Filtering - In/Out

[FreshStartMA]

Thanks for the newbie help everyone. I've got Signal K running and connected/playing historical N2K and N0183 data files. OpenCpn is connected to it and displaying running navigation data on the chart (i.e, the boat is moving). OpenCpn and SK server both running on my windows box. I'll pretty much take a break here until the boat gets back in the water in the spring and I'll try to repeat these basic steps in a real environment probably using a YDEN-02 NMEA 2000 Ethernet Gateway.

I did want to ask one question about filtering. I assume that any filtering is applied to the input stream so if something is filtered, it never makes it into the Signal K environment. Is there any opportunity to filter data that is supplied to downstream clients? The usage case that I'm imagining is that a gateway device sends all data to Signal K. A given type of data is desired in the SK environment (maybe to use as input to a gauge widget), but that data is not desired/causing problems in the downstream client (OpenCpn). So, can the input side of SK be wide-opened and the output side filtered or some combination in between? My understanding is that OpenCpn does not have the capability to filters its incoming Signal K stream, but even if it did it seems to me that Signal K is the more natural environment for data i/o control (well I guess unless their are multiple clients with conflicting needs).

Thanks,
--Howie

p.s. I was going to try to filter some data, but with FileStream no filtering options seemed to be available. Is there a technical reason that is disabled? Would have been nice to learn about that in a simulated environment.

[tkurki]

There are several different options for filtering, depending on your particular use case.

1. Source priorities if you have several different sources for the same data, the canonical example being multiple GNSSs (GPSs), you can define priorities and timeouts for them so that as long as a higher prio source is producing data input from lower prio sources is discarded. This is by path.
2. Subscriptions a WebSocket client can subscribe to just some of the paths. Subscriptions allow also downsampling the data.
3. NMEA output filtering you can select what data you want converted to NMEA 0183 (with signalk-to-nmea0183 plugin) and what data a given output (like a serial connection to your autopilot) is outputting by fiddling with the events in the server. Filtering NMEA 2000 output / outputting only some of the data to NMEA 2000 connection is conceptually the same.
4. Access control lists if you want to filter data that a WebSocket client sees you need to enable authentication (so that we can distinguish ws clients) and use Access Control Lists to allow access only to some paths for the client. This is much less used and is configured via configuration files. This requires also that the client supports authentication.
5. Custom logic with Node-RED Node-RED integration supports filtering and altering incoming data and you have the full power of Node-RED at your disposal, allowing basically whatever you want. A practical example could be a separate WebSocket server with output filtered in Node-RED.

[FreshStartMA]

Thanks for info, plenty to dig into. The multiple GPS's may definitely come into play and seems fairly straightforward. The rest I'll look into. I'm anticipating (hoping) that getting the chart plotter functionality won't be too much of a challenge. Once I have that, I'll work on the autopilot which seems like it might be a bigger hurdle. Most crucial next step is getting the boat back in the water in the spring.