The Streaming Protocol
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You can find a list of all supported client libraries . 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 to exchange protocols between the client and the server.
The NATS streaming protocol sits atop the core NATS protocol and uses . 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.
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 .
The following table briefly describes the NATS streaming protocol messages.
Click the name to see more detailed information, including usage:
Message Name
Sent By
Description
Client
Request to connect to the NATS Streaming Server
Server
Result of a connection request
Client
Request sent to subscribe and retrieve data
Server
Result of a subscription request
Client
Unsubscribe from a subject
Client
Publish a message to a subject
Server
An acknowledgement that a published message has been processed on the server
Server
A message from the NATS Streaming Server to a subscribing client
Client
Acknowledges that a message has been received
Client
Ping sent to server to detect connection loss
Server
Result of a Ping
Client
Request sent to close the connection to the NATS Streaming Server
Server
Result of the close request
The following sections explain each protocol message.
Description
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
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
Description
Message Structure
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
Description
Message Structure
ackInbox: subject the client sends message acknowledgements to the NATS Streaming Server
error: error string, empty/omitted if no error
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
subject: Subject for the subscription
inbox: Inbox subject to identify subscription
durableName: Optional durable name which survives client restarts
Description
Message Structure
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
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
Description
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
Description
Message Structure
subject: Subject of the message being acknowledged
sequence: Sequence of the message being acknowledged
Description
Message Structure
connID: The connection ID
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
Description
Message Structure
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
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 message to the reply subject specified in the NATS request message.
A SubscriptionRequest is published on the subject returned in the subRequests field of a , and creates a subscription to a subject on the NATS Streaming Server. This will return a message to the reply subject specified in the NATS protocol request message.
clientID: Client ID originally provided in the
The SubscriptionResponse message is the response from the SubscriptionRequest. After a client has processed an incoming message, it must send an acknowledgement to the ackInbox subject provided here.
clientID: Client ID originally provided in the
The PubMsg protocol message is published from a client to the NATS Streaming Server. The GUID must be unique, and is returned in the message to correlate the success or failure of storing this particular message.
clientID: Client ID originally provided in the
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 message within the duration specified by the ackWaitInSecs field of the subscription request will be redelivered.
An Ack message is an acknowledgement from the client that a message has been considered received. It is published to the ackInbox field of the .
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 protocol.
A CloseRequest message is published on the closeRequests subject from the , 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.
clientID: Client ID originally provided in the