When you want to run a local version of a service, you start it locally as normal and configure either local stubs of dependencies and dependents, or you can hook up the actual “production” services to your local service process. If doing the latter, you can create special user accounts and events that change how the networking routing happens. Events from those fake users pass through regular production apps up until the service you’re testing, then they are instead routed to your local version, and you can continue the calls to other prod services.

Uber employees’ apps are special and allow us to log in as these fake users and create fake rides or deliveries, and then we can look at the traces and logs to debug and stuff.