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NATS protocol

The NATS wire protocol is a simple, text-based publish/subscribe style protocol. Clients connect to and communicate with gnatsd (the NATS server) through a regular TCP/IP socket using a small set of protocol operations that are terminated by newline.

Unlike traditional messaging systems that use a binary message format that require an API to consume, the text-based NATS protocol makes it easy to implement clients in a wide variety of programming and scripting languages.

The NATS server implements a zero allocation byte parser that is fast and efficient.

NATS protocol conventions

Subject names: Subject names, including reply subject (INBOX) names, are case-sensitive and must be non-empty alphanumeric strings with no embedded whitespace, and optionally token-delimited using the dot character (.), e.g.:

FOO, BAR, foo.bar, foo.BAR, FOO.BAR and FOO.BAR.BAZ are all valid subject names

FOO. BAR, foo. .bar andfoo..bar are not valid subject names

Wildcards: NATS supports the use of wildcards in subject subscriptions.

  • The asterisk character (*) matches any token at any level of the subject.
  • The greater than symbol (>), also known as the full wildcard, matches one or more tokens at the tail of a subject, and must be the last token. The wildcarded subject foo.> will match foo.bar or foo.bar.baz.1, but not foo.
  • Wildcards must be separate tokens (foo.*.baz or foo.> are syntactically valid; foo*.bar, f*o.b*r and foo> are not)

For example, the wildcard subscriptions foo.*.quux and foo.> both match foo.bar.quux, but only the latter matches foo.bar.baz. With the full wildcard, it is also possible to express interest in every subject that may exist in NATS: sub > 1.

Field Delimiters: The fields of NATS protocol messages are delimited by whitespace characters ‘’ (space) or \t (tab). Multiple whitespace characters will be treated as a single field delimiter.

Newlines: Like other text-based protocols, NATS uses CR followed by LF (CR+LF, \r\n, 0x0D0A) to terminate protocol messages. This newline sequence is also used to mark the beginning of the actual message payload in a PUB or MSG protocol message.

NATS protocol messages

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

Click the name to see more detailed information, including syntax:

OP Name Sent By Description
INFO Server Sent to client after initial TCP/IP connection
CONNECT Client Sent to server to specify connection information
PUB Client Publish a message to a subject, with optional reply subject
SUB Client Subscribe to a subject (or subject wildcard)
UNSUB Client Unsubscribe (or auto-unsubscribe) from subject
MSG Server Delivers a message payload to a subscriber
PING Both PING keep-alive message
PONG Both PONG keep-alive response
+OK Server Acknowledges well-formed protocol message in verbose mode
-ERR Server Indicates a protocol error. May cause client disconnect.

The following sections explain each protocol message.

INFO

Syntax

INFO {["option_name":option_value],...}

The valid options are as follows:

  • server_id: The unique identifier of the NATS server
  • version: The version of the NATS server
  • go: The version of golang the NATS server was built with
  • host: The IP address of the NATS server host
  • port: The port number the NATS server is configured to listen on
  • auth_required: If this is set, then the client should try to authenticate upon connect.
  • ssl_required: If this is set, then the client must authenticate using SSL.
  • max_payload: Maximum payload size that the server will accept from the client.
  • connect_urls : An optional list of server urls that a client can connect to.

Description

As soon as the server accepts a connection from the client, it will send information about itself and the configuration and security requirements that are 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.

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":"1ec445b504f4edfb4cf7927c707dd717","version":"0.6.6","go":"go1.4.2","host":"0.0.0.0","port":4222,"auth_required":false,"ssl_required":false,"max_payload":1048576}

CONNECT

Syntax

CONNECT {["option_name":option_value],...}

The valid options are as follows:

  • verbose: Turns on +OK protocol acknowledgements.
  • pedantic: Turns on additional strict format checking, e.g. for properly formed subjects
  • ssl_required: Indicates whether the client requires an SSL connection.
  • auth_token: Client authorization token
  • user: Connection username (if auth_required is set)
  • pass: Connection password (if auth_required is set)
  • name: Optional client name
  • lang: The implementation language of the client.
  • version: The version of the client.
  • protocol: optional int. Sending 0 (or absent) indicates client supports original protocol. Sending 1 indicates that the client supports dynamic reconfiguration of cluster topology changes by asynchronously receiving INFO messages with known servers it can reconnect to.

Description

The CONNECT message is analogous to 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.

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}]\r\n

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

Syntax

PUB <subject> [reply-to] <#bytes>\r\n[payload]\r\n

where:

  • subject: The destination subject to publish to
  • reply-to: The reply inbox subject that subscribers can use to send a response back to the publisher/requestor
  • #bytes: The payload size in bytes
  • payload: The message payload data

Description

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.

