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Publish–subscribe pattern
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<p>In <a href="/facts/Software_architecture/1Hokp0Ed">software architecture</a>, the publish–subscribe pattern (pub/sub) is a <a href="/facts/Messaging_pattern/ItHgSCSR">messaging pattern</a> in which message senders, called publishers, categorize <a href="/facts/Message_passing/RYf3hD3J">messages</a> into classes (or <i>topics</i>), and send them without needing to know which components will receive them. Message recipients, called subscribers, express interest in one or more classes and only receive messages in those classes, without needing to know the identity of the publishers. </p><p>This pattern decouples the components that produce messages from those that consume them, and supports asynchronous, many-to-many communication. The publish–subscribe model is commonly contrasted with <a href="/facts/Message_queue/Kq7kX86w">message queue</a>-based and <a href="/facts/Point-to-point_(telecommunications)/RmGgN1Pj">point-to-point</a> messaging models, where producers send messages directly to consumers. </p><p>Publish–subscribe is a sibling of the message queue paradigm, and is typically a component of larger <a href="/facts/Message-oriented_middleware/jI3NVb5b">message-oriented middleware</a> systems. Many modern messaging frameworks and protocols, such as the <a href="/facts/Java_Message_Service/aTzyVj3n">Java Message Service</a> (JMS), <a href="/facts/Apache_Kafka/DVzgoEmu">Apache Kafka</a>, and <a href="/facts/MQTT/JltGCjue">MQTT</a>, support both the pub/sub and queue-based models. </p><p>This pattern provides greater network <a href="/facts/Scalability/jh7JomPa">scalability</a> and supports more dynamic <a href="/facts/Network_topology/rBYE0kke">topologies</a>, but can make it harder to modify the publisher’s logic or the structure of the published data. Compared to synchronous patterns like <a href="/facts/Remote_procedure_call/uMMb9CQw">RPC</a> and point-to-point messaging, publish–subscribe provides the highest level of <a href="/facts/Loose_coupling/uXRb4qBJ">decoupling</a> among architectural components. However, it can also lead to <a href="/facts/Coupling_(computer_programming)/QEeSaXF3">semantic or format coupling</a> between publishers and subscribers, which may cause systems to become entangled or <a href="/facts/Software_brittleness/w3sfqkvz">brittle</a> over time. </p>