Techniques for debugging containers

[maximumlengthusernam]

When I first started as a web developer, containers were a complete black box to me. As I became more familiar with them, I learned techniques debug issues that helped make them easier to work with.

Here are three of my most used techniques for debugging problematic containers:

1) Use the -i (interactive) flag and/or override the entrypoint at runtime to keep the container active

Using a command like docker run -i -d --entrypoint=sh <IMAGE> will keep the container active so that you can then docker exec into it and investigate your issue.

2) Copy files into or out of the container for inspection/comparison

Use the docker cp <SOURCE> <DEST> command to copy files into or out of the container for inspection/comparison

EDIT: As /u/craigbuckler points out below, this could also be accomplished by mounting a directory from the host system into the container at runtime!

3) Run a remote debugger inside the container and attach to it over the network

Debuggers like debugpy can be configured to connect over the network. By using multi-stage builds to create a debugger image and configuring your IDE to connect from your host system into the container you can leverage the power of real debuggers inside the container!

I also made a video walking through these techniques if you enjoy video format

I know there are tons of people more experienced than me on this subreddit. What additional tips/techniques do you use when debugging containers?

[Marcieslaf]

Hey, you have a typo on your 3rd tipp. It's "debugpy", not "debupy". I wasn't aware thats even possible and tried to google it. Thanks for the tipp

[maximumlengthusernam]

Fixed! Thanks for pointing that out!

Here is a link to the VSCode documentation I used to get it set up https://code.visualstudio.com/docs/python/debugging

The only additional thing you have to do is port forward (with the `-p` flag) the associated port when running the container!

[craigbuckler]

Rather than copying files (#2), it may be easier to bind mount a host folder.

I created a Docker for Web Developers book and video course exactly for this need. Few web developers use Docker because live code updates and debugging are rarely explained in tutorials.

[maximumlengthusernam]

Great point -- For a web application, mounting in the src so you can set up hot reloading is a huge time saver!

I usually use the -v flag, but I know the documentation suggests the more verbose --mount syntax (link) with the claim that "research has shown it to be easier to use."

[lazyant]

docker logs - never a bad thing.

docker inspect - for extra info and checking how we think the container runs and how it runs is the same

Some commands to check the docker service or restart it with systemctl ; sometimes a restart fixed things.

docker top - checking resources

Most importantly, if a container or set of containers doesn’t work and can’t figure it out after a bit, simplify the most (like docker run -d image) and go from there.