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Subject-based Messaging

Fundamentally NATS is about publishing and listening for messages. Both of these depend heavily on Subjects which scope messages into streams or topics. At its simplest, a subject is just a string of characters that form a name the publisher and subscriber can use to find each other.


digraph g {
  rankdir=LR
  publisher [shape=box, style="rounded", label="PUB time.us"];
  subject [shape=circle, fixedsize="true", width="1.0", height="1.0", label="nats-server"];
  sub1 [shape=box, style="rounded", label="SUB time.us"];
  sub2 [shape=box, style="rounded", label="SUB time.us"];

  publisher -> subject [label="msg"];
  subject -> sub1 [label="msg"];
  subject -> sub2 [label="msg"];
}

The NATS server reserves a few characters as special, and the specification says that only "alpha-numeric" characters plus the "." should be used in subject names. Subjects are case-sensitive and cannot contain whitespace. For safety across clients, ASCII characters should be used, although this is subject to change in the future.

Subject Hierarchies

The . character is used to create a subject hierarchy. For example, a world clock application might define the following to logically group related subjects:

time.us
time.us.east
time.us.east.atlanta
time.eu.east
time.eu.warsaw

Wildcards

NATS provides two wildcards that can take the place of one or more elements in a dot-separated subject. Subscribers can use these wildcards to listen to multiple subjects with a single subscription but Publishers will always use a fully specified subject, without the wildcard.

Matching A Single Token

The first wildcard is * which will match a single token. For example, if an application wanted to listen for eastern time zones, they could subscribe to time.*.east, which would match time.us.east and time.eu.east.


digraph g {
  rankdir=LR
  publisher [shape=box, style="rounded", label="PUB time.us.east"];
  subject [shape=circle, fixedsize="true", width="1.0", height="1.0", label="nats-server"];
  sub1 [shape=box, style="rounded", label="SUB time.*.east"];
  sub2 [shape=box, style="rounded", label="SUB time.us.east"];

  publisher -> subject [label="msg"];
  subject -> sub1 [label="msg"];
  subject -> sub2 [label="msg"];
}

Matching Multiple Tokens

The second wildcard is > which will match one or more tokens, and can only appear at the end of the subject. For example, time.us.> will match time.us.east and time.us.east.atlanta, while time.us.* would only match time.us.east since it can't match more than one token.


digraph g {
  rankdir=LR
  publisher [shape=box, style="rounded", label="PUB time.us.east.atlanta"];
  subject [shape=circle, fixedsize="true", width="1.0", height="1.0", label="nats-server"];
  sub1 [shape=box, style="rounded", label="SUB time.us.east.atlanta"];
  sub2 [shape=box, style="rounded", label="SUB time.us.*"];
  sub3 [shape=box, style="rounded", label="SUB time.us.>"];

  publisher -> subject [label="msg"];
  subject -> sub2 [style="invis"];
  subject -> sub1 [label="msg"];
  subject -> sub3 [label="msg"];
}

Monitoring and Wire Taps

Subject to your security configuration, wildcards can be used for monitoring by creating something sometimes called a wire tap. In the simplest case you can create a subscriber for >. This application will receive all messages -- again, subject to security settings -- sent on your NATS cluster.

Subject-Based Messaging