3.1 KiB
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 used to find each other.
digraph g {
rankdir=LR
publisher [shape=box, style="rounded", label="PUB time.us"];
subject [shape=circle, label="gnatsd"];
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 can not 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.us.east.warsaw
to logically group related subjects.
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, label="gnatsd"];
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, label="gnatsd"];
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"];
}