Compose: `docker-compose up` not attaching, but only for some containers

[tedkornish]

I have a trivial Haskell webservice in a container and a trivial Python webservice in a container. When I run docker-compose up haskell-web-service, the process hangs on Attaching to haskellwebservice_haskell-web-service_1 and doesn't give any further output, which isn't the expected behavior. However, when I start up the Flask app, it behaves as expected.

I disproved the hypothesis that the process wasn't able to run properly in the container by opening up a shell inside the container (docker-compose run haskell-web-service bash) and starting up the app manually (haskell-web-service), where it worked as expected.

Further explanation and code for reproducing (or not, which would also be interesting) here: https://github.com/tedkornish/docker-compose-hanging.

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Would be curious to see if anyone else can reproduce. Everything is included in the repository, so it should be as simple as

git clone https://github.com/tedkornish/docker-compose-hanging.git
cd docker-compose-hanging
docker-compose up

And seeing if the Haskell container hangs on attaching but the Python one doesn't. (warning: Haskell app may take 10ish min to download and compile dependencies)

[dnephin]

What happens if you try to curl the port of the haskell service? Can you confirm that it's actually running (and the only issue is that you aren't seeing the stdout/stderr) ?

If that's the case, it's very likely a buffering issue. It might be that after enough output you'll start to see it. There might be a way to tell the haskell app to force flush the buffer more often

[tedkornish]

Great call, that's exactly what it was - I just had to hammer on the endpoint repeatedly and the buffer eventually came through and flushed to stdout.