Companies like Facebook, Google, Quora, PayPal etc have huge codebase. I agree they must be having good test coverage, but how do they make sure that everything works as expected after each deployment?
What does the deployment pipeline look like?
At Google we had 3 test servers that were for automated testing and 1 human-based testing that some early Google users have access to. After there isn't any bugs etc in the bug reporting service + they don't spot any bugs (which is actually very rare, despite the huge amount of testing), the code is pushed to production.
Here is what Facebook is doing: facebook.com/FacebookforDevelopers/videos/1015280…
Recorded at the Facebook developer conference 'f8' this year.
Jan Vladimir Mostert
Idea Incubator
Netflix is using Spinnaker, see techblog.netflix.com/2015/11/global-continuous-de… if you have a look in a deployed instance of Spinnaker, there are options for manual testing as well as automated testing to complete the pipeline- so they probably use both.
If you have a look at how Pivotal's cloud works, they simply monitor for errors, if the number of errors rapidly increase, they automatically rollback to the previous version.