Debugging linked docker containers when using docker-compose

Question

Lets assume I have the following docker-compose.yml file running two different python apps in parallel (e.g. via flask):

app1:
  command: python app.py
  build: app1/

app2:
  command: python app.py
  build: app2/
  links:
    - app1

app2 is linked to app1 since I want to get particular data from app1 within it. Now my problem is a certain scenario where I want to debug this link. I can easily debug app1 and app2 as standalone containers (via docker-compose run --service-ports ... python app.py and placing pdb somewhere in the code). My problem is when I want to debug app1 in case the request comes from app2. If I start app1 with docker-compose run, then app2 is not able to resolve the link. This issue becomes even more a problem with more apps/services "talking" to each other depending on their links.

Is there a good way to handle this? How do you approach the debug problem with linked containers in general (not necessarily python specifc)? Thanks for the input.

Answer 1

If you're doing development locally on the same machine then you can add a net: 'host' into your configuration which does the following:

Tells Docker to skip placing the container inside of a separate network stack. In essence, this choice tells Docker to not containerize the container's networking!

For more info see the documentation

app1:
  command: python app.py
  build: app1/
  net: 'host'

app2:
  command: python app.py
  build: app2/
  net: 'host'

Additionally you should start your app1 in daemon mode and app2 in the foreground mode for debugging purposes:

docker-compose up -d app1
docker-compose run app2

As soon as you get a request from app1 to app2 you will drop down to (pdb) in app2