That C++ compiler is a single product (okay, you might have built a linker, and an assembler as well - say 3-5 products). In even medium enterprises (say, 500 employees, about 250 developers) you might have upwards of 35 different products, each of which with a 5-6 year active history.

Enterprise source control can be ugly - particularly if you have non-text resources (Art, Firmware Binaries, tools) that need to be checked in and version managed as well.

With all that said - I don't really understand why all the code is in a single repository. Surely a company of Facebook's size would experience some fairly great benefits from compartmentalization and published service interfaces. I guess I agree with the parent - sounds like a lot of intertwined spaghetti code. :-)

There're costs and benefits both ways. AFAIK, Microsoft and Amazon both use the separate repositories model, and Google and Facebook use a single large repository. Most people I know that have worked at both of these styles prefer the Google/Facebook style.

The biggest advantage of a single repository is pretty intangible - it's cultural. When anyone can change anything or can use any code, people feel like the whole company belongs to them, and they're responsible for Google/Facebook's success as a whole. People will spontaneously come together to accomplish some user need, and they can refactor to simplify things across component boundaries, and you don't get the sort of political infighting that tends to plague large organizations where people in subprojects never interact with each other.

I think if it were my company, I'd want the single repository model, but there need to be tools and practices to manage API complexity. I dunno what those tools & practices would look like; there are some very smart people in Google that are grappling with this problem though.

Why is a single repo required for everybody to see all the code? Tools like gerrit and github can handle multiple repos and provide commit access for multiple repos among a large group of people. If it were my company, I would keep separate repos but allow read and merge requests for all employees. That keeps everybody involved in projects across the entire company, but also allows them to notice when individual projects get spaghettified and thereby deserving of some cleanup/breakup into components. A GB-scale codebase does not help smart, new employees grok what the hell they can contribute.

It's not a matter of being able to see all the code, it's a matter of being able to see and modify all the code. It allows you to have a "just fix it" culture when people see something's broken, and it lets you write changes that span multiple projects without worrying about how your change will behave when it can't be committed atomically.