PublishSubscribe

Publish-subscribe is a messaging pattern used in distributed systems where publishers do not send messages directly to specific receivers. Instead, messages are published to topics or channels, and subscribers express interest in one or more topics. A broker or messaging middleware routes messages from publishers to interested subscribers, enabling asynchronous, decoupled communication between components.

In operation, publishers emit messages to a topic, and subscribers subscribe to topics. The broker tracks subscriptions and may apply filters to determine which subscribers receive a given message. When a message is published, the broker delivers it to all subscribers of that topic according to the system’s delivery semantics. Delivery can be best-effort, at-least-once, or exactly-once, and messages can be persisted for durability or discarded after delivery. Durable subscriptions allow offline subscribers to receive messages published while they were unavailable.

Pub/sub architectures can be brokered, with a central component handling routing, or brokerless, where peers communicate directly. Common implementations include Apache Kafka, which uses a distributed log; RabbitMQ, a traditional broker supporting topics and fan-out delivery; Google Cloud Pub/Sub; and MQTT, a lightweight protocol designed for sensing and IoT devices. Differences among systems include guarantees around delivery, message ordering, scalability, and topic semantics. Some integrations combine pub/sub with event streaming or data pipelines to enable real-time analytics.

Use cases include event-driven architectures, real-time notifications, telemetry collection, and decoupled service integrations. Benefits of publish-subscribe include loose coupling, improved scalability, and resilience to failures; trade-offs can include potential message duplicates, out-of-order delivery, operational complexity, and the need for robust security and authorization mechanisms.