I guess large is a relative question. For me ~600K lines of C++ is a pretty large code base. Apparently Facebook Messenger just on Android is 10M lines of Kotlin in another comment. To me that seems like they are probably doing something wrong.
But to address your question here is my recollection of what happened, now more than 20 years ago fwiw. We were given the code and I spent maybe 6-7 days, all day, reading it and analyzing it with this tool I had, called Source Navigator [1]. Then we spent 1 full work week at the other companies HQ, mostly in meetings asking questions on different modules and classes. Then when we returned to our offices it took me another 2 weeks of work to get the system setup and deploy a few components inside our own middleware system. I was the primary c++ expert, there was another business analyst and I had a more junior developer who worked with me. So in comparison to Twitter certainly a much much smaller scale situation. The team that had written the system was around 15 people.
We definitely used the code and I don’t recall it being much a problem at all that other people had written it. Plenty of open source projects have random contributors show up and work fine in their code base.
I think a lot of SWEs have pretty big egos and tend to overestimate how special or unique their particular projects are based on my own professional experience. This particular situation was an example but there have been plenty others. When I fix or find bugs in other’s code sometimes they are surprised which for me is always surprising. Why are you so surprised I can debug your code?
But to address your question here is my recollection of what happened, now more than 20 years ago fwiw. We were given the code and I spent maybe 6-7 days, all day, reading it and analyzing it with this tool I had, called Source Navigator [1]. Then we spent 1 full work week at the other companies HQ, mostly in meetings asking questions on different modules and classes. Then when we returned to our offices it took me another 2 weeks of work to get the system setup and deploy a few components inside our own middleware system. I was the primary c++ expert, there was another business analyst and I had a more junior developer who worked with me. So in comparison to Twitter certainly a much much smaller scale situation. The team that had written the system was around 15 people.
We definitely used the code and I don’t recall it being much a problem at all that other people had written it. Plenty of open source projects have random contributors show up and work fine in their code base.
I think a lot of SWEs have pretty big egos and tend to overestimate how special or unique their particular projects are based on my own professional experience. This particular situation was an example but there have been plenty others. When I fix or find bugs in other’s code sometimes they are surprised which for me is always surprising. Why are you so surprised I can debug your code?
[1] https://sourcenav.sourceforge.net/
(I’d be curious if people have a favorite more modern version of a tool like Source Navigator)