[Distutils] Serverside Dependency Resolution and Virtualenv Build Server

Thomas Güttler guettliml at thomas-guettler.de
Thu Nov 12 03:55:49 EST 2015

Am 11.11.2015 um 13:59 schrieb Donald Stufft:
> On November 11, 2015 at 1:30:57 AM, Thomas Güttler (guettliml at thomas-guettler.de) wrote:
>> Maybe I am missing something, but still think server side dependency resolution is possible.
> I don’t believe it’s possible nor desirable to have the server handle dependency resolution, at least not without
> removing some currently supported features and locking out some future features from ever happening.

I can understand you, if you say it is not desirable.

I like the general concept of simple clients and solving complicated stuff at the server.

Now to "possible":

  - What features are not supported if you do resolve dependencies on the server?
  - What features are not possible in the future?

> Currently pip can be configured with multiple repository locations that it will use when resolving dependencies. By
> default this only includes PyPI but people can either remove that, or add additional repository locations. In order
> to support this we need a resolver that can union multiple repositories together before doing the resolving. If the
> repository itself was the one handling the resolution than we are locked into a single repository per invocation of
> pip.

I am aware of that. In our company the CI system has no access to pypi.org. All packages come from our package server 
which contains a mirror of some pypi packages.

If this can be done on the client side today, I see no problem doing this on the server-side tomorrow.

> Additionally, pip can also be configured to use a simple directory full of files as a repository. Since this is just
> a simple directory, there *is* no server process running that would allow for a server side resolver to happen and
> pip either *must* handle the resolution itself in this case or it must disallow these feature all together.

Same as above: can be done on a server, too.

> Additionally, the fact that we currently treat the server as a “dumb” server, means that someone can implement a PEP
> 503 compatible repository very trivially with pretty much any web server that supports static files and automatically
> generating an index for static files. Switching to server side resolution would require removing this capability and
> force everyone to run a dedicated repository software that can handle that resolution.

You currently treat the server as a "dump" server. That's ok.

Did I think I want to replace your server with my idea? I am very sorry if you thought this way.

My solution is optional and just an idea. I never meant that pypi.or or the new wheel server should use my idea.

You use the word "force". Nobody gets forced just because there is an alternative.

> Additionally, we want there to be as little variance in the requests that people make to the repository as possible.
> We utilize a caching CDN layer which handles > 80% of the total traffic to PyPI which is the primary reason we’ve
> been able to scale to handling 5TB and ~50 million requests a day with a skeleton crew of people. If we move to a
> server side dependency resolution than we reduce our ability to ensure that as many requests as possible are served
> directly out of the cache rather than having to be go back to our backend servers.

Your thoughts were too fast. There are a lot of private package hostings servers in intranets of companies.

In this context the load can be handled very well. And if you have CI-Systems asking for the same stuff
over and over again, caching could improve the speed very much. You can do caching at high level: all
projects going through CI in one company benefit.

> Finally, we want to move further away from trusting the actual repository where we can. In the future we’ll be
> allowing package signing that will make it possible to survive a compromise of the repository. However there is no
> way to do that if the repository needs to be able to dynamically generate a list of packages that need to be
> installed as part of a resolution process because by definition that needs to be done on the fly and thus must be
> signed by a key that the repository has access too if it’s signed at all. However, since the metadata for a package
> can be signed once and then it never changes, that can be signed by a human when they are uploading to PyPI and than
> pip can verify the signature on that metadata before feeding it into the resolver. This would allow us to treat PyPI
> as just an untrusted middleman instead of something that is essentially going to be allowed to force us to execute
> arbitrary code whenever someone does a pip install (because it’ll be able to instruct us to install any package, and
> packages can contain arbitrary code).

My idea is made of two parts which don't depend on each other.

The main (first) part is dep resolution on server:

Input: install_requires list with fuzzy version requirements
Output: version pinned package list.

If the server was hacked. What could a black hat hacker have done?
He could send you an evil line in the result. Instead of "Django==1.8.3" he could
send you "Django-with-my-evil-hacks-included==1.8.3".

It is still up to the client if he install the requirements that the server gave you.
These packages can be downloaded individually and checked with the way you want
pip the check packages in the future.

I understand you fear for the second part: Creating one package from a list
of version-pinned requirements.

> Hopefully that answers your question about why it’s unlikely that we’ll ever move to a server side dependency
> resolver because even though it is possible to do so, doing it would severely regress a number of very important
> features.

I just wanted to share my idea: https://github.com/guettli/virtualenv-build-server

The idea is in the public domain. I will happily coach developers who
want to implement it. I won't implement the idea myself :-)

   Thomas Güttler

Thomas Guettler http://www.thomas-guettler.de/

More information about the Distutils-SIG mailing list