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.

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.

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.

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.

Mix Wildcards

The wildcard * can appear multiple times in the same subject. Both types can be used as well. For example, *.*.east.> will receive time.us.east.atlanta.