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hivemq/hivemq-community-edition
HiveMQ CE is a Java-based open source MQTT broker that fully supports MQTT 3.x and MQTT 5. It is the foundation of the HiveMQ Enterprise Connectivity and Messaging Platform
HiveMQ CE is a production-grade MQTT broker in Java, covering the full MQTT 3.x and 5.0 specs. It's the open-source core of the HiveMQ commercial product, so it carries real enterprise pedigree. Useful for IoT backends, device telemetry pipelines, or anywhere you need a standards-compliant MQTT broker you can run yourself.
Full MQTT 5 support including user properties, message expiry, shared subscriptions, and topic aliases — not a partial implementation. Embedded mode is genuinely useful: drop it into a Java app as a library dependency and wire extensions programmatically without touching the filesystem. The extension SDK is well-structured — authentication, authorization, packet interceptors, and client lifecycle hooks are all first-class extension points. Active maintenance with regular releases (June 2026 push, CalVer versioning) and Snyk security scanning in CI.
No clustering in the community edition — that's the enterprise paywall. If you need horizontal scale or high availability, you're either rolling your own or buying. Documentation lives in a GitHub wiki, which is harder to search and version than proper docs. RocksDB is used for persistence by default but requires manual InternalConfigurations hacks to swap out, which is leaking implementation details through the API. The Java 11 runtime requirement for running but Java 21 for compiling is an odd split that will catch people out.