How to send both image(ndarray) and string data in single ZMQ send request

Question

For sending string data, following codes works :

context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.bind("tcp://*:5667")
socket.send_string("my string data")

For sending image(ndarray) following code works :

def send_array(socket, img, flags=0, copy=True, track=False):
    """send a numpy array with metadata"""
    md = dict(
        dtype = str(img.dtype),
        shape = img.shape,
    )
    socket.send_json(md, flags|zmq.SNDMORE)
    return socket.send(img, flags, copy=copy, track=track)

context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.bind("tcp://*:5667")
send_array(socket, my_ndarray_image )

But I do need to send both the string message along with the image file. Is there any way to append the message in the same request ?

Any ideas are welcomed ! Thanks

Genseric

What do you mean of "request"? Why don't you add a key, val to you dictionary for sending your image and your string then parse it in SUB side? – Belen

@BenyaminJafari By single "request", I meant single message queue request. Whats the need ? ---> As i mentioned, in my application I need to send an image & an associated string message along with it. If I send them in 2 separate requests. Great ! works good for single user instance. But, say there are 2 users who simultaneously run my code. If both requests are done at same time, there's no way to know which Image & String are together. Possible wrong combinations ( IMAGE1 + STRING2 ) instead of correct ( IMAGE1 + STRING1 ). I know, its has gone complicated. Feel free to query back. – Genseric

@BenyaminJafari Secondly, adding image & string in key, value doesn't works out, I've already tried. It can't be encoded together at the sender end itself. So, no point I can parse it at SUBSCRIBER side – Genseric

Answer 1

I guess you are looking for multipart messages, which allow you to compose a message out of several frames. The Python implementation pyzmq already provides us with a nice wrapper for multipart messages. Here is an example for the server sending a multipart message:

context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.bind("tcp://*:5555")

time.sleep(0.2)  # wait for socket to be properly bound

socket.send_multipart([b"first part", b"second part"])

And the client receiving a multipart message:

context = zmq.Context()
socket = context.socket(zmq.SUB)
socket.setsockopt(zmq.SUBSCRIBE, b"")
socket.connect("tcp://localhost:5555")

print(socket.recv_multipart())

Please consider the following when working with multipart messages:

• All the message parts will only ever be sent after you send the final part.
• The receiver will always either receive all message parts or none at all.

---

In your specific example, you are already composing a multipart messages in the send_array function by using the flag zmq.SNDMORE. We can extend your example by also adding the string data with the zmq.SNDMORE flag. Here is the server side:

def send_array_and_str(socket, img, string, flags=0):
    md = dict(dtype = str(img.dtype), shape=img.shape)

    socket.send_string(string, flags | zmq.SNDMORE)
    socket.send_json(md, flags | zmq.SNDMORE)
    return socket.send(img, flags)

context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.bind("tcp://*:5667")
time.sleep(0.2)

my_ndarray = np.array([1, 2, 3])
my_string = "Hello World"
send_array_and_str(socket, my_ndarray, my_string)

And the client code receiving the message:

def recv_array_and_str(socket, flags=0, copy=True, track=False):
    string = socket.recv_string(flags=flags)
    md = socket.recv_json(flags=flags)
    msg = socket.recv(flags=flags, copy=copy, track=track)

    img = np.frombuffer(bytes(memoryview(msg)), dtype=md['dtype'])
    return string, img.reshape(md['shape'])

context = zmq.Context()
socket = context.socket(zmq.SUB)
socket.setsockopt(zmq.SUBSCRIBE, b"")
socket.connect("tcp://localhost:5667")

print(recv_array_and_str(socket))

This code is based on the example Serializing messages with PyZMQ and adapted to work with Python 3. For Python 2, consider using buffer(msg) instead of bytes(memoryview(msg)).

Minh

Do you mean was "The receiver will ..." instead of "The sender will always either receive all message parts or none at all."? – Belen

@BenyaminJafari exactly, thanks for pointing out this error! :) – Minh