Defining a wsgi app deployment standard

Next month at Pycon, we'll have a web summit and I'm invited there to talk about how I deploy web applications. This is not a new topic, as it was already discussed a bit last year -- see Ian Bicking's thought on the topic.

My presentation at the summit will be in two parts. I want to 1/ explain how I organized our Python deployments at Mozilla (using RPMs) 2/ make an initial proposal for a deployment standard that would work for the community at large - I intend to work on this during Pycon and later on the dedicated SIG.

Here's an overview of the deployment standard idea...

How we deploy usually

If I want to roughly summarize how people deploy their web applications these days, from my knowledge I'd say that there are two main categories.
1. Deployments that need to be done in the context of an existing packaging system -- like RPM or DPKG 2. Deployments that are done in no particular context, where we want it to just work. -- like a directory containing a virtualenv and all the dependencies needed.

In both cases, preparing a deployment usually consists of fetching Python packages at PyPI and maybe compile some of them. These steps are usually done using tools like zc.buildout or virtualenv + pip, and in the case of Mozilla Services, a custom tool that transforms all dependencies into RPMs.

In one case we end up with a directory filled with everything needed to run the application, except the system dependencies, and in the other case with a collection of RPMs that can be deployed on the target system.

But in both cases, we end up using the same thing: a complete list of Python dependencies.

The trick with using tools like zc.buildout or pip is that from an initial list of dependencies, you end up pulling indirect dependencies. For instance, the Pyramid package will pull the Mako package and so on. A good practice is to have them listed in a single place and to pin each package to a specific version before releasing the app. Both pip and zc.buildout have tools to do this.

Deployments practices I have seen so far:
- a collection of rpms/debian packages/etc are built using tools like bdist_rpms etc. - a virtualenv-based directory is created in-place in production or as a pre-build binary release that's archived and copied in production - a zc-buildout-based directory is created in-place in production or as a pre-build binary release that's archived and copied in production

The part that's still fuzzy for everyone that is not using RPMs or Debian packages is how to list system-level dependencies. We introduced in PEP 345 the notion of hint where you can define system level dependencies which name may not be the actual name on the target system. So if you say you need libxml-dev, which is valid under Debian, people that deploy your system will know they'll need libxml-devel under Fedora. Yeah no magic here, it's a tough issue. see Requires-External.

The Standard

EDIT : Ian has a much more rich standard proposal here. (see the comments)

The standard I have in mind is a very lightweight standard that could be useful in all our deployment practices - it's a thin layer on the top of the WSGIstandard.

A wsgi application is a directory containing:
- a text file located in the directory at dependencies.txt, listing all dependencies - possibly reusing Pip's requirements format - a text file located in the directory at external-dependencies.txt, listing all system dependencies - possibly reusing PEP 345 format - a Python script located it the directory at bin/wsgiapp with an "application" variable. The shebang line of the Python script might also point to a local Python interpreter (a virtualenv version)

From there we have all kind of possible scenarios where the application can be built and/or run with the usual set of tools

Here's one example of a deployment from scratch :
- The repository of the project is cloned - A virtualenv is created in the repository clone - pip, which gets installed with virtualenv, is used to install all dependencies describes in dependencies.txt - gunicorn is used to run the app locally using "cd bin; gunicorn wsgiapp:application" - the directory is zipped and sent in production - the directory is unzipped - virtualenv is run again in the directory - the app is hooked to Apache+mod_wsgi

Another scenario I'd use in our RPM environment:
- The repository of the project is cloned - a RPM is built for each package in dependencies.txt - if possible, external-dependencies.txt is used to feed a spec file. - the app is deployed using the RPM collection

That's the idea, roughly -- a light standard to point a wsgi app and a list of dependencies.

Comments !