Start docker container on incoming HTTP request

Question

I have a server running multiple web applications inside separate docker containers. I am using Traefik as a reverse proxy. So, whenever a container is idle for, say 15 mins, I stop the container from inside (end the running process which causes the container to stop). How can I restart the container on demand, i.e., when there is an incoming request for the stopped container?

As asked, I am not using any cluster manager or anything like that. Basically, I have an API server which uses the docker-py library to create images and containers. Traefik listens to the docker events and generates configuration whenever a container is created to route URLs to respective containers.

I tried systemd socket activation. Here are the socket and service files.

app.socket

[Unit]
Description=App Socket

[Socket]
ListenStream=3000
Accept=yes

[Install]
WantedBy=sockets.target

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[Unit]
Description=App Service
Requires=app.socket

[Service]
Type=simple
ExecStart=/usr/bin/npm start --prefix /path/to/dir
StandardInput=socket
StandardError=journal
TimeoutStopSec=5

[Install]
WantedBy=multi-user.target

This is my current approach. My containers are running node apps. So, I end the node process inside the containers. While ending the node process, I'll enable and start app.socket. And when there is incoming traffic on port 3000, my apps will be started by socket activation.

But nothing happens when I try to access that port. I've confirmed that socket activation is working. When I execute the command date | netcat 127.0.0.1 3000, the app seems to start and then immediately stops without any error.

Maybe socket activation doesn't work how I'm expecting it to. I can see that a process init with PID 1 is running on port 3000 after enabling app.socket. As soon as traffic comes on port 3000, I want to start my node app inside the container. But how can the app start on 3000, if there is already a process running on that port?

Perhaps there's some way to do it with Traefik since it is the reverse proxy I am using. Is there some functionality which can allow me to execute a command or script whenever 404 occurs?

Answer 1

It would be more helpful if you can tell how are you managing your docker containers ( k8 or swarm or anything else ). But based on your initial input, I guess you are looking for Inetd or systemd socket activation. This post can be helpful https://www.reddit.com/r/docker/comments/72sdyf/startrun_a_container_when_incoming_traffic/