# Publish-subscribe Core NATS has one fundamental operation: a client publishes a message to a subject, and every client subscribed to that subject right now gets a copy. This page examines that operation in detail. The concept primer already says *what* publish-subscribe is. This page shows *how* it behaves on the wire: where the copies come from, what happens when nobody is listening, and exactly what core NATS promises about delivery. ## The scenario Acme runs an order platform. Three things happen to an order, and each one is a message on its own subject: ``` orders.created orders.shipped orders.cancelled ``` Every message in this chapter carries the same small JSON payload: ``` { "order_id": "ord_8w2k", "customer": "acme-co", "total_cents": 4200, "ts": "2026-05-22T10:14:22Z" } ``` Three services care about these messages. The `warehouse` service packs the box when an order is created. The `notifications` service emails the customer. The `analytics` service counts everything. None of them knows the others exist. You need one local `nats-server` running for the rest of this chapter: ``` nats-server ``` That's the whole deployment, with no flags, no persistence, and no cluster. Leave it running and add services to it as the chapter grows. ## Publishing a message A **publisher** is a client that sends a message to a subject. The warehouse doesn't subscribe to anything yet, so start by publishing one `orders.created` message: #### CLI ``` #!/bin/bash # Publish one order to the orders.created subject. The publish is # fire-and-forget: nats pub hands the message to the server and exits. # It does not wait for, or report, any subscriber. nats pub orders.created '{"order_id":"ord_8w2k","customer":"acme-co","total_cents":4200,"ts":"2026-05-22T10:14:22Z"}' ``` The publish call returns immediately. It doesn't wait for a subscriber, and it doesn't tell you how many subscribers received the message, or whether any did. This is **fire-and-forget**: the publisher hands the message to the server and moves on. A publisher always names a fully-qualified subject. `orders.created` is a concrete subject, not a pattern. Wildcards belong to subscribers, and we cover them on the [next page](/learn/core-nats/subjects-and-wildcards.md). ## Subscribing to a subject A **subscriber** is a client that registers interest in a subject and receives a copy of every matching message. Start the warehouse service as a subscriber on `orders.created`: #### CLI ``` #!/bin/bash # Subscribe as the warehouse service to orders.created. Each matching # message is printed as it arrives; Ctrl-C to stop. To receive every # order subject at once instead, subscribe to the wildcard orders.> nats sub orders.created ``` Now publish again with the snippet above. The warehouse receives the message. Run a second subscriber for `notifications` and a third for `analytics`, each on `orders.created`, and every one of them receives its own copy of the next publish. A subscriber doesn't consume or remove the message. Subscribing isn't taking from a queue. Each subscriber gets an independent copy, and one subscriber receiving a message takes nothing away from another. If you want one subscriber to see *all* order subjects at once, it can subscribe to `orders.>` instead of a single subject. That `>` is a wildcard, and the [next page](/learn/core-nats/subjects-and-wildcards.md) is where we explain it. ## The interest graph The server tracks who's subscribed to what in an in-memory structure called the **interest graph**. Each subscription adds an entry; each disconnect removes it. **Message flow — Publish / Subscribe (animated):** Animated publish/subscribe: a publisher emits messages; NATS delivers a copy to every matching subscriber. * Publisher → NATS (subject: updates) * NATS → Subscriber 1 (subject: updates) * NATS → Subscriber 2 (subject: updates) When a message arrives, the server looks up the subject in the interest graph, finds the matching subscribers, and sends each one a copy. With three services subscribed to `orders.created`, one publish produces three deliveries. The publisher did nothing different; the fan-out happened entirely on the server. The interest graph is the source of the decoupling. The publisher holds no list of subscribers. It publishes to a subject, and the server resolves interest at the moment the message arrives. This is also why services can come and go freely. Start a fourth subscriber and it joins the graph; the next publish reaches it. Stop one and its entry is gone; the next publish skips it. Nobody coordinates, and the publisher never changes. ## When nobody is listening Publish to `orders.created` while no service is subscribed. The publish still succeeds, and the message is dropped. The server finds no matching entry in the interest graph, so there's nothing to deliver to, and the message is discarded. This is the behavior to internalize: a publish with no interest isn't an error and isn't a stored backlog. It is a silent no-op. The publisher can't tell the difference between "delivered to three subscribers" and "delivered to nobody", because both look like a successful publish. That gap matters for orders. If the warehouse is restarting when an `orders.created` message is published, that message is gone, and no restart brings it back. Remembering messages for absent subscribers is exactly what core NATS does not do. The [JetStream deep dive](/learn/jetstream/your-first-stream.md#why-a-stream) is the layer that adds it. ## At-most-once delivery Core NATS delivers each message **at-most-once**. A subscriber that's connected and interested when the message is published gets it once. A subscriber that's absent, slow, or