Signal K has a new format for specifying model changes. Now, instead of sending the entire model whenver a value changes, we can send just the changes using the format below. An example of a delta message is presented below.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
{
  "context": "vessels.230099999",
  "updates": [
    {
      "source": {
        "pgn": "128275",
        "device" : "/dev/actisense",
        "timestamp": "2014-08-15T16:00:05.538Z",
        "src": "115"
      },
      "values": [
        {
          "path": "navigation.logTrip",
          "value": 43374
        },
        {
          "path": "navigation.log",
          "value": 17404540
        }
      ]
    },
    {
      "source": {
        "device" : "/dev/actisense",
        "timestamp":"2014-08-15T16:00:00.081Z",
        "src":"115",
        "pgn":"128267"
      },
      "values": [
        {
          "path": "navigation.courseOverGroundTrue",
          "value": 172.9
        },
        {
          "path": "navigation.speedOverGround",
          "value": 3.85
        }
      ]
    }
  ]
}

The first attribute of the delta object is context. The context attribute identifies the specific source system which the delta update applies to. In the example above, the context is a vessel with an MMSI of 230099999. If this context doesn’t exist (i.e. this is a new source system), a new context will be created in the model at the specified path.

Next, there is an array called updates which contains 1 or more update objects. The update object consists of source and values where the source describes the device which generated the update (this could be a sensor, chart plotter, computer or whatever) and the values array is an array of update objects which consists of a path and a value.

Paths are dot delimited and it is expected that the consumer propagates the source timestamp down all of these paths.

This update is available on GitHub in the master branch of SignalK/n2k-signalk.