python websocket-client, only receive most recent message in queue

Question

I'm running websocket-client on a raspberry pi to control a servo using data sent by a leap motion websocket server. The volume of data sent over the websocket connection by the leap motion is so high that there is a long delay with the pi catching up to the most recent message which only gets worse the longer it runs.

How can I discard all the old messages in a queue and just use the most recent one when checking? Currently I'm doing this:

ws = create_connection("ws://192.168.1.5:6437")
    while True:
        result = ws.recv()
        resultj = json.loads(result)
        if "hands" in resultj and len(resultj["hands"]) > 0:
                # print resultj["hands"][0]["palmNormal"]
                rotation = math.atan2(resultj["hands"][0]["palmNormal"][0], -resultj["hands"][0]["palmNormal"][1])
                dc = translate(rotation, -1, 1, 5, 20)
                pwm.ChangeDutyCycle(float(dc))

You need a thread to receive into a local queue and a thread to process the newest message from the queue (discarding all others) once the previous move finishes. — DisappointedByUnaccountableMod, Jul 07 '16 at 20:10
Will the other thread not suffer from the same problem and get nowhere near the front of the queue of websocket messages? — Tom, Jul 07 '16 at 20:38
when you do the recv, before actioning it can you test see if there is another message, read them all and action the last message? — DisappointedByUnaccountableMod, Jul 08 '16 at 07:35

score 1 · Answer 1 · answered Jul 13 '16 at 17:35

Since the Leap service sends data at roughly 110 frames per second, you have about 9ms to do any processing. If you exceed this, then you will get behind. The easiest solution is probably to create an artificial app frame rate. For each loop, if you haven't reached the time for the next update, then you just get the next message and discard it.

ws = create_connection("ws://192.168.1.5:6437")
allowedProcessingTime = .1 #seconds
timeStep = 0 
while True:
    result = ws.recv()
    if time.clock() - timeStep > allowedProcessingTime:
        timeStep = time.clock()
        resultj = json.loads(result)
        if "hands" in resultj and len(resultj["hands"]) > 0:
                # print resultj["hands"][0]["palmNormal"]
                rotation = math.atan2(resultj["hands"][0]["palmNormal"][0], -resultj["hands"][0]["palmNormal"][1])
                dc = translate(rotation, -1, 1, 5, 20)
                pwm.ChangeDutyCycle(float(dc))
        print "processing time = " + str(time.clock() - timeStep) #informational

(I haven't run this modification to your code, so the usual caveats apply)

python websocket-client, only receive most recent message in queue

1 Answers1