Publish/subscribe messaging (pub/sub) is an asynchronous pattern in which publishers send messages to a topic or channel, and messaging infrastructure routes them to subscribers interested in that topic. The publisher does not need to know who the subscribers are or where they run.
How publish/subscribe messaging works
Pub/sub has four basic roles:
- Publisher: creates and sends a message or event.
- Topic or channel: identifies a category or stream of messages.
- Subscriber: registers interest in a topic, channel, or filter.
- Broker or router: tracks subscriptions and routes matching messages to subscribers.
Usually, subscribers register first. A publisher then sends a message to the topic or intermediary, and the infrastructure selects the matching subscriptions and delivers it. The result is often one-to-many distribution, though routing and delivery details depend on the system.
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Example: an order event
When an order service publishes an order-placed event, inventory, fulfillment, and analytics services could each subscribe to it. The messaging infrastructure distributes the event to those subscribers, while the order service remains unaware of their identities. A new subscriber can be added without changing the publisher.
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What pub/sub is useful for
Pub/sub separates message production from consumption. Publishers and subscribers can evolve, scale, and operate more independently, and subscribers can process events asynchronously. A single publication can also trigger multiple independent workflows in parallel.
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Topic- or content-based filters can direct messages to interested subscribers rather than delivering every message to every consumer. This pattern is useful when a state change should notify several components, when work does not need to finish before the sender continues, or when a broadcast does not require an immediate reply.
How pub/sub differs from a queue and request-reply
| Pattern | Typical purpose | What the sender should expect |
|---|---|---|
| Pub/sub | Distribute a publication to multiple interested subscribers | Usually asynchronous delivery; publishing alone does not provide a synchronous answer |
| Queue | Hold work until it can be processed, often by a consumer taking responsibility for a message | Typically work-oriented processing rather than fan-out to every interested subscriber |
| Request-reply | Obtain an answer to a request | A response path is part of the design when the sender needs a reply |
These are common distinctions, not rigid rules for every product. Some messaging services combine or blur queue and pub/sub behavior, so check the chosen service’s routing and delivery semantics. If the sender needs an immediate, synchronous response, use a request-reply design rather than treating publication as a call that returns an answer.
What pub/sub does not guarantee by itself
The pattern alone does not ensure that every subscriber is available or receives every message. A message may be transient, delivery may be delayed, and subscribers may miss messages while disconnected. Delayed delivery can also leave parts of a system temporarily out of sync, a condition known as eventual consistency.
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If subscribers must recover missed work, check whether the implementation supports persistence, acknowledgements, retries, replay, and message retention. Delivery guarantees and message time-to-live (TTL) vary. Pub/sub is a poor fit when a workflow requires an immediate, strongly consistent answer from all receivers.
Questions to check before choosing a pub/sub system
- Delivery: What guarantees are offered? How are retries, acknowledgements, and duplicate messages handled?
- Ordering: Is message order guaranteed, and is ordering needed per key or across the entire system?
- Recovery: What persistence, retention, TTL, and replay options are available for disconnected subscribers?
- Routing: What fan-out capacity, filtering, and subscription-management features are provided?
- Security: How are publishers and subscribers authenticated and authorized, and how is message data protected in transit and at rest?
- Operations: Do latency, expected scale, operational effort, and cost fit the workload?
These are implementation-specific comparison points, not features guaranteed by the pub/sub pattern.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.MQTT is one protocol that uses pub/sub
MQTT is a protocol built around publish/subscribe messaging, not another name for the pattern. The OASIS MQTT Version 5.0 Standard, published on 7 March 2019, describes MQTT as a client-server publish/subscribe messaging transport protocol designed for contexts that include constrained environments. It specifies three quality-of-service levels: at most once, at least once, and exactly once. Those delivery levels are MQTT specification details and should not be assumed of other pub/sub systems.
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