How is Apache Pulsar different from Apache Kafka?

Apache Pulsar and Apache Kafka are both distributed streaming platforms, but they differ significantly in architecture, messaging models, and operational characteristics. Pulsar is designed as a cloud-native system with a layered architecture, separating compute (brokers) from storage (Apache BookKeeper), while Kafka uses a unified broker model where each node handles both data serving and storage. This fundamental difference impacts scalability, fault tolerance, and flexibility in managing large-scale data streams.

Architecture: Pulsar’s separation of brokers and storage allows it to scale independently. Brokers manage message routing and lightweight computation, while BookKeeper handles durable, low-latency storage. This design enables Pulsar to scale brokers horizontally without data rebalancing, as storage nodes remain decoupled. In contrast, Kafka brokers store partitions directly on their disks, meaning scaling requires redistributing data across new brokers, which can be slow and complex. For example, adding a Kafka broker involves reassigning partitions manually or using automated tools, while Pulsar simply adds brokers that fetch data from BookKeeper on demand. Pulsar’s architecture also provides faster recovery from failures: if a broker fails, another can immediately take over without waiting for data replication, as storage is already fault-tolerant via BookKeeper’s ledger-based design.

Messaging and Features: Pulsar supports a broader set of messaging patterns out of the box. It natively handles queuing (via shared subscriptions), pub-sub (via exclusive subscriptions), and event streaming, while Kafka focuses primarily on streaming. For instance, Pulsar’s shared subscriptions allow multiple consumers to process messages from a single topic in a round-robin fashion, similar to traditional message queues, whereas Kafka requires custom client-side logic to achieve similar behavior. Pulsar also offers built-in tiered storage, letting you offload older data to cloud storage (e.g., S3) while retaining access, whereas Kafka requires third-party tools or manual management for this. Additionally, Pulsar provides multi-tenancy with granular namespace and tenant-level policies, making it easier to manage shared clusters across teams.

Operational Considerations: Kafka’s maturity means it has a more extensive ecosystem, including Kafka Connect for data integration and Kafka Streams for stream processing. Pulsar counters with integrated features like Pulsar Functions (lightweight serverless processing) and simpler geo-replication. For example, Pulsar’s replication can be configured at the namespace level with a few CLI commands, while Kafka relies on tools like MirrorMaker. Pulsar’s stateless brokers also simplify operations: there’s no need to manage disk usage or rebalance partitions when scaling. However, Kafka’s simplicity appeals in smaller deployments, where its single-layer architecture reduces operational overhead. Developers must weigh these trade-offs: Pulsar excels in elasticity and multi-use-case scalability, while Kafka remains a solid choice for teams prioritizing ecosystem integration and operational familiarity.