disconnected at that instant gets it zero times. There's no second attempt. At-most-once is a precise promise, so it's worth stating what it rules out. Core NATS doesn't retry a missed message, doesn't detect or suppress duplicates, and doesn't guarantee that two subscribers see messages in the same order under load. Each of those is a property you add with [JetStream](/learn/jetstream/.md), not something core provides. At-most-once is the right guarantee for a large class of messages: telemetry you sample, cache invalidations, a live dashboard feed. For those, a missed message costs nothing because another is already on the way. For an order that must be packed exactly once, it isn't enough. That's the boundary this chapter respects. ## The 1 MB payload limit A message payload has a maximum size. By default the server caps it at **1 MB** (`max_payload`, `1048576` bytes). The server announces this limit to every client when the connection opens, so the client knows the ceiling before it ever publishes. Exceeding the limit is not a soft failure. If a client publishes a payload larger than `max_payload`, the server rejects it and closes the connection. The Acme order payload is a few hundred bytes, so this never bites here, but a service that tries to ship a large blob inside a message will hit it. The fix is not to use a bigger payload. For large data, publish a reference (an object-store key or a URL) and let the receiver fetch the bytes out of band. Subjects have low overhead, while large messages do not. The wire-level `PUB`/`SUB`/`MSG` protocol is documented in [Reference → Client protocol](/reference/protocols/client.md). We only need the behavior here. ## Try it in two terminals Observe fire-and-forget and at-most-once directly. Open two terminals against the running server. ``` # Terminal 1 — the warehouse subscribes nats sub orders.created ``` ``` # Terminal 2 — publish three orders nats pub orders.created '{"order_id":"ord_8w2k","customer":"acme-co","total_cents":4200,"ts":"2026-05-22T10:14:22Z"}' nats pub orders.created '{"order_id":"ord_2zr9","customer":"globex","total_cents":7800,"ts":"2026-05-22T10:14:25Z"}' nats pub orders.created '{"order_id":"ord_5kq1","customer":"initech","total_cents":1500,"ts":"2026-05-22T10:14:29Z"}' ``` Terminal 1 prints each message the instant it's published. Now stop the subscriber in Terminal 1 with Ctrl-C, publish a fourth message, and restart the subscriber. The fourth message never appears. It was published into an empty interest graph and discarded, which is at-most-once delivery in action. ## Pitfalls A few problems show up the first time you build on publish-subscribe. None of them is a bug; each is a direct consequence of the model this page just described. **Publishing over the limit drops your connection.** A payload larger than `max_payload` isn't truncated or queued: the server rejects it and closes the connection. The Acme order payload is tiny, but a service that tries to ship a large blob inside a message hits this. Don't guess the ceiling: ask the server for it, then keep payloads under it and pass a reference for anything large. #### CLI ``` #!/bin/bash # Ask the server for its limits before sizing a message. The "Maximum # Payload" row is the max_payload ceiling (1 MB by default). Publish # anything larger and the server rejects it and closes the connection, # so size your payloads under this number rather than discovering it the # hard way. nats server info # A safe order publish stays far under the ceiling. nats pub orders.created '{"order_id":"ord_8w2k","customer":"acme-co","total_cents":4200,"ts":"2026-05-22T10:14:22Z"}' ``` **Exiting before the publish leaves the client drops it.** The publish call returns immediately because the client buffers the message and sends it in the background. A short-lived publisher that exits right after the call can quit before that buffered message reaches the server, and the message is gone. Flush (or drain) before you exit so the client waits for the server to acknowledge the buffered publishes: ``` nc.Publish("orders.created", payload) nc.Flush() // wait until the server has the message, then exit nc.Close() ``` **A slow subscriber gets cut off.** A subscriber that can't keep up with the rate of matching messages builds a backlog on the server. Past a threshold the server stops protecting it, logs `Slow Consumer Detected`, and closes its connection. The other subscribers are unaffected. The fix lives in the client: process messages fast enough, or hand them to a worker. [Resilient clients → Slow consumers](/learn/resilient-clients/slow-consumers.md) covers the tuning and recovery. ## Where you are You have one local `nats-server` running, and the Acme order services talking over core publish-subscribe: * A publisher sends `orders.created` messages fire-and-forget. * The `warehouse`, `notifications`, and `analytics` subscribers each receive their own copy. * A message published with no interest is discarded, and delivery is at-most-once. ## What's next Right now every service subscribes to one exact subject. The next page, [Subjects & wildcards](/learn/core-nats/subjects-and-wildcards.md), shows how subjects form a hierarchy and how a subscriber uses `*` and `>` to match many subjects at once, including regional orders like `orders.us.created`. ## See also * [Core Concepts → Publish-subscribe](/concepts/pub-sub-basics.md) — the five-minute overview of this pattern. * [Learn → JetStream → Why a stream](/learn/jetstream/your-first-stream.md#why-a-stream) — the layer that remembers messages core NATS discards. * [Reference → Client protocol](/reference/protocols/client.md) — the wire-level `PUB`/`SUB`/`MSG` details.