Client Protocol

The protocol used to communicate between the NATS server and clients is a simple, text-based protocol. The default transport is a TCP/IP socket, however WebSockets are supported as well as a UNIX domain socket if the server is embedded within a Go process.

Overview

The following table briefly describes the NATS protocol messages. NATS protocol operation names are case insensitive, thus SUB foo 1␍␊ and sub foo 1␍␊ are equivalent.

The following sections explain each protocol message.

INFO

A client will need to start as a plain TCP connection, then when the server accepts a connection from the client, it will send information about itself, the configuration and security requirements necessary for the client to successfully authenticate with the server and exchange messages.

When using the updated client protocol (see CONNECT below), INFO messages can be sent anytime by the server. This means clients with that protocol level need to be able to asynchronously handle INFO messages.

Syntax

INFO {"option_name":option_value,...}␍␊

The valid options are as follows, encoded as JSON:

connect_urls

The connect_urls field is a list of urls the server may send when a client first connects, and when there are changes to server cluster topology. This field is considered optional, and may be omitted based on server configuration and client protocol level.

When a NATS server cluster expands, an INFO message is sent to the client with an updated connect_urls list. This cloud-friendly feature asynchronously notifies a client of known servers, allowing it to connect to servers not originally configured.

The connect_urls will contain a list of strings with an IP and port, looking like this: "connect_urls":["10.0.0.184:4333","192.168.129.1:4333","192.168.192.1:4333"]

Example

Below you can see a sample connection string from a telnet connection to the demo.nats.io site.

telnet demo.nats.io 4222

Trying 107.170.221.32...
Connected to demo.nats.io.
Escape character is '^]'.
INFO {"server_id":"Zk0GQ3JBSrg3oyxCRRlE09","version":"1.2.0","proto":1,"go":"go1.10.3","host":"0.0.0.0","port":4222,"max_payload":1048576,"client_id":2392}

CONNECT

The CONNECT message is the client version of the INFO message. Once the client has established a TCP/IP socket connection with the NATS server, and an INFO message has been received from the server, the client may send a CONNECT message to the NATS server to provide more information about the current connection as well as security information.

Syntax

CONNECT {"option_name":option_value,...}␍␊

The valid options are as follows, encoded as JSON:

Example

Here is an example from the default string of the Go client:

CONNECT {"verbose":false,"pedantic":false,"tls_required":false,"name":"","lang":"go","version":"1.2.2","protocol":1}␍␊

Most clients set verbose to false by default. This means that the server should not confirm each message it receives on this connection with a +OK back to the client.

PUB

The PUB message publishes the message payload to the given subject name, optionally supplying a reply subject. If a reply subject is supplied, it will be delivered to eligible subscribers along with the supplied payload. Note that the payload itself is optional. To omit the payload, set the payload size to 0, but the second CRLF is still required.

Syntax

PUB <subject> [reply-to] <#bytes>␍␊[payload]␍␊

where:

Example

To publish the ASCII string message payload "Hello NATS!" to subject FOO:

PUB FOO 11␍␊Hello NATS!␍␊

To publish a request message "Knock Knock" to subject FRONT.DOOR with reply subject JOKE.22:

PUB FRONT.DOOR JOKE.22 11␍␊Knock Knock␍␊

To publish an empty message to subject NOTIFY:

PUB NOTIFY 0␍␊␍␊

HPUB

The HPUB message is the same as PUB but extends the message payload to include NATS headers. Note that the payload itself is optional. To omit the payload, set the total message size equal to the size of the headers. Note that the trailing CR+LF is still required.

NATS headers are similar, in structure and semantics, to HTTP headers as name: value pairs including supporting multi-value headers. Headers can be mixed case and NATS will preserve case between message publisher and message receiver(s). See also ADR-4 NATS Message Headers.

Syntax

HPUB <subject> [reply-to] <#header bytes> <#total bytes>␍␊[headers]␍␊␍␊[payload]␍␊

where:

Example

To publish the ASCII string message payload "Hello NATS!" to subject FOO with one header Bar with value Baz:

HPUB FOO 22 33␍␊NATS/1.0␍␊Bar: Baz␍␊␍␊Hello NATS!␍␊

To publish a request message "Knock Knock" to subject FRONT.DOOR with reply subject JOKE.22 and two headers:

HPUB FRONT.DOOR JOKE.22 45 56␍␊NATS/1.0␍␊BREAKFAST: donut␍␊LUNCH: burger␍␊␍␊Knock Knock␍␊

To publish an empty message to subject NOTIFY with one header Bar with value Baz:

HPUB NOTIFY 22 22␍␊NATS/1.0␍␊Bar: Baz␍␊␍␊␍␊

To publish a message to subject MORNING MENU with one header BREAKFAST having two values and payload "Yum!"