Example

To publish the string message payload “Hello NATS!” to subject FOO:

PUB FOO 11\r\nHello NATS!\r\n

To publish a request message “Knock Knock” to subject FRONT.DOOR with reply subject INBOX.22:

PUB FRONT.DOOR INBOX.22 11\r\nKnock Knock\r\n

To publish an empty message to subject NOTIFY:

PUB NOTIFY 0\r\n\r\n

SUB

Syntax

SUB <subject> [queue group] <sid>\r\n

where:

  • subject: The subject name to subscribe to
  • queue group: If specified, the subscriber will join this queue group
  • sid: A unique alphanumeric subscription ID

Description

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

Example

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

SUB FOO 1\r\n

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

SUB BAR G1 44\r\n

UNSUB

Syntax

UNSUB <sid> [max_msgs]

where:

  • sid: The unique alphanumeric subscription ID of the subject to unsubscribe from
  • max_msgs: Number of messages to wait for before automatically unsubscribing

Description

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

Example

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

UNSUB 1\r\n

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

UNSUB 1 5\r\n

MSG

Syntax

MSG <subject> <sid> [reply-to] <#bytes>\r\n[payload]\r\n

where:

  • subject: Subject name this message was received on
  • sid: The unique alphanumeric subscription ID of the subject
  • reply-to: The inbox subject on which the publisher is listening for responses
  • #bytes: Size of the payload in bytes
  • payload: The message payload data

Description

The MSG protocol message delivers a message to the client.

Example

The following message delivers a message from subject FOO.BAR:

MSG FOO.BAR 9 11\r\nHello World\r\n

Deliver the same message along with a reply inbox:

MSG FOO.BAR 9 INBOX.34 11\r\nHello World\r\n

PING/PONG

Description

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:

telnet demo.nats.io 4222

Trying 107.170.221.32...
Connected to demo.nats.io.
Escape character is '^]'.
INFO {"server_id":"ad29ea9cbb16f2865c177bbd4db446ca","version":"0.6.8","go":"go1.5.1","host":"0.0.0.0","port":4222,"auth_required":false,"ssl_required":false,"max_payload":1048576}
PING
PING
-ERR 'Stale Connection'
Connection closed by foreign host.

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.

+OK/ERR

Syntax

+OK

-ERR <error message>

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.

Some protocol errors result in the server closing the connection. Upon recieving these errors, the connection is no longer valid and the client should clean up relevant resources. These errors include:

  • -ERR 'Unknown Protocol Operation': Unknown protocol error
  • -ERR 'Attempted To Connect To Route Port': Client attempted to connect to a route port instead of the client port
  • -ERR 'Authorization Violation': Client failed to authenticate to the server with credentials specified in the CONNECT message
  • -ERR 'Authorization Timeout': Client took too long to authenticate to the server after establishing a connection (default 1 second)
  • -ERR 'Invalid Client Protocol': Client specified an invalid protocol version in the CONNECT message
  • -ERR 'Maximum Control Line Exceeded': Message destination subject and reply subject length exceeded the maximum control line value specified by the max_control_line server option. The default is 1024 bytes.
  • -ERR 'Parser Error': Cannot parse the protocol message sent by the client
  • -ERR 'Secure Connection - TLS Required': The server requires TLS and the client does not have TLS enabled.
  • -ERR 'Stale Connection': PING/PONG interval expired.
  • -ERR 'Maximum Connections Exceeded‘: This error is sent by the server when creating a new connection and the server has exceeded the maximum number of connections specified by the max_connections server option. The default is 64k.
  • -ERR 'Slow Consumer': The server pending data size for the connection has reached the maximum size (default 10MB).
  • -ERR 'Maximum Payload Violation': Client attempted to publish a message with a payload size that exceeds the max_payload size configured on the server. This value is supplied to the client upon connection in the initial INFO message. The client is expected to do proper accounting of byte size to be sent to the server in order to handle this error synchronously.

Protocol error messages where the connection remains open are listed below. The client should not close the connection in these cases.

  • -ERR 'Invalid Subject': Client sent a malformed subject (e.g. sub foo. 90)
  • -ERR 'Permissions Violation for Subscription to <subject>': The user specified in the CONNECT message does not have permission to subscribe to the subject.
  • -ERR 'Permissions Violation for Publish to <subject>': The user specified in the CONNECT message does not have permissions to publish to the subject.

Protocol demo

Refer to the topic NATS Protocol Demo to demo the NATS protocol for yourself.