此Kafka提供两种策略去删除旧数据。一是基于时间，二是基于partition文件大小。例如可以通过配置$KAFKA_HOME/config/server.properties ，让Kafka删除一周前的数据，也可通过配置让Kafka在partition文件超过1GB时删除旧数据，如下所示。
############################# Log Retention Policy #############################
# The following configurations control the disposal of log segments. The policy can
# be set to delete segments after a period of time, or after a given size has accumulated.
# A segment will be deleted whenever *either* of these criteria are met. Deletion always happens
# from the end of the log.
# The minimum age of a log file to be eligible for deletion
log.retention.hours=168
# A size-based retention policy for logs. Segments are pruned from the log as long as the remaining
# segments don't drop below log.retention.bytes.
#log.retention.bytes=1073741824
# The maximum size of a log segment file. When this size is reached a new log segment will be created.
log.segment.bytes=1073741824
# The interval at which log segments are checked to see if they can be deleted according
# to the retention policies
log.retention.check.interval.ms=300000
# By default the log cleaner is disabled and the log retention policy will default to
#just delete segments after their retention expires.
# If log.cleaner.enable=true is set the cleaner will be enabled and individual logs
#can then be marked for log compaction.
log.cleaner.enable=false
Kafka会为每一个consumer group保留一些metadata信息—当前消费的消息的position，也即offset。这个offset由consumer控制。正常情况下consumer会在消费完一条消息后线性增加这个offset。当然，consumer也可将offset设成一个较小的值，重新消费一些消息。因为offet由consumer控制，所以Kafka broker是无状态的，它不需要标记哪些消息被哪些consumer过，不需要通过broker去保证同一个consumer group只有一个consumer能消费某一条消息，因此也就不需要锁机制，这也为Kafka的高吞吐率提供了有力保障。