The Streaming Protocol

You can find a list of all supported client libraries here. There are also links to community contributed clients.

In the event you would want to write your own NATS Streaming library, you could have a look at existing libraries to understand the flow. But you need to use Google Protocol Buffers to exchange protocols between the client and the server.

NATS Streaming Protocol

The NATS streaming protocol sits atop the core NATS protocol and uses Google's Protocol Buffers. Protocol buffer messages are marshaled into bytes and published as NATS messages on specific subjects described below. In communicating with the NATS Streaming Server, the NATS request-reply pattern is used for all protocol messages that have a corresponding reply.

NATS streaming 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 streaming does not support wildcards in subject subscriptions

Protocol definition: The fields of NATS streaming protocol messages are defined in the NATS streaming client protocol file.

NATS streaming protocol messages

The following table briefly describes the NATS streaming protocol messages.

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

Protocols

Message Name	Sent By	Description
ConnectRequest	Client	Request to connect to the NATS Streaming Server
ConnectResponse	Server	Result of a connection request
SubscriptionRequest	Client	Request sent to subscribe and retrieve data
SubscriptionResponse	Server	Result of a subscription request
UnsubscribeRequest	Client	Unsubscribe from a subject
PubMsg	Client	Publish a message to a subject
PubAck	Server	An acknowledgement that a published message has been processed on the server
MsgProto	Server	A message from the NATS Streaming Server to a subscribing client
Ack	Client	Acknowledges that a message has been received
Ping	Client	Ping sent to server to detect connection loss
PingResponse	Server	Result of a Ping
CloseRequest	Client	Request sent to close the connection to the NATS Streaming Server
CloseResponse	Server	Result of the close request

The following sections explain each protocol message.

ConnectRequest

Description

A connection request is sent when a streaming client connects to the NATS Streaming Server. The connection request contains a unique identifier representing the client, and an inbox subject the client will listen on for incoming heartbeats. The identifier must be unique; a connection attempt with an identifier currently in use will fail. The inbox subject is the subject where the client receives incoming heartbeats, and responds by publishing an empty NATS message to the reply subject, indicating it is alive. The NATS Streaming Server will return a ConnectResponse message to the reply subject specified in the NATS request message.

More advanced libraries can set the protocol to 1 and send a connection ID which in combination with ping interval and ping max out allows the library to detect that the connection to the server is lost.

This request is published to a subject comprised of the <discover-prefix>.cluster-id. For example, if a NATS Streaming Server was started with a cluster-id of mycluster, and the default prefix was used, the client publishes to _STAN.discover.mycluster

Message Structure

• clientID: A unique identifier for a client

• heartbeatInbox: An inbox to which the NATS Streaming Server will send heartbeats for the client to process

• protocol: Protocol the client is at

• connID: Connection ID, a way to uniquely identify a connection (no connection should ever have the same)

• pingInterval: Interval at which client wishes to send PINGs (expressed in seconds)

• pingMaxOut: Maximum number of PINGs without a response after which the connection can be considered lost

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ConnectResponse

Description

After a ConnectRequest is published, the NATS Streaming Server responds with this message on the reply subject of the underlying NATS request. The NATS Streaming Server requires the client to make requests and publish messages on certain subjects (described above), and when a connection is successful, the client saves the information returned to be used in sending other NATS streaming protocol messages. In the event the connection was not successful, an error is returned in the error field.

Message Structure

• pubPrefix: Prefix to use when publishing

• subRequests: Subject used for subscription requests

• unsubRequests: Subject used for unsubscribe requests

• closeRequests: Subject for closing a connection

• error: An error string, which will be empty/omitted upon success

• subCloseRequests: Subject to use for subscription close requests

• pingRequests: Subject to use for PING requests

• pingInterval: Interval at which client should send PINGs (expressed in seconds).

• pingMaxOut: Maximum number of PINGs without a response after which the connection can be considered lost

• protocol: Protocol version the server is at

• publicKey: Reserved for future use

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SubscriptionRequest

Description

A SubscriptionRequest is published on the subject returned in the subRequests field of a ConnectResponse, and creates a subscription to a subject on the NATS Streaming Server. This will return a SubscriptionResponse message to the reply subject specified in the NATS protocol request message.

Message Structure

• clientID: Client ID originally provided in the ConnectRequest

• subject: Formal subject to subscribe to, e.g. foo.bar

• qGroup: Optional queue group

• inbox: Inbox subject to deliver messages on

• maxInFlight: Maximum inflight messages without an acknowledgement allowed

• ackWaitInSecs: Timeout for receiving an acknowledgement from the client

• durableName: Optional durable name which survives client restarts

• startPosition: An enumerated type specifying the point in history to start replaying data

• startSequence: Optional start sequence number

• startTimeDelta: Optional start time

StartPosition enumeration

• NewOnly: Send only new messages

• LastReceived: Send only the last received message

• TimeDeltaStart: Send messages from duration specified in the startTimeDelta field.

• SequenceStart: Send messages starting from the sequence in the startSequence field.

• First: Send all available messages

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SubscriptionResponse

Description

The SubscriptionResponse message is the response from the SubscriptionRequest. After a client has processed an incoming MsgProto message, it must send an acknowledgement to the ackInbox subject provided here.

Message Structure

• ackInbox: subject the client sends message acknowledgements to the NATS Streaming Server

• error: error string, empty/omitted if no error

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UnsubscribeRequest

Description

The UnsubscribeRequest closes or unsubcribes the subscription from the specified subject. The inbox specified is the inbox returned from the NATS Streaming Server in the SubscriptionResponse. Depending on which subject this request is sent, the action will result in close (if sent to subject subCloseRequests) or unsubscribe (if sent to subject unsubRequests)

Message Structure

• clientID: Client ID originally provided in the ConnectRequest

• subject: Subject for the subscription

• inbox: Inbox subject to identify subscription

• durableName: Optional durable name which survives client restarts

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PubMsg

Description

The PubMsg protocol message is published from a client to the NATS Streaming Server. The GUID must be unique, and is returned in the PubAck message to correlate the success or failure of storing this particular message.

Message Structure

• clientID: Client ID originally provided in the ConnectRequest

• guid: a guid generated for this particular message

• subject: subject

• data: payload

• connID: Connection ID. For servers that know about this field, clientID can be omitted

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PubAck

Description

The PubAck message is an acknowledgement from the NATS Streaming Server that a message has been processed. The message arrives on the subject specified on the reply subject of the NATS message the PubMsg was published on. The GUID is the same GUID used in the PubMsg being acknowledged. If an error string is present, the message was not persisted by the NATS Streaming Server and no guarantees regarding persistence are honored. PubAck messages may be handled asynchronously from their corresponding PubMsg in the client.

Message Structure

• guid: GUID of the message being acknowledged by the NATS Streaming Server

• error: An error string, empty/omitted if no error

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MsgProto

Description

The MsgProto message is received by client from the NATS Streaming Server, containing the payload of messages sent by a publisher. A MsgProto message that is not acknowledged with an Ack message within the duration specified by the ackWaitInSecs field of the subscription request will be redelivered.

Message Structure

• sequence: Globally ordered sequence number for the subject's channel

• subject: Subject

• data: Payload

• timestamp: Time the message was stored in the server. Represented as Unix time (number of nanoseconds elapsed since January 1, 1970 UTC)

• redelivered: Flag specifying if the message is being redelivered

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Ack

Description

An Ack message is an acknowledgement from the client that a MsgProto message has been considered received. It is published to the ackInbox field of the SubscriptionResponse.

Message Structure

• subject: Subject of the message being acknowledged

• sequence: Sequence of the message being acknowledged

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Ping

Description

A Ping message is sent to the server at configured interval to check that the connection ID is still valid. This should be used only if client is at protocol 1, and has sent a connID in the ConnectRequest protocol.

Message Structure

• connID: The connection ID

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PingResponse

Description

This is a response from the server to a Ping from the client. If the content is not empty, it will be the error indicating to the client why the connection is no longer valid.

Message Structure

• error: Error string, empty/omitted if no error

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CloseRequest

Description

A CloseRequest message is published on the closeRequests subject from the ConnectResponse, and notifies the NATS Streaming Server that the client connection is closing, allowing the server to free up resources. This message should always be sent when a client is finished using a connection.

Message Structure

• clientID: Client ID originally provided in the ConnectRequest

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CloseResponse

Description

The CloseResponse is sent by the NATS Streaming Server on the reply subject of the CloseRequest NATS message. This response contains any error that may have occurred with the corresponding close call.

Message Structure

• error: error string, empty/omitted if no error

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