# Publish Subscribe

NATS implements a publish subscribe message distribution model as a one-to-many communication. A publisher sends a message on a subject and any active subscriber listening on that subject receives the message. Subscribers can also register interest in wildcard subjects that work a bit like a regular expression (but only a bit). NATS and NATS Streaming combine to offer two qualities of service:

- **At Most Once Delivery (NATS w/TCP reliability)** - In the basic NATS platform, if a subscriber is not listening on the subject (no subject match), or is not active when the message is sent, the message is not received. NATS is a fire-and-forget messaging system. If you need higher levels of service, you can either use NATS Streaming, or build the additional reliability into your client(s) yourself.

- **At Least Once Delivery (NATS Streaming)** -  Some applications require higher levels of service and more stringent delivery guarantees but at the potential cost of lower message throughput and higher end-to-end delivery latency. These applications rely on the underlying messaging transport to ensure that messages are delivered to subscribers irrespective of network outages or whether or not a subscriber is offline at a particular point in time.

<div class="graphviz"><code data-viz="dot">
digraph nats_pub_sub {
  rankdir=LR
  publisher [shape=box, style="rounded", label="Publisher"];
  subject [shape=circle, label="Subject"];
  sub1 [shape=box, style="rounded", label="Subscriber"];
  sub2 [shape=box, style="rounded", label="Subscriber"];
  sub3 [shape=box, style="rounded", label="Subscriber"];

  publisher -> subject [label="msg1"];
  subject -> sub1 [label="msg1"];
  subject -> sub2 [label="msg1"];
  subject -> sub3 [label="msg1"];
}
</code></div>

Try NATS publish subscribe on your own, using a live server by walking through the [pub-sub tutorial](../tutorials/pubsub.md).