Client Protocol

Client Protocol

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

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. In fact, refer to the topic NATS Protocol Demo to play with the NATS protocol for yourself using telnet.

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

Protocol conventions

Control Line with Optional Content: Each interaction between the client and server consists of a control, or protocol, line of text followed, optionally by message content. Most of the protocol messages don’t require content, only PUB, MSG, HPUB, and HMSG include payloads.

Field Delimiters: The fields of NATS protocol messages are delimited by whitespace characters `(space) or `(tab). Multiple whitespace characters will be treated as a single field delimiter.

Newlines: NATS uses ␍ followed by ␊ (␍␊, 0x0D0A) to terminate protocol messages. This newline sequence is also used to mark the end of the message payload in PUB, MSG, HPUB, and HMSG protocol messages.

Subject names: Subject names, including reply subject names, are case-sensitive and must be non-empty alphanumeric strings with no embedded whitespace. All UTF-8 characters except spaces/tabs and separators which are . and > are allowed. Subject names can be 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

A subject is comprised of 1 or more tokens. Tokens are separated by . and can be any non whitespace UTF-8 character. The full wildcard token > is only valid as the last token and matches all tokens past that point. A token wildcard, * matches any token in the position it was listed. Wildcard tokens should only be used in a wildcard capacity and not part of a literal token.

Character Encoding: Subject names should be UTF-8 compatible.

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

• The asterisk character (*) matches a single 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 a separate token (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, limited of course by authorization settings.

Protocol messages

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.

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

The following sections explain each protocol message.

INFO

Description

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

Description

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

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, 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

Description

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

Description

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

Description

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

Description

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

Description

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

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.

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

Description

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>␍␊

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. 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': The server hasn’t received a message from the client, including a PONG in too long.
• -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.