This system was actually built incrementally over several months and did not start out as a full VCS -- it started as a read-only service that ingested data from our Git repo to take pressure off of CI and other tools needing to materialize checkouts for certain types of repo/graph questions. Even when we did decide to go all the way, we were operating in a hybrid Git/Scion world for months during the migration and the path to get there made the system and code very unique to our infrastructure.
This is very similar to the way SCMQuery existed before Mononoke and eventually became served by it. The source control team at Meta was, and is, quite familiar with hybrid deployments of the kind you describe, though of course at longer timescales.
This system was actually built incrementally over several months and did not start out as a full VCS -- it started as a read-only service that ingested data from our Git repo to take pressure off of CI and other tools needing to materialize checkouts for certain types of repo/graph questions. Even when we did decide to go all the way, we were operating in a hybrid Git/Scion world for months during the migration and the path to get there made the system and code very unique to our infrastructure.