While the NATS server has many flags that allow for simple testing of features, the NATS server products provide a flexible configuration format that combines the best of traditional formats and newer styles such as JSON and YAML.

The NATS configuration file supports the following syntax:

• Lines can be commented with # and //

• Values can be assigned to properties with:

  • Equals sign: foo = 2

  • Colon: foo: 2

  • Whitespace: foo 2

• Arrays are enclosed in brackets: ["a", "b", "c"]

• Maps are enclosed in braces: {foo: 2}

• Maps can be assigned with no key separator

• Semicolons can be used as terminators

Strings and Numbers

The configuration parser is very forgiving, as you have seen:

• values can be a primitive, or a list, or a map

• strings and numbers typically do the right thing

String values that start with a digit can create issues. To force such values as strings, quote them.

BAD Config:

listen: 127.0.0.1:4222
authorization: {
    # BAD!
    token: 3secret
}

Fixed Config:

listen: 127.0.0.1:4222
authorization: {
    token: "3secret"
}

Variables

Server configurations can specify variables. Variables allow you to reference a value from one or more sections in the configuration.

Variables:

• Are block-scoped

• Are referenced with a $ prefix.

• Can be resolved from environment variables having the same name

If the environment variable value begins with a number you may have trouble resolving it depending on the server version you are running.

# Define a variable in the config
TOKEN: "secret"
​
# Reference the variable
authorization {
    token: $TOKEN
}

A similar configuration, but this time, the value is in the environment:

# TOKEN is defined in the environment
authorization {
    token: $TOKEN
}

export TOKEN="hello"; nats-server -c /config/file

Include Directive

The include directive allows you to split a server configuration into several files. This is useful for separating configuration into chunks that you can easily reuse between different servers.

Includes must use relative paths, and are relative to the main configuration (the one specified via the -c option):

server.conf:

listen: 127.0.0.1:4222
include ./auth.conf

Note that include is not followed by = or :, as it is a directive.

auth.conf:

authorization: {
    token: "f0oBar"
}

> nats-server -c server.conf

Configuration Properties

Configuration Reloading

A server can reload most configuration changes without requiring a server restart or clients to disconnect by sending the nats-server a signal:

> nats-server --signal reload