Stream temperature changes

[tobyweston]

Using inotifywait (), watch for file changes of the temperature sensor and send straight to the server.

Update the sever to stream current temperatures via web sockets rather than HTTP GET.

Update the UI to respond reactively to streamed temperature changes. Potentially update the chart in real time.

Note that RRD has a rate limit of 1 second, so might need the debounce.

[tobyweston]

See the BlazeWebSocketExample.scala from Http4s (server side).

See react-webscoket for the client side

On the server, could use java's WatcherService to update a queue of some sort then in the TemperatureEndpoint create a continuous Process to read the queue:

   val source = scalaz.stream.time.duration.map{ d =>
      Text( ... read the queue and encode as json ... )
   }

Raspbian supports inotify (the presence of /proc/sys/fs/inotify/ confirms it):

$ ll /proc/sys/fs/inotify/
total 0
-rw-r--r-- 1 root root 0 Nov 12 20:50 max_queued_events
-rw-r--r-- 1 root root 0 Nov 12 20:50 max_user_instances
-rw-r--r-- 1 root root 0 Nov 12 20:50 max_user_watches
$ cat /proc/sys/fs/inotify/max_user_watches 
8192

Looks like http4s doesn't support websockets from the client side, perhaps Async Http Client would be better? or nv-websocket-client?

[tobyweston]

See issue_6 branch and issue_6_of_temperature_machine for the UI.

Bit of a blocker: on the Pi, the sensor file's stat doesn't seem to update 😢

https://raspberrypi.stackexchange.com/questions/75167/ds18b20-w1-slave-file-stamp-doesnt-change

[tobyweston]

This is basically done now. There are some caveats:

• Because of the underlying driver, we can't "stream" temperature changes directly from the temperature sensor hardware. We are forced to poll.
• We therefore poll every second from the client, sending data to the server which is stored in memory
• The server exposes this data via websocket for average and all sensor data
• The client UI creates a websocket to read the values displaying them as they change

There may be weird behaviour when client machines drop in and out of the pool. It looked like when a new machine was added, the UI didn't update but I need to confirm if this is the case.