Moving from Docker Containers to Cloud Foundry containers

An old question, but a lot has changed since this was posted:

Recently I started to practice Dockers. Basically, I am running a C application on Docker container. Now, I want to try cloud foundry, therefore, trying to understand the difference between the two.

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The last option I have is to use docker containers in Cloud foundry, but I want to understand if I can achieve what I described above.

There's nothing wrong with using Docker containers on CF. If you've already got everything set up to run inside a Docker container, being able to run that on CF give you yet another place you can easily deploy your workload.

While these are pretty minor, there are a couple requirements for your Docker container, so it's worth checking those to make sure it's possible to run on CF.

https://docs.cloudfoundry.org/devguide/deploy-apps/push-docker.html#requirements

Anyways, I am not working on this now as CF is not suitable for the project. It's an SIP application and CF only accepts HTTP/S requests.

OK, the elephant in the room. This is no longer true. CF has support for TCP routes. These allow you to receive TCP traffic directly to your application. This means, it's no longer just HTTP/S apps that are suitable for running on CF.

Instructions to set up your CF environment with TCP routing: https://docs.cloudfoundry.org/adminguide/enabling-tcp-routing.html

Instructions to use TCP routes as a developer: https://docs.cloudfoundry.org/devguide/deploy-apps/routes-domains.html#create-route-with-port

Now, moving on to Cloud Foundry, the first thing I was not able to find is 'How to deploy C application' then I found a C build pack and a binary build pack option.

Picking a buildpack is an important step. The buildpack takes your app and prepares it to run on CF. A C buildpack might sound nice as it would take your source code, build and run it, but it's going to get tricky because your C app likely depends on libraries. Libraries that may or may not be installed.

If you're going to go this route, you'll probably need to use CF's multi-buildpack support. This lets you run multiple buildpacks. If you pair this with the Apt buildpack, you can install the packages that you need so that any required libraries are available for your app as it's compiled.

https://docs.cloudfoundry.org/buildpacks/use-multiple-buildpacks.html

https://github.com/cloudfoundry/apt-buildpack

Using the binary buildpack is another option. In this case, you'd build your app locally. Perhaps in a docker container or on an Ubuntu VM (it needs to match the stack being used by your CF provider, i.e. cf stacks, currently Ubuntu Trusty or Ubuntu Bionic). Once you have a binary or binary + set of libraries, you can simply cf push the compiled artifacts. The binary buildpack will "run" (it actually does nothing) and then your app will be started with the command you specify.

My $0.02 only, but the binary buildpack is probably the easier of the two options.

what I am not able to understand how I can provide a startup script to a cloud foundry container or in brief how to achieve what I was doing with Dockers.

There's a few ways you can do this. The first is to specify a custom start command. You do this with cf push -c 'command'. This would normally be used to just start your app, like './my-app', but you could also use this to do other things.

Ex: cf push -c './prep-my-app.sh && ./my-app'

Or even just call your start script:

Ex: cf push -c './start-my-app.sh'.

CF also has support for a .profile script. This can be pushed with your app (at the root of the files you push), and it will be executed by the platform prior to your application starting up.

https://docs.cloudfoundry.org/devguide/deploy-apps/deploy-app.html#profile

Normally, you'd want to use a .profile script as you'd want to let the buildpack decide how to start your app (setting -c will override the buildpack), but in your case with the C or binary buildpack's, it's unlikely the buildpack will be able to do that, so you'll end up having to set a custom start command anyway.

For this specific case, I'd suggest using cf push -c as it's slightly easier, but for all other cases and apps deployed with other buildpacks, I'd suggest a .profile script.

Hope that helps!