JESSE TOTH SAYS that upgrading an Internet service is like building a new bridge across San Francisco Bay. In building the new eastern span of the Bay Bridge, engineers didn’t tear down the old one and erect the new one in its place. They built the new span alongside the old one, before making sure the new bridge could handle the same traffic. Only then did they switch all the cars over and start tearing down the old span. As Toth explains, when it comes time to rebuild software that underpins a service like Google or Facebook or Uber, the process should work in much the same way. “You battle-test this new bridge—this new code path—while the original one is still being used,” she says.