# How to set up bidirectional communication between a Docker container and its host machine using TCP sockets

Sun Nov 15 2020

## Introduction

(I wrote this to document the specifics of a feature I shipped at IMDA. I have reproduced it here because it may be useful to some (especially future me), but I haven't intended it for public consumption. So it's probably not going to be well-motivated or interesting to you, and you may be missing some critical context: but read on if you want to know how to set this up for yourself ).

I wrote simple sockets code to allow a host machine and a Docker container running on it to communicate bidirectionally. I needed to do this because our machine learning model ran on a Docker container and we needed to send data to/receive data from it.

## The problem

We want the machine learning model to be in a Dockerfile to make setup and configuration easy. But this introduces a new challenge: how do we send data from the Jetson (host) to the machine learning model (Docker container)?

If the code for the machine learning model were not in a Docker, we could simply import the module into the webcam stream code, something like this:

# webcam_stream.pyimport ml_modelimg = get_img_from_webcam_stream()bounding_boxes = ml_model.detect_objects_in(img)

But we can't do this now because ml_model is running in a Docker container. We need some way to pass the img from the webcam stream into the Docker container, and get the bounding_boxes back. So how? I thought a lot about this. There were a few different candidates:

1. Spin up HTTP server on the Docker container and make requests
2. Use a message broker like Kafka or RabbitMQ
3. Mount a host directory on the docker and read/write to files on that directory
4. Expose a port and build bidirectional communication over TCP

It seemed like a very simple task to send data to and from the same machine, so I wanted to avoid the overhead of spinning up a HTTP server or message broker. This left the last two options.

### Writing to and from files in a directory

Suppose you have a webcam_stream.py and ml_model.py that both have access to a shared folder. If both Python files watch the folder for changes we can get kind-of real-time communication. Let webcam_stream.py write frame_1.ndarray to the folder. Then ml_model.py can read that file in, detect the objects in that first frame, and then write a frame_1_bbox.ndarray file to the folder. webcam_stream.py can then read that file in.

This is a pretty simple idea: the downsides are that i) file I/O is slow, and ii) you will have to write code to handle deleting old files. It also didn't seem very elegant to have to constantly create and delete files. Therefore, I chose not to use this approach in the end. But since everything in Unix is a file [1], couldn't we use Unix sockets?

### Building bidirectional communication over TCP

Suppose we had an open port on localhost:6000 and a way to send and receive data over that connection. Then we could make ml_model.py constantly listen on that port for image data from webcam_stream.py. After receiving the image data it would then run it through the ML model and send the bounding boxes back through the same connection.

Some pseudocode here:

# webcam_stream.pywhile True:    img = get_img_from_webcam_stream()    bounding_boxes = send_img_to_ml_model(img, port=6000)
# ml_model.pywhile True:    img = recv_img_from_stream(port=6000)    bounding_boxes = detect_objects_in(img)    send_img_to_stream(bounding_boxes, port=6000)

## How to build bidirectional communication over TCP

I used a low-level Python library called socket. This library directly calls the Unix socket syscalls (again, something I don't quite understand).

We first establish a connection using socket.connect before sending any data over.

Server-side: First we start a listener on the server side. We bind the socket to an open port (in this case 6000) and start listening for connections forever in a while loop, calling socket.accept to accept incoming connections on that socket. Something to note here is that the socket.accept call actually returns a new socket when a client connects to it: a new socket is created for the connection ready to send and recv, and the original socket continues to listen for new connections [2].

Client-side: The client side is simple: we just use socket.connect to connect to the remote socket.

Here's what that looks like on the server side:

# ml_model.pywith socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)    server_address = ('localhost', 6000)    sock.bind(server_address)    sock.listen(1)    try:        while True:            print('waiting for a connection')            connection, client_address = sock.accept()            ...

And on the client-side:

# webcam_stream.pyserver_address = ('localhost', 6000)with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:    sock.connect(server_address)    ...

Once a connection has been accepted we can start the bidirectional sending. Sadly it's not as simple as sending the image and receiving the bounding boxes. There are two main complications: serialisation and deserialisation of the data, and knowing when the message has ended.

