Pulsar Terminology

Here is a glossary of terms related to Apache Pulsar:

Concepts

Pulsar

Pulsar is a distributed messaging system originally created by Yahoo but now under the stewardship of the Apache Software Foundation.

Message

Messages are the basic unit of Pulsar. They're what producers publish to topicsand what consumers then consume from topics.

Topic

A named channel used to pass messages published by producers to consumers whoprocess those messages.

Partitioned Topic

A topic that is served by multiple Pulsar brokers, which enables higher throughput.

Namespace

A grouping mechanism for related topics.

Namespace Bundle

A virtual group of topics that belong to the same namespace. A namespace bundleis defined as a range between two 32-bit hashes, such as 0x00000000 and 0xffffffff.

Tenant

An administrative unit for allocating capacity and enforcing an authentication/authorization scheme.

Subscription

A lease on a topic established by a group of consumers. Pulsar has three subscriptionmodes (exclusive, shared, and failover).

Pub-Sub

A messaging pattern in which producer proccesses publish messages on topics thatare then consumed (processed) by consumer processes.

Producer

A process that publishes messages to a Pulsar topic.

Consumer

A process that establishes a subscription to a Pulsar topic and processes messages publishedto that topic by producers.

Reader

Pulsar readers are message processors much like Pulsar consumers but with two crucial differences:

• you can specify where on a topic readers begin processing messages (consumers always begin with the latestavailable unacked message);
• readers don't retain data or acknowledge messages.

Cursor

The subscription position for a consumer.

Acknowledgment (ack)

A message sent to a Pulsar broker by a consumer that a message has been successfully processed.An acknowledgement (ack) is Pulsar's way of knowing that the message can be deleted from the system;if no acknowledgement, then the message will be retained until it's processed.

Negative Acknowledgment (nack)

When an application fails to process a particular message, it can sends a "negative ack" to Pulsarto signal that the message should be replayed at a later timer. (By default, failed messages arereplayed after a 1 minute delay)

Unacknowledged

A message that has been delivered to a consumer for processing but not yet confirmed as processed by the consumer.

Retention Policy

Size and/or time limits that you can set on a namespace to configure retention of messagesthat have already been acknowledged.

Multi-Tenancy

The ability to isolate namespaces, specify quotas, and configure authentication and authorizationon a per-tenant basis.

Architecture

Standalone

A lightweight Pulsar broker in which all components run in a single Java Virtual Machine (JVM) process. Standaloneclusters can be run on a single machine and are useful for development purposes.

Cluster

A set of Pulsar brokers and BookKeeper servers (aka bookies).Clusters can reside in different geographical regions and replicate messages to one anotherin a process called geo-replication.

Instance

A group of Pulsar clusters that act together as a single unit.

Geo-Replication

Replication of messages across Pulsar clusters, potentially in different datacentersor geographical regions.

Configuration Store

Pulsar's configuration store (previously known as configuration store) is a ZooKeeper quorum thatis used for configuration-specific tasks. A multi-cluster Pulsar installation requires just oneconfiguration store across all clusters.

Topic Lookup

A service provided by Pulsar brokers that enables connecting clients to automatically determinewhich Pulsar cluster is responsible for a topic (and thus where message traffic forthe topic needs to be routed).

Service Discovery

A mechanism provided by Pulsar that enables connecting clients to use just a single URL to interactwith all the brokers in a cluster.

Broker

A stateless component of Pulsar clusters that runs two other components: an HTTP serverexposing a REST interface for administration and topic lookup and a dispatcher thathandles all message transfers. Pulsar clusters typically consist of multiple brokers.

Dispatcher

An asynchronous TCP server used for all data transfers in-and-out a Pulsar broker. The Pulsardispatcher uses a custom binary protocol for all communications.

Storage

BookKeeper

Apache BookKeeper is a scalable, low-latency persistent log storageservice that Pulsar uses to store data.

Bookie

Bookie is the name of an individual BookKeeper server. It is effectively the storage server of Pulsar.

Ledger

An append-only data structure in BookKeeper that is used to persistently store messages in Pulsar topics.

Functions

Pulsar Functions are lightweight functions that can consume messages from Pulsar topics, apply custom processing logic, and, if desired, publish results to topics.