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Google has tons of services internally that talk via RPC. The monolithic repo means that it's much easier to hunt down and find people that are (say) using an outdated RPC method and help them fix their code.

(Just one example of how it's useful even when things are mostly services.)

To add to your point: let's differentiate between a monolithic REPOSITORY vs a monolithic CODE BASE. Having all the code in a monolithic REPOSITORY allows developers to more efficiently explore and transform the code with a better understanding of the breadth of their impact.

Whether the CODE BASE is monolithic or not is orthogonal to the repository's nature. I was G for a couple years and I'd say they've done an ok job of breaking things up into libraries and services. Certainly there are interfaces that have done a better or worse job of setting the code up for open sourcing but because of the nature of the repository, large scale refactoring is more efficiently accomplished.

This discourages you from ever making breaking changes to an API. On the face of it that sounds good but sometimes you do have to make breaking changes. My guess is that there are many duplicate projects in the Google code base for when a breaking change is needed. This is a way to sidestep the problem.

I think sometimes it's actually the opposite. I was able to make a breaking change to an API and update all of the callers to use the new one in a single commit.

Tests would run on all the clients and, since in my workspace the server was updated simultaneously, I could be more sure it would work.

That's fine for small breaking changes like an API being renamed, but some times breaking changes require actually refactoring code which is hard/impossible to do without intimate knowledge of a codebase.

Think python 2->3 or Angular 1->2. These types of changes do happen, and I bet they happen at Google. I don't think anyone is rewriting a downstream app when they make these changes. Most likely they are doing something like forking the library and renaming it, which is just another form of versioning.

A talk and a paper on how we do large-scale refactoring in the C++ parts of the codebase

https://isocpp.org/blog/2015/05/cppcon-2014-large-scale-refa...

http://research.google.com/pubs/pub41342.html

python 2->3 and angular 1->2 indeed need a different strategy and the strategy varies per case. For Angular both versions will have to be supported (basically just with different directory name) and switch most probably won't happen in terms of single applications, but by writing completely new applications on new stack. Lifespan of web application is rather short, so we can afford this.

As someone who's responsible for migrating the users of a large breaking change (~300 projects), it does discourage you from making a breaking change. I'm not convinced that this is a bad thing though. Breaking changes can be both expensive and tempting. By making the author of such a change feel the cost of it you prevent the migration work from becoming an externality.

It also encourages writing tools for automating this stuff. A big part of the motivation for polyup (https://github.com/PolymerLabs/polyup) was the fact that we'd be responsible for getting people to upgrade.

I'd be interested on an analysis of the meta-differences between those, but I think one of the main ones is a monolithic codebase makes it massively harder to open source components. I'm sure that, working on GitLab, you can agree with that - if GitLab were a massive, monolithic codebase and you wanted to open source specific parts of it, it'd be a huge pain.

Indeed, if you have a monolithic codebase releasing your code for other people use is much harder. In git it is possible to create a separate repository for it (using subtree and filter-branch). But the harder part is versioning and release management. At Google they avoid the cost of versioning their services and the release management around that, but this makes it really hard to have the outside world use their code. Maybe this is hard anyway since their code probably depends on a lot of services common to Google (GFS, BigTable). This is probably why they can't release Borg/Omega but have to make Kubernetes instead.

I think that advantages mentioned in the presentation about the monolithic codebase can be achieved if you have one source code server that is open to everyone (for example GitLab with most projects set to internal). Some of the tools will be easier to write for one repository than iterating over many, but that seems solvable.

The biggest advantage seems to be that when you are an author of a dependency you can propose upgrades to all services that use your application. It is not clear to me but it seems that for small changes you can just force that change on the code owners. This ensures that the dependency author incurs the cost of a change (as is done for API changes in the Linux kernel) and that you do not need to version the API of the dependency.

Interestingly Google recently started marking API's private by default. So they are moving in the direction of explicit API management.

As soon as you work with people that are outside your control (as is common in open source) you would need to version the API as well in my opinion.