### Serialising and deserialising the data

The first main complication is how to send numpy arrays over the TCP connection, because a TCP connection only accepts bytes. One way to do it would be to simply convert the numpy arrays into a string, convert that into a bytestring using bytes(), and send that.

There's a better way to do it though. I use Numpy's built-in np.save and np.load to write and read to a BytesIO object, which is basically a file AFAIK. The BytesIO file can simply be file.read() and sent over.

Take note that writing to a file places a pointer to the end of the file (not sure how this works) so we'll need to do file.seek(0)to seek to the start of the file in order to read the file with file.read.

Here's what that looks like:

message = BytesIO()np.save(message, img)message.seek(0)sock.sendall(message.read()) # sock is the connected socket

When receiving a response we simply use file.write() to write to a new BytesIO object. and np.load to get it back into an numpy array.

### Knowing when the message has ended

TCP connections are open-ended, which means that the receiving server doesn't know when to stop receiving.

This Python sockets guide puts it nicely:

...you need to realize that there is no EOT on a socket... If the connection has not been broken, you may wait on a recv forever, because the socket will not tell you that there’s nothing more to read (for now). Now if you think about that a bit, you’ll come to realize a fundamental truth of sockets: messages must either be fixed length (yuck), or be delimited (shrug), or indicate how long they are (much better), or end by shutting down the connection.

The simplest way to solve this problem is to simply close the connection after we've sent out the image. But we don't want to close the connection because we want to get a response back from the server (namely, the bounding boxes). So we'll have to specify the message length instead. [3] We'll simply prepend the message with the message length. The problem is that the message length itself can also vary, so we'll need to pad it to ensure the first N bytes will be guaranteed to be the header. If you don't pad the message length then you might read part of the array in as your message length.

First we save the numpy array into a BytesIO object using np.save. We can get the message length using message.getbuffer().nbytes, and can pad the message length string using Python's string formatting.

# webcam_stream.pymsg_len = message.getbuffer().nbytesHEADER_SIZE = 10b_msg_len = bytes(f"{msg_len:<{HEADER_SIZE}}", 'utf-8')

The :<{HEADER_SIZE} thing right-pads the formatted string with zeros so that it is exactly 10 characters long. Here's an example of what that looks like:

>>> f"{12345}"'12345'>>> f"{12345:<10}"'12345     '>>> bytes(f"{12345:<10}", 'utf-8')b'12345     '>>> bytes(f"{12345:<10}", 'utf-8').decode('utf-8')'12345     '# on the server side>>> int(bytes(f"{12345:<10}", 'utf-8').decode('utf-8'))12345

On the Docker container we first receive the message length and decode it. (Note that since we are using UTF-8 encoding, a numeric string of length 10 will be 10 bytes long!) We then keep receiving until the number of bytes we've received equals the message length. We then send it to the object detection model (calling the detect function) and send the bounding boxes back.

# ml_model.pyHEADER_SIZE = 10# First receive the header to know how long the message is:b_msg_len = connection.recv(HEADER_SIZE)msg_len = int(b_msg_len.decode("utf-8"))# Then receive the data in small chunks until we receive msg_len bytesfull_data = BytesIO()while (full_data.getbuffer().nbytes < msg_len):    data = connection.recv(4096)    full_data.write(data)full_data.seek(0)print("All data received!")# Now loading and using the object detection modelimg = np.load(full_data, allow_pickle=True)bboxes = detect(img, 0.3, 0.4)[0]print("Sending data back to the client")bbox_bytes = BytesIO()np.save(bbox_bytes, bboxes)bbox_bytes.seek(0)connection.sendall(bbox_bytes.read())

This solution has its limitations: it won't work if we wanted to send multiple images in a single connection. If we sent multiple messages then there would be no guarantee that the first 10 (or whatever) bytes of the message buffer would be the message length. But since we close the socket and re-open a new one after each frame, this is not a problem.

## Conclusion

1. Don't quite understand the implications of this fact, have to do more research ↩︎

2. From https://beej.us/guide/bgnet/html/ ↩︎

3. HTTP specifies message length using a "Content Length" or "Transfer Encoding" header field: see this SO link for details. ↩︎