Automatic port forwarding

[boukeversteegh]

Description

As a user, I would like to run commands like npm run serve or mkdocs serve without being required to manually map the port to the container.

This way, the commands work more like native commands, and it makes dockerized much more convenient.

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Requirements:

1. The user does not always need to specify -p <PORT> when running dockerized.
2. Multiple commands can still run in parallel (e.g. npm run serve and npm run lint, using the same npm command).
3. User is not tied to specific ports, and can use the command as flexibly as the native command.
4. Should work equally well on each platform (dockerized should not have features that only work on some platforms)

Out of scope:

1. Commands that open multiple ports should expose all of them.

Possible solutions:

• No mapping
  • Use the host network directly
    • ❌ Not possible on Windows/Mac
• Static mapping
  Predefine ports in built-in docker-compose.yml (e.g. always expose port 8080 for npm)
  • ❌ will cause port-mapping conflicts, e.g. running npm run serve and npm run lint
  • ❌ prevents user from choosing another port
• Post-mapping
  Automatically detect that a port was opened in the container, and map it afterwards
  • ❔ Open port detection within the container:
    • https://stackoverflow.com/questions/40350456/docker-any-way-to-list-open-sockets-inside-a-running-docker-container
  • Mapping the ports after container started:
    • ❔ Exposing the port after starting the container, using native docker networking code. Seems not possible.
    • ❌ Exposing the port to a random host-port, at start. Then tunneling it on the host.
      • ❌ We wouldn't know which container port to expose
    • Map a random port to the container for communication, then map them both on the host, and within the container
      HOST:application port ⇄ HOST:$RANDOM ⇄ CONTAINER:$RANDOM ⇄ CONTAINER:application port
    • 🙁 Ugly solution (might be unreliable, slow, complex), but might work.
• Pre-mapping
  Detect which port will be needed, and then expose/map it before starting the container
  • 🤔 This will be command specific, regardless of the solution, so not as scalable.
  • ✅ Detecting port parameters, i.e. mkdocs serve --dev-addr=0.0.0.0:8000 reveals that port 8000 should be opened.
    • Possible, with predefined (regex) patterns per command. For mkdocs, this could be --dev-addr=[^:]+:(?P<port>\d+)
    • Patterns can be stored as labels in the docker-compose file: com.dockerized.port
  • Detecting ports from configuration files, e.g. dev_addr within mkdocs.yml
    • Will work for simple single-file static configuration
    • Multi-file, environment dependent configuration (e.g. npm run serve --env=production using a different port)
  • Detecting ports from environment variables, e.g. some commands may natively use $PORT to determine the port to run on.
• Two-phase mapping
  • Concept:
    1. Run the command
    2. Detect the opened ports
    3. Allow user to re-run the command with the ports mapped
  • Variations
    • ✅ Opened ports will be remembered, and automatically mapped the next run.
      • Mapping can be stored in projects dockerized.env per command
        • ❌ Prevents running the same command twice (requirement 2)
      • ❌ Can be stored in a global cache, based on path (breaks 2)
      • Cache based on all command arguments and working directory
        • 🤔 won't work for dynamic arguments, such as <command> foo $(date)
          • But, will work in many cases. Covering a bunch of them will be an improvement.
    • Opened ports are remembered, but the user needs to confirm somehow
    • Immediately kill the command, and re-run it with ports mapped
      • ❌ Not so nice. Not all opened ports will be essential to the user, and sometimes a long build-process may happen before serving.