pubsub

Pub/Sub, short for publish-subscribe, is a messaging pattern that decouples producers of information from consumers. Producers publish messages to named channels called topics, while consumers subscribe to topics of interest to receive messages. A broker or message broker mediates between publishers and subscribers, enabling asynchronous communication and scalable distribution.

Key elements include topics, publishers, subscribers, and subscriptions. Publishers emit messages to a topic; subscribers register subscriptions to that topic; the broker delivers messages to all active subscriptions. Messages may be delivered immediately or retained for later delivery, depending on configuration. Delivery guarantees vary; common modes include at-least-once, at-most-once, and exactly-once semantics, though exactly-once is complex to implement. Subscriptions can be durable, surviving broker restarts, or non-durable. The system may support filtering, enabling subscribers to receive only relevant messages.

Uses and considerations: pub/sub is widely used in event-driven architectures, real-time notifications, microservices communication, and IoT. Advantages include loose coupling, scalability, and easier integration of independent components. Drawbacks can include eventual consistency challenges, message ordering across distributed systems, and the need for careful handling of duplicates. Related technologies and protocols include MQTT, AMQP, and cloud services such as Google Cloud Pub/Sub. Some systems implement pub/sub as a distributed log, like Kafka, which determines ordering and durability differently.