Build an MQTT Broker

You build an MQTT broker: a publish/subscribe message server that accepts client connections, decodes the binary MQTT control packets, routes published messages to subscribers via topic matching including wildcards, honors the protocol's quality-of-service levels, stores retained messages, and tracks per-client sessions, all on a single-threaded epoll-style event loop. It is worth building because it combines low-level binary protocol parsing with the concurrency model that powers high-throughput servers, and pub/sub is the backbone of IoT and event-driven systems. It is resume-defining because it shows you can implement a real wire protocol against its specification and build a non-blocking server that holds thousands of connections without a thread per client. You finish fluent in both protocol design and the event-loop architecture behind production network services.