Lukas Klein

Python and Django developer. Docker enthusiast. This is my blog.

Building a realtime audio transcoder with Docker, FFmpeg and Flask

Recently I was faced with the issue of having a device capable of playing MP3s from the network, but I only had MP4s available. Since the source files were too many and changing daily, transcoding them in advance and effectively mirroring them was not an option. The only solution would be to do it on demand. A great opportunity to get fancy with Docker.

I quickly found a good ffmpeg Docker image, cellofellow/ffmpeg. But how would I set it all up? I’m working on a Mac with boot2docker and docker-compose. I thought about starting the ffmpeg containers over the remote api on my Mac’s Docker instance, but I wasn’t really feeling comfortable with this setup. In my opinion, the ffmpeg containers should somehow stay inside the application’s boundaries.

Meet Docker-in-Docker

Docker introduced the privileged mode in version 0.6. It allows you to run containers with many of the host machine’s capabilities, regarding kernel features and device access. jpetazzo/dind makes it really easy to run a Docker host inside Docker. You can even run Docker inside Docker inside Docker inside… Oh my.

The docker-compose.yml now looks something like this:

    image: jpetazzo/dind
    privileged: true
    - PORT=4444

We’re going to use Flask in conjunction with gunicorn and a nginx reverse proxy for the app itself.

Create a new folder nginx with this Dockerfile:

FROM tutum/nginx
RUN rm /etc/nginx/sites-enabled/default
ADD sites-enabled/ /etc/nginx/sites-enabled

And the nginx/sites-enabled/default:

server {
    listen 80;
    charset utf-8;

    location / {
        proxy_pass http://transcoder:8000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

And finally add it to the docker-compose.yml:

  restart: always
  build: ./nginx/
    - "8080:80"
    - transcoder

The web frontend itself will be a Flask application named transcoder, we can add it to the docker-compose.yml already:

  build: transcoder
  restart: always
  command: /usr/local/bin/gunicorn -w 2 -b :8000 app:app
    - docker

The Dockerfile for the transcoder app is quite simple. We’re making use of the python:3.4.3-onbuild image, which handles copying the requirements.txt and pip installing it. The final Dockerfile in transcoder/Dockerfile is simply:

FROM python:3.4.3-onbuild

To communicate with the Docker host, we’re using the docker-py. Apart from flask and gunicorn that’s our only requirement. Our requirements.txt:


At first, we create a Docker client:

from docker import Client
docker_client = Client(base_url='http://docker:4444')

In the handler for our transcode endpoint, we then create and start the ffmpeg container:

container = docker_client.create_container(
    command=['ffmpeg', '-hide_banner', '-loglevel', 'quiet', '-i', url, '-codec:a', 'libmp3lame', '-b:a', '160k', '-f', 'mp3', '-'],

To get the output of ffmpeg, we’re attaching to our container using the stream=True option, in order to get back a generator rather than a string:

output = docker_client.attach(

Since Flask supports streaming responses out of the box, getting the result to the user is as easy as:

return Response(output, mimetype='audio/mpeg')

That’s all! You can now call our microservice with the url of an audio file and get back a streamed transcoded MP3 version of it.

Our final app looks like this: