Apache Pulsar
Open-source distributed messaging and streaming platform built for the cloud. Top-10 Apache Software Foundation project (740+ contributors, current version 4.2). Multi-tenant by design (tenants, namespaces, topics, subscriptions), supports both queue and pub/sub messaging models, layered architecture separates stateless brokers from BookKeeper storage. Includes Pulsar Functions (serverless processing) and Pulsar IO (connectors).