Streaming API

WebSocket API: /signalk/«version»/stream

Initiates a WebSocket connection that will start streaming the server’s updates as Signal K delta messages. You can specify the contents of the stream by using the subscribe query parameter.

• ws://hostname/signalk/«version»/stream?subscribe=self
• ws://hostname/signalk/«version»/stream?subscribe=all
• ws://hostname/signalk/«version»/stream?subscribe=none

With no query parameter the default is self, which will stream the data related to the self object. all will stream all the updates the server sees and none will stream only the heartbeat, until the client issues subscribe messages in the WebSocket stream.

A server may send the latest values it has cached when a client connects via WebSocket. A client can control this behavior with query parameter sendCachedValues. false will suppress sending the values and true force it. With no sendCachedValues parameter the server should send them.

If a server does not support some streaming options listed in here it must respond with HTTP status code 501 Not Implemented.

See Subscription Protocol for more details.

Connection Hello

Upon connection the server MUST send a 'hello' JSON message, for example:

{
    "name": "foobar marine server",
    "version": "1.0.4",
    "timestamp": "2018-06-21T15:09:16.704Z",
    "self": "vessels.urn:mrn:signalk:uuid:c0d79334-4e25-4245-8892-54e8ccc8021d",
    "roles": [
        "master",
        "main"
    ]
}

This response is defined by the hello.json schema.

The server MUST provide:

• roles which specifies which roles the server is capable of providing. See roles for details about possible server roles.
• version which specifies the version of the SignalK schema and APIs that the server is using. See versioning for details about version strings.

The server SHOULD provide:

• timestamp but only if the server is equipped with a time source and it has been set.

The server MAY provide:

• self is the unique identifier of the vessel using the URN format specified for the uuid field in the Signal K schema. It may also use the URN format specified for the mmsi field in the Signal K schema if it exists. This is only provided if the server relates to a specific vessel, aircraft, aid to navigation or sar.
• name is the name of the Signal K server software, e.g. signalk-server

name, self and roles MUST return the same values as provided in the swname, self and roles properties within the DNS-SD advertisement (if implemented).

version MUST be the same value as version within the associated endpoints list provided by the http GET request to /signalk within the REST API (if implemented).

History playback

The server MAY support history playback from a certain point in time with a specified rate.

To create a WebSocket connection that plays back data the client uses the request parameter startTime to specify the start timestamp and the optional request parameter playbackRate to specify the rate. Rate value parameter is a floating point value with value 1 equal to real time playback and for example 0.5 to half the real time rate and 5 to five times real time rate. Omitting the playbackRate will result in real time playback.

The playback api is located at /signalk/v1/playback. An example url for history playback streaming: wss://localhost:3443/signalk/v1/playback?subscribe=self&startTime=2018-08-24T15:19:09Z&playbackRate=5.

The hello message for a history playback stream MUST NOT contain the timestamp property and MUST include the properties startTime and playbackRate. The delta stream format for history playback is the normal streaming format. Timestamps indicate the time data was originally captured.

{
    "name": "foobar marine server",
    "version": "1.1.4",
    "startTime": "2018-08-24T15:19:09Z",
    "playbackRate": 1,
    "self": "vessels.urn:mrn:signalk:uuid:c0d79334-4e25-4245-8892-54e8ccc8021d",
    "roles": [
        "master",
        "main"
    ]
}

A server MAY respond with 501 Not Implemented status code if it does not support history playback and with 400 Bad Request if it does not have data to play back for the given time period. A 404 Not Found response is also acceptable to be backwards compatible.

Streaming over TCP

A server MAY provide streaming delta service over TCP. See Urls and Ports and Discovery and Connection Establishment for more details.

The messages MUST be serialised as JSON with one message per line using line terminator \r\n (carriage return and newline).

As there is no way to specify the subscription policy using url parameters as when opening a WebSocket connection the initial subscription policy is none, no active subscriptions. The client can modify the subscriptions after connection is established.

Connection hello is the same as when using WebSockets.