Where I work, we have several dozen repos, on for each component, and are thinking about merging it into a monorepo soon. That will end up giving us a repo with several different root commits.

In fact, for at least one of our components, we've already done this. We had something that solved the same problem for a couple of different permutations of software versions and platforms. The person who had originally written it had used separate repos, with common code copied and pasted between them, and later on we merged the repos together and factored out the common code so that could be shared. So we already have at least one repo which has 4 root commits, I believe.

Actually, I just checked and it has 7 root commits. I'd forgotten that we had also had similar problems that needed to be solved for some other software, and had some interns who had done those in separate repos as well, which also eventually got merged in to share all of the common code:

  $ git log --max-parents=0 --pretty="format:%h %cd %s" --date=short
  666c0bf 2012-07-09 Initial tree.
  528ca3a 2012-06-12 first commit
  50f1b18 2012-06-12 first commit
  86d66e0 2011-11-17 initial
  76a7789 2011-03-21 initial commit
  58f7afc 2010-05-20 initial
  deb50b7 2009-10-29 initial commit

That's a fantastic idea. Do you have any photos of the hoodies?

Yep, here's one: https://i.imgur.com/LEI4BAA.jpg

It makes for a great conversation piece, even around non-tech crowds.

My company got to ~20 services in separated repos prior to moving them all into one monorepo, so we've ended up with 20 root commits and an enormous commit graph around the time where it all converged together. We did do them all separately though - rather than as an octopus merge.

Has been particularly interesting to balance the different languages in the same repo. Few painful bits, but overall has worked out really well.

We also build the entire thing into a single Docker container containing every service. You just start the particular one you want via the command (the entrypoint is a little shell script to invoke the correct executable).

What's the advantage of having all that in one repository? And what's the advantage of provisioning every container with functionality it will never need?

I don't know about that container strategy, but the thing I like about monorepos in that situation is that you can enforce having every commit to master represent a state where all your services and tools work together.

Otherwise if you ever need to do a rollback or post mortem or similar you have to figure out what commit out of the 20 different projects was actually being used at the time.

Of course that comes at a cost and there are other ways to get the same benefit, but my experience is that this is an easy way to keep things consistent.

In my opinion (I work at said company) the best thing about the monorepo is that 1 feature that touches several parts of the infrastructure can be in a single PR, and therefore is much easier to review.

This. Where I work it's common for features to involve 3-4 repos and it makes testing and deploying a huge pain. This leads to people sticking code in the wrong place as a work around, defeating the purpose of splitting the code into services in the first place.

There are a few main benefits of a monorepo:

- much less admin, don’t need to maintain/update so many repos

- easier to coordinate changes between systems at the same time

- everything is on a single version of each library - you don’t have some services on one version, some on another, etc

Benefits of having a single Docker image:

- much faster (overall) build times - doing everything in one go is way faster than doing each one individually

- much faster deployment - only have to download the image once per host, rather than many variations

- most services have a similar set of the same base dependencies, so reduces duplication

- overall the difference in image size between including one service and including all of them is probably only 5-10MB anyway

Those are just the things which immediately come to mind. There are probably many others.

All of the items in your mono-container advantage list go away if you consider the alternative of having a bunch of separate images that share a common base image.

Image layers are immutable and get deduplicated/cached individually when stored or sent over the network. For example if you pull 10 images with a common base to the same host, the first image might be slow to download but after that docker only fetches the deltas between it and the rest.

I am well aware of that. That doesn’t change the significant decrease in admin time spent, or the improvement in usability for our engineers.

Pulling in an unrelated git repo's objects in order to integrate that software with yours is an everyday occurrence. I suspect, lots of git repos have multiple root commits; some probably much more than just two.