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Pulsar Terminology

Here is a glossary of terms related to Apache Pulsar:



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

Namespace Bundle

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


Namespace is a virtual grouping of topics, under a specific tenant. A namespace is defined by a string name, such as my-tenant/my-namespace.


Pub-sub is a messaging pattern in which producer processes publish messages on topics that are then consumed (processed) by consumer processes.


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 latest available unacked message);
  • readers don't retain data or acknowledge messages.


Cursor is the subscription position for a consumer.


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

Retention Policy

Retention policy is the size and time limits that you can set on a namespace to configure retention of messages that have already been acknowledged.


Multi-tenancy is the ability to isolate namespaces, specify quotas, and configure authentication and authorization on a per-tenant basis.

Failure Domain

Failure domain is a logical domain under a Pulsar cluster. Each logical domain contains a pre-configured list of brokers.

Anti-Affinity Namespaces

Anti-affinity namespaces are a group of namespaces that have anti-affinity to each other.



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

Topic Lookup

Topic lookup is a service provided by Pulsar brokers that enables connecting clients to automatically determine which Pulsar cluster is responsible for a topic (and thus where message traffic for the topic needs to be routed).


Dispatcher is an asynchronous TCP server used for all data transfers in and out of a Pulsar broker. The Pulsar dispatcher uses a custom binary protocol for all communications.


Broker is a Pulsar server that receives, acknowledges, and delivers messages to consumers. A Pulsar cluster can have one or more brokers.



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