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When you push a Docker container to Gstack, there is few mystery about how Gstack runs it. It just brings the conntainer up and runs it.
When you push an applications to Gstack, it will also end up as a runnable Docker container. But this one will be generated on the fly. We detail here how this is done.
The stack below your application
gk push an application to Gstack, three layers will be stacked to
bring it online, up and running.
- A Docker container, that provides isolation to the runtime binaries that power your application.
- A root filesystem, that provides basics for your application runtime to be compiled and run.
- A buildpack, that provides compilation, packaging, and runtime.
We detail these 3 layers in reverse order, starting with the “Buildpack” layer that is the nearest from your applications.
A “buildpack” is just a bunch of tools that are able to package your application in a standard way, so that it can then be run. Gstack provides some of the most popular buildpacks: Go, Java, Node.js, PHP, Python, and Ruby. Each one of these supports all the major frameworks and dialects of the central technology it addresses.
The design of buildpacks is very simple, which participated to their outstanding success.
- A buildpack is either packaged as a git repository, or a zip archive.
- It must provide these 3 executable scripts:
bin/detectthat tells whether the buildpack can build and run a given application.
bin/compilethat builds the application, after this has run, your application is completely ready to be started.
bin/releasethat details how to run the application: which command line to execute, and which environement variables to provide.
- The 3 scripts above will be called in sequence.
- As input, they take specific arguments.
- As output, they return specific exit codes, print logs, or dump some YAML-formatted properties.
Buildpack make Gstack incredibly polyglot, because dozen of them have been
written so far. Even the most funkyest web technologies have a dedicated
buildpack! Besides the standard Gstack buildpacks, you can choose to use your
own very easily when you
gk push you applications. You just need to specify
a mere Github URL on the command line or in the
manifest.yml. Gstack will
then proceed downloading and using it for your app.
Buildpacks were first designed by Heroku. Considering their success, they were later adopted by others and they are now available in Gstack. For more details, go watch the Heroku Buildpacks presentation, it’s brilliant.
A Root Filesystem
The root filesystem is a
.tgz archive containing a basic collection of
files. There are the base system utilities and configs that the buildpack and
the application will have access to.
For example, it provides basic C and C++ compilers, and script engines like
sh. This is meant to be enough for you to
download and build any missing technology, whether mainstream like
Go, or quite funky like Pike
The standard root filesystem in Gstack is called
cflinuxfs2. It is based on
a stable Ubuntu Linux distribution and contains something like 21,000+ files.
You can see which root filesystems are available in Gstack with
A Docker container
Your application as built, augmented and packaged by the buildpack will run inside a Docker container.
Now that you know how applications are deployed in Gstack, you might be interested in writing your own.
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