Add PEP 481 - Migrate Some Supporting Repositories to Git and Github
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Donald Stufft
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PEP: 481
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Title: Migrate Some Supporting Repositories to Git and Github
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Version: $Revision$
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Last-Modified: $Date$
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Author: Donald Stufft <donald@stufft.io>
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Status: Draft
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Type: Process
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Content-Type: text/x-rst
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Created: 29-Nov-2014
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Post-History: 29-Nov-2014
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Abstract
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========
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This PEP proposes migrating to Git and Github for certain supporting
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repositories (such as the repository for Python Enhancement Proposals) in a way
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that is more accessible to new contributors, and easier to manage for core
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developers. This is offered as an alternative to PEP 474 which aims to achieve
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the same overall benefits but while continuing to use the Mercurial DVCS and
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without relying on a commerical entity.
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In particular this PEP proposes changes to the following repositories:
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* https://hg.python.org/devguide/
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* https://hg.python.org/devinabox/
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* https://hg.python.org/peps/
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This PEP does not propose any changes to the core development workflow for
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CPython itself.
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Rationale
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=========
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As PEP 474 mentions, there are currently a number of repositories hosted on
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hg.python.org which are not directly used for the development of CPython but
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instead are supporting or ancillary repositories. These supporting repositories
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do not typically have complex workflows or often branches at all other than the
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primary integration branch. This simplicity makes them very good targets for
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the "Pull Request" workflow that is commonly found on sites like Github.
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However where PEP 474 wants to continue to use Mercurial and wishes to use an
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OSS and self-hosted and therefore restricts itself to only those solutions this
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PEP expands the scope of that to include migrating to Git and using Github.
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The existing method of contributing to these repositories generally includes
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generating a patch and either uploading them to bugs.python.org or emailing
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them to peps@python.org. This process is unfriendly towards non-comitter
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contributors as well as making the process harder than it needs to be for
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comitters to accept the patches sent by users. In addition to the benefits
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in the pull request workflow itself, this style of workflow also enables
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non techincal contributors, especially those who do not know their way around
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the DVCS of choice, to contribute using the web based editor. On the committer
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side the Pull Requests enable them to tell, before merging, whether or not
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a particular Pull Request will break anything. It also enables them to do a
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simple "push button" merge which does not require them to check out the
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changes locally. Another such feature that is useful in particular for docs,
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is the ability to view a "prose" diff. This Github specific feature enables
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a committer to view a diff of the rendered output which will hide things like
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reformatting a paragraph and show you what the actual "meat" of the change
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actually is.
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Why Git?
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--------
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Looking at the variety of DVCS which are available today it becomes fairly
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clear that git has gotten the vast mindshare of people who are currently using
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it. The Open Hub (Previously Ohloh) statistics [#openhub-stats]_ show that
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currently 37% of the repositories Open Hub is indexing is using git which is
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second only to SVN (which has 48%) while Mercurial has just 2% of the indexed
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repositories (beating only bazaar which has 1%). In additon to the Open Hub
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statistics a look at the top 100 projects on PyPI (ordered by total download
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counts) shows us that within the Python space itself there is a majority of
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projects using git:
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=== ========= ========== ====== === ====
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Git Mercurial Subversion Bazaar CVS None
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=== ========= ========== ====== === ====
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62 22 7 4 1 1
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=== ========= ========== ====== === ====
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Chosing a DVCS which has the larger mindshare will make it more likely that any
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particular person who has experience with DVCS at all will be able to
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meaningfully use the DVCS that we have chosen without having to learn a new
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tool.
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In addition to simply making it more likely that any individual will already
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know how to use git, the number of projects and people using it means that the
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resources for learning the tool are likely to be more fully fleshed out and
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when you run into problems the liklihood that someone else had that problem
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and posted a question and recieved an answer is also far likelier.
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Thirdly by using a more popular tool you also increase your options for tooling
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*around* the DVCS itself. Looking at the various options for hosting
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repositories it's extremely rare to find a hosting solution (whether OSS or
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commerical) that supports Mercurial but does not support Git, on the flip side
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there are a number of tools which support Git but do not support Mercurial.
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Therefore the popularity of git increases the flexibility of our options going
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into the future for what toolchain these projects use.
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Also by moving to the more popular DVCS we increase the likelhood that the
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knowledge that the person has learned in contributing to these support
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repositories will transfer to projects outside of the immediate CPython project
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such as to the larger Python community which is primarily using Git hosted on
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Github.
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In previous years there was concern about how well supported git was on Windows
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in comparison to Mercurial. However git has grown to support Windows as a first
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class citizen. In addition to that, for Windows users who are not well aquanted
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with the Windows command line there are GUI options as well.
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On a techincal level git and Mercurial are fairly similar, however the git
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branching model is signifcantly better than Mercurial "Named Branches" for
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non-comitter contributors. Mercurial does have a "Bookmarks" extension however
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this isn't quite as good as git's branching model. All bookmarks live in the
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same namespace so it requires individual users to ensure that they namespace
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the branchnames themselves lest the risk collision. It also is an extension
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which requires new users to first discover they need an extension at all and
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then figure out what they need to do in order to enable that extension. Since
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it is an extension it also means that in general support for them outside of
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Mercurial core is going to be less than 100% in comparison to git where the
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feature is built in and core to using git at all. Finally users who are not
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used to Mercurial are unlikely to discover bookmarks on their own, instead they
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will likely attempt to use Mercurial's "Named Branches" which, given the fact
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they live "forever", are not often what a project wants their contributors to
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use.
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Why Github?
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-----------
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There are a number of software projects or web services which offer
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functionality similar to that of Github. These range from commerical web
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services such as a Bitbucket to self-hosted OSS solutions such as Kallithea or
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Gitlab. This PEP proposes that we move these repositories to Github.
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There are two primary reasons for selecting Github: Popularity and
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Quality/Polish.
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Github is currently the most popular hosted repository hosting according to
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Alexa where it currently has a global rank of 121. Much like for Git itself by
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choosing the most popular tool we gain benefits in increasing the likelhood
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that a new contributor will have already experienced the toolchain, the quality
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and availablity of the help, more and better tooling being built around it, and
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the knowledge transfer to other projects. A look again at the top 100 projects
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by download counts on PyPI shows the following hosting locations:
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====== ========= =========== ========= =========== ==========
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GitHub BitBucket Google Code Launchpad SourceForge Other/Self
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====== ========= =========== ========= =========== ==========
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62 18 6 4 3 7
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====== ========= =========== ========= =========== ==========
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In addition to all of those reasons, Github also has the benefit that while
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many of the options have similar features when you look at them in a feature
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matrix the Github version of each of those features tend to work better and be
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far more polished. This is hard to quantify objectively however it is a fairly
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common sentiment if you go around and ask people who are using these services
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often.
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Finally a reason to choose a web service at all over something that is
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self-hosted is to be able to more efficiently use volunteer time and donated
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resources. Every additional service hosted on the PSF infrastruture by the
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PSF infrastructure team further spreads out the amount of time that the
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volunteers on that team have to spend and uses some chunk of resources that
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could potentionally be used for something where there is no free or affordable
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hosted solution available.
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One concern that people do have with using a hosted service is that there is a
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lack of control and that at some point in the future the service may no longer
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be suitable. It is the opinion of this PEP that Github does not currently and
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has not in the past engaged in any attempts to lock people into their platform
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and that if at some point in the future Github is no longer suitable for one
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reason or another than at that point we can look at migrating away from Github
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onto a different solution. In other words, we'll cross that bridge if and when
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we come to it.
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Example: Scientific Python
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--------------------------
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One of the key ideas behind the move to both git and Github is that a feature
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of a DVCS, the repository hosting, and the workflow used is the social network
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and size of the community using said tools. We can see this is true by looking
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at an example from a sub-community of the Python community: The Scientific
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Python community. They have already migrated most of the key pieces of the
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SciPy stack onto Github using the Pull Request based workflow starting with
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IPython and as more projects moved over it became a natural default for new
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projects.
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They claim to have seen a great benefit from this move, where it enables casual
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contributors to easily move between different projects within their
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sub-community without having to learn a special, bespoke workflow and a
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different toolchain for each project. They've found that when people can use
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their limited time on actually contributing instead of learning the different
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tools and workflows that not only do they contribute more to one project, that
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they also expand out and contribute to other projects. This move is also
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attributed to making it commonplace for members of that community to go so far
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as publishing their research and educational materials on Github as well.
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This showcases the real power behind moving to a highly popular toolchain and
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workflow, as each variance introduces yet another hurdle for new and casual
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contributors to get past and it makes the time spent learning that workflow
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less reusable with other projects.
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Migration
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=========
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Through the use of hg-git [#hg-git]_ we can easily convert a Mercurial
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repository to a Git repository by simply pushing the Mercurial repository to
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the Git repository. People who wish to continue to use Mercurual locally can
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then use hg-git going into the future using the new Github URL, however they
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will need to re-clone their repositories as using Git as the server seems to
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trigger a one time change of the changeset ids.
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As none of the selected repositories have any tags, branches, or bookmarks
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other than the ``default`` branch the migration will simply map the ``default``
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branch in Mercurial to the ``master`` branch in git.
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In addition since none of the selected projects have any great need of a
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complex bug tracker, they will also migrate their issue handling to using the
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GitHub issues.
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In addition to the migration of the repository hosting itself there are a
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number of locations for each particular repository which will require updating.
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The bulk of these will simply be changing commands from the hg equivilant to
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the git equivilant.
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In particular this will include:
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* Updating www.python.org to generate PEPs using a git clone and link to
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Github.
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* Updating docs.python.org to pull from Github instead of hg.python.org for the
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devguide.
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* Enabling the ability to send an email to python-checkins@python.org for each
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push.
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* Enabling the ability to send an IRC message to #python-dev on Freenode for
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each push.
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* Migrate any issues for these projects to their respective bug tracker on
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Github.
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This will restore these repositories to similar functionality as they currently
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have. In addition to this the migration will also include enabling testing for
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each pull request using Travis CI [#travisci]_ where possible to ensure that
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a new PR does not break the ability to render the documentation or PEPs.
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User Access
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===========
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Moving to Github would involve adding an additional user account that will need
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to be managed, however it also offers finer grained control, allowing the
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ability to grant someone access to only one particular repository instead of
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the coarser grained ACLs available on hg.python.org.
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References
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==========
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.. [#openhub-stats] `Open Hub Statistics <https://www.openhub.net/repositories/compare>`
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.. [#hg-git] `hg-git <https://hg-git.github.io/>`
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.. [#travisci] `Travis CI <https://travis-ci.org/>`
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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