Configuring NATS Server

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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
The NATS configuration file is parsed with UTF-8 encoding.
Note
The NATS configuration in the file can also be rendered as a JSON object (with comments!), but to combine it with variables the variables still have to be unquoted.
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
numbers support units such as, 1K for 1000, 1KB for 1024
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. They have to be unquoted when being referenced, for example an assigment like foo = "$example" will result in foo being the literal string "$example".
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
Connectivity
Property
Description
Default
host
Host for client connections.
0.0.0.0
port
Port for client connections.
4222
listen
Listen specification <host>:<port> for client connections. Either use this or the options host and/or port.
same as host, port
client_advertise
Alternative client listen specification <host>:<port> or just <host> to advertise to clients and other server. Useful in cluster setups with NAT.
Advertise what host and port specify.
​tls​
Configuration map for tls for client and http monitoring.
​
​cluster​
Configuration map for cluster.
​
​gateway​
Configuration map for gateway.
​
​leafnode​
Configuration map for a leafnode.
​
​mqtt​
Configuration map for mqtt.
​
​websocket​
Configuration map for websocket.
​
Connection Timeouts
Property
Description
Default
ping_interval
Duration at which pings are sent to clients, leaf nodes and routes. In the presence of client traffic, such as messages or client side pings, the server will not send pings. Therefore it is recommended to keep this value bigger than what clients use.
"2m"
ping_max
After how many unanswered pings the server will allow before closing the connection.
2
write_deadline
Maximum number of seconds the server will block when writing. Once this threshold is exceeded the connection will be closed. See slow consumer on how to deal with this on the client.
"10s"
Limits
Property
Description
Default
max_connections
Maximum number of active client connections.
64K
max_control_line
Maximum length of a protocol line (including combined length of subject and queue group). Increasing this value may require client changes to be used. Applies to all traffic.
4KB
max_payload
Maximum number of bytes in a message payload. Reducing this size may force you to implement chunking in your clients. Applies to client and leafnode payloads. It is not recommended to use values over 8MB but max_payload can be set up to 64MB. The max payload must be equal or smaller to the max_pending value.
1MB
max_pending
Maximum number of bytes buffered for a connection Applies to client connections. Note that applications can also set 'PendingLimits' (number of messages and total size) for their subscriptions.
64MB
max_subscriptions
Maximum numbers of subscriptions per client and leafnode accounts connection.
0, unlimited
JetStream
You can enable JetStream in the server's configuration by simply adding a jetstream {} map. By default, the JetStream subsystem will store data in the /tmp directory, but you can specify the directory to use via the store_dir, as well as the limits for JetStream storage (a value of 0 means no limit).
Property
Description
Default
store_dir
Directory to use for JetStream storage.
/tmp/nats/jetstream
max_memory_store
Maximum size of the 'memory' storage
75% of available memory
max_file_store
Maximum size of the 'file' storage
Up to 1TB if available
Here's an example minimal file that will store data in a local "nats" directory with some limits.
$ nats-server -c js.conf
# js.conf
jetstream {
   store_dir=nats
   
   // 1GB
   max_memory_store: 1073741824
​
   // 10GB
   max_file_store: 10737418240
}
Normally JetStream will be run in clustered mode and will replicate data, so the best place to store JetStream data would be locally on a fast SSD. One should specifically avoid NAS or NFS storage for JetStream. Note that each JetStream enabled nats-server should use its own individual storage directory.
Authentication and Authorization
Centralized Authentication and Authorization
Property
Description
​authorization​
Configuration map for client authentication/authorization.
​accounts​
Configuration map for multi tenancy via accounts.
​no_auth_user​
​Username present in the authorization block or an account. A client connecting without any form of authentication will be associated with this user, its permissions and account.
Decentralized Authentication and Authorization
The Configuration options here refer to JWT based authentication and authorization.
Property
Description
​operator​
Path to an operator JWT.
​resolver​
The built-in NATS resolver, MEMORY for static or URL(<url>) to use an external account server. (When the operator JWT contains an account URL, it will be used as default. In this case resolver is only needed to overwrite the default.)
​resolver_tls​
​tls configuration map for tls connections to the resolver. (This is for an outgoing connection and therefore does not use timeout, verify and map_and_verify)
​resolver_preload​
​Map to preload account public keys and their corresponding JWT. Keys consist of <account public nkey>, value is the <corresponding jwt>.
Runtime Configuration
Property
Description
Default
disable_sublist_cache
If true disable subscription caches for all accounts. This is saves resources in situations where different subjects are used all the time.
false, cache enabled
lame_duck_duration
In lame duck mode the server rejects new clients and slowly closes client connections. After this duration is over the server shuts down. This value cannot be set lower than 30 seconds. Start lame duck mode with: nats-server --signal ldm.
"2m"
lame_duck_grace_period
This is the duration the server waits, after entering lame duck mode, before starting to close client connections
"10s"
Monitoring and Tracing
Property
Description
Default
server_name
The servers name, shows up in logging. Defaults to the server's id. When JetStream is used, withing a domain, all server names need to be unique.
Generated Server ID
trace
If true enable protocol trace log messages. Excludes the system account.
false, disabled
trace_verbose
If true enable protocol trace log messages. Includes the system account.
false, disabled
debug
If true enable debug log messages
false, disabled
logtime
If set to false, log without timestamps
true, include timestamp
log_file
Log file name, relative to...
No log file
​log_size_limit​
Size in bytes after the log file rolls over to a new one
0, unlimited
max_traced_msg_len
Set a limit to the trace of the payload of a message.
0, unlimited
syslog
Log to syslog.
false, disabled
remote_syslog
​Syslog server address.
​
​http_port​
http port for server monitoring.
​
​http​
Listen specification <host>:<port>for server monitoring.
​
​https_port​
https port for server monitoring. This is influenced by the tls property.
​
​http_base_path​
base path for monitoring endpoints.
/
​https​
Listen specification <host>:<port>for TLS server monitoring.
​
system_account
Name of the system account. Users of this account can subscribe to system events. See System Accounts for more details.
​
pid_file
File containing PID, relative to ... This can serve as input to nats-server --signal​
​
port_file_dir
Directory to write a file containing the servers open ports to, relative to ...
​
connect_error_reports
Number of attempts at which a repeated failed route, gateway or leaf node connection is reported. Connect attempts are made once every second.
3600, approx every hour
reconnect_error_reports
Number of failed attempt to reconnect a route, gateway or leaf node connection. Default is to report every attempt.
1, every failed attempt
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