Pulsar Clients | Apache Pulsar (2024)

Pulsar exposes a client API with language bindings for Java, C++, Go, Python, Node.js and C#. The client API optimizes and encapsulates Pulsar's client-broker communication protocol and exposes a simple and intuitive API for use by applications.

Pulsar client libraries support transparent reconnection and/or connection failover to brokers, queuing of messages until acknowledged by the broker, and heuristics such as connection retries with backoff.

Client setup phase​

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

1. 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.
2. Once the client library has the broker address, it creates a TCP connection (or reuses an existing connection from the pool) and authenticates it. Within this connection, the client and broker exchange binary commands from a custom protocol. At this point, the client sends a command to create producer/consumer to the broker, which will comply after having validated the authorization policy.

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.

Producer​

A producer is a process that attaches to a topic and publishes messages to a Pulsar broker. The Pulsar broker processes the messages.

Send mode​

Producers send messages to brokers synchronously (sync) or asynchronously (async).

Access mode​

You can have different types of access modes on topics for producers.

You can set producer access mode through Java Client API. For more information, see ProducerAccessMode in ProducerBuilder.java file.

Consumer​

A consumer is a process that attaches to a topic via a subscription and then receives messages.

A consumer sends a flow permit request to a broker to get messages. There is a queue at the consumer side to receive messages pushed from the broker. You can configure the queue size with the receiverQueueSize parameter. The default size is 1000). Each time consumer.receive() is called, a message is dequeued from the buffer.

Receive mode​

Messages are received from brokers either synchronously (sync) or asynchronously (async).

Listener​

Client libraries provide listener implementation for consumers. For example, the Java client provides a MesssageListener interface. In this interface, the received method is called whenever a new message is received.

Reader​

In Pulsar, the "standard" consumer interface involves using consumers to listen on topics, process incoming messages, and finally acknowledge those messages when they are processed. Whenever a new subscription is created, it is initially positioned at the end of the topic (by default), and consumers associated with that subscription begin reading with the first message created afterward. Whenever a consumer connects to a topic using a pre-existing subscription, it begins reading from the earliest message un-acked within that subscription. In summary, with the consumer interface, subscription cursors are automatically managed by Pulsar in response to message acknowledgments.

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. When connecting to a topic, the reader interface enables you to begin with:

• The earliest available message in the topic.
• The latest available message in the topic.
• Some other messages between the earliest and the latest. If you select this option, you'll need to explicitly provide a message ID. Your application will be responsible for "knowing" this message ID in advance, perhaps fetching it from a persistent data store or cache.

The reader interface is helpful for use cases like using Pulsar to provide effectively-once processing semantics for a stream processing system. For this use case, the stream processing system must be able to "rewind" topics to a specific message and begin reading there. The reader interface provides Pulsar clients with the low-level abstraction necessary to "manually position" themselves within a topic.

Internally, the reader interface is implemented as a consumer using an exclusive, non-durable subscription to the topic with a randomly-allocated name.

TableView​

The TableView interface serves an encapsulated access pattern, providing a continuously updated key-value map view of the compacted topic data. Messages without keys will be ignored.

With TableView, Pulsar clients can fetch all the message updates from a topic and construct a map with the latest values of each key. These values can then be used to build a local cache of data. In addition, you can register consumers with the TableView by specifying a listener to perform a scan of the map and then receive notifications when new messages are received. Consequently, event handling can be triggered to serve use cases, such as event-driven applications and message monitoring.

The following figure illustrates the dynamic construction of a TableView updated with newer values of each key.