Pulsar Clients

底层实现上，目前官方版的Pulsar客户端支持对用户透明的连接重连、故障切换、未ack消息的缓冲、消息重传。

Before an application creates a producer/consumer, the Pulsar client library needs to initiate a setup phase including two steps:

The client attempts to determine the owner of the topic by sending an HTTP lookup request to the broker. The request could reach one of the active brokers which, by looking at the (cached) zookeeper metadata knows who is serving the topic or, in case nobody is serving it, tries to assign it to the least loaded broker.

Whenever the TCP connection breaks, the client immediately re-initiates this setup phase and keeps trying with exponential backoff to re-establish the producer or consumer until the operation succeeds.

Reader 接口

The reader interface for Pulsar enables applications to manually manage cursors. When you use a reader to connect to a topic—-rather than a consumer—-you need to specify which message the reader begins reading from when it connects to a topic. 当连接到一个 topic 时，reader 接口支持的开始位置包括：

The earliest available message in the topic
如果你想开始的位置在最早和最新之间，则需要显示的指定消息 ID。你的应用程序将需要提前“知道”这个消息 ID，可能要从持久化存储或缓存中获取。

Reader 接口对流处理系统中，需要用到 effectively-once(仅仅一次) 语义的场景是很有帮助的。 Pulsar能够将主题的消息进行重放，并从重放后的位置开始读取消息，是满足流处理的场景的重要基础。 Reader 接口为 Pulsar 客户端在 Topic 内提供了一种能“手动管理起始位置”的底层抽象。

Reader 接口内部是作为一个使用独占、非持久化订阅的被随机命名的一个消费者来实现的。

[ IMPORTANT ]

Please also note that a reader can have a “backlog”, but the metric is only used for users to know how behind the reader is. The metric is not considered for any backlog quota calculations.

下面是一个Java语言实现的从主题上最早可用消息的位置开始消费的例子

To create a reader that reads from the latest available message: