Mercurial and Git both experience scaling pains as the number of heads in a repository approaches infinity. Operations like push and pull slow to a crawl and everyone gets frustrated.

This is the problem Mozilla's Try repository has been dealing with for years. We know the solution doesn't scale. But we've been content kicking the can by resetting the repository (blowing away data) to make the symptoms temporarily go away.

One of my official goals is to ship a scalable Try solution by the end of 2014.

Today, I believe I finally have enough code cobbled together to produce a working concept. And I could use your help testing it.

I would like people to push their Try, code review, and other miscellaneous heads to a special repository. To do this:

$ hg push -r . -f ssh://hg@hg.gregoryszorc.com/gecko-headless

That is:

• Consider the changeset belonging to the working copy
• Allow the creation of new heads
• Send it to the gecko-headless repo on hg.gregoryszorc.com using SSH

Here's what happening.

I have deployed a special repository to my personal server that I believe will behave very similarly to the final solution.

When you push to this repository, instead of your changesets being applied directly to the repository, it siphons them off to a Mercurial bundle. It then saves this bundle somewhere along with some metadata describing what is inside.

When you run hg pull -r on that repository and ask for a changeset that exists in the bundle, the server does some magic and returns data from the bundle file.

Things this repository doesn't do:

• This repository will not actually send changesets to Try for you.
• You cannot hg pull or hg clone the repository and get all of the commits from bundles. This isn't a goal. It will likely never be supported.
• We do not yet record a pushlog entry for pushes to the repository.
• The hgweb HTML interface does not yet handle commits that only exist in bundles. People want this to work. It will eventually work.
• Pulling from the repository over HTTP with a vanilla Mercurial install may not preserve phase data.

The purpose of this experiment is to expose the repository to some actual traffic patterns so I can see what's going on and get a feel for real-world performance, variability, bugs, etc. I plan to do all of this in the testing environment. But I'd like some real-world use on the actual Firefox repository to give me peace of mind.

Please report any issues directly to me. Leave a comment here. Ping me on IRC. Send me an email. etc.

Update 2014-11-21: People discovered a bug with pushed changesets accidentally being advanced to the public phase, despite the repository being non-publishing. I have fixed the issue. But you must now push to the repository over SSH, not HTTP.

http://gregoryszorc.com/blog/2014/11/20/test-drive-the-new-headless-try-repository