kafka接口协议

The Kafka protocol is fairly simple, there are only six client requests APIs.

1. Metadata - Describes the currently available brokers, their host and port information, and gives information about which broker hosts which partitions.获得活着的broker，他们的host和port，broker包含的partition
2. Send - Send messages to a broker  发送消息到broker
3. Fetch - Fetch messages from a broker, one which fetches data, one which gets cluster metadata, and one which gets offset information about a topic.  从broker上获得消息，集群元数据和topic的offset
4. Offsets - Get information about the available offsets for a given topic partition.获得topic的partition的offset
5. Offset Commit - Commit a set of offsets for a consumer group 写入consumer group消费的offset
6. Offset Fetch - Fetch a set of offsets for a consumer group 读取consumer group消费的offset