HPUB MORNING.MENU 47 51␍␊NATS/1.0␍␊BREAKFAST: donut␍␊BREAKFAST: eggs␍␊␍␊Yum!␍␊

SUB

SUB initiates a subscription to a subject, optionally joining a distributed queue group.

Syntax

SUB <subject> [queue group] <sid>␍␊

where:

Example

To subscribe to the subject FOO with the connection-unique subscription identifier (sid) 1:

SUB FOO 1␍␊

To subscribe the current connection to the subject BAR as part of distribution queue group G1 with sid 44:

SUB BAR G1 44␍␊

UNSUB

UNSUB unsubscribes the connection from the specified subject, or auto-unsubscribes after the specified number of messages has been received.

Syntax

UNSUB <sid> [max_msgs]␍␊

where:

Example

The following examples concern subject FOO which has been assigned sid 1. To unsubscribe from FOO:

UNSUB 1␍␊

To auto-unsubscribe from FOO after 5 messages have been received:

UNSUB 1 5␍␊

MSG

The MSG protocol message is used to deliver an application message to the client.

Syntax

MSG <subject> <sid> [reply-to] <#bytes>␍␊[payload]␍␊

where:

Example

The following message delivers an application message from subject FOO.BAR:

MSG FOO.BAR 9 11␍␊Hello World␍␊

To deliver the same message along with a reply subject:

MSG FOO.BAR 9 GREETING.34 11␍␊Hello World␍␊

HMSG

The HMSG message is the same as MSG, but extends the message payload with headers. See also ADR-4 NATS Message Headers.

Syntax

HMSG <subject> <sid> [reply-to] <#header bytes> <#total bytes>␍␊[headers]␍␊␍␊[payload]␍␊

where:

Example

The following message delivers an application message from subject FOO.BAR with a header:

HMSG FOO.BAR 34 45␍␊NATS/1.0␍␊FoodGroup: vegetable␍␊␍␊Hello World␍␊

To deliver the same message along with a reply subject:

HMSG FOO.BAR 9 BAZ.69 34 45␍␊NATS/1.0␍␊FoodGroup: vegetable␍␊␍␊Hello World␍␊

PING/PONG

PING and PONG implement a simple keep-alive mechanism between client and server. Once a client establishes a connection to the NATS server, the server will continuously send PING messages to the client at a configurable interval. If the client fails to respond with a PONG message within the configured response interval, the server will terminate its connection. If your connection stays idle for too long, it is cut off.

If the server sends a ping request, you can reply with a pong message to notify the server that you are still interested. You can also ping the server and will receive a pong reply. The ping/pong interval is configurable.

The server uses normal traffic as a ping/pong proxy, so a client that has messages flowing may not receive a ping from the server.

Syntax

PING␍␊

PONG␍␊

Example

The following example shows the demo server pinging the client and finally shutting it down.

telnet demo.nats.io 4222

Trying 107.170.221.32...
Connected to demo.nats.io.
Escape character is '^]'.
INFO {"server_id":"Zk0GQ3JBSrg3oyxCRRlE09","version":"1.2.0","proto":1,"go":"go1.10.3","host":"0.0.0.0","port":4222,"max_payload":1048576,"client_id":2392}
PING
PING
-ERR 'Stale Connection'
Connection closed by foreign host.

+OK/ERR

When the verbose connection option is set to true (the default value), the server acknowledges each well-formed protocol message from the client with a +OK message. Most NATS clients set the verbose option to false using the CONNECT message

The -ERR message is used by the server indicate a protocol, authorization, or other runtime connection error to the client. Most of these errors result in the server closing the connection.

Handling of these errors usually has to be done asynchronously.

Syntax

+OK␍␊

-ERR <error message>␍␊

Errors

The table below lists the possible errors that can occur. Some protocol errors result in the server closing the connection. Upon receiving these errors, the connection is no longer valid and the client should clean up relevant resources. This is denoted by the Recoverable column.