(fuzzy...) Mainframes and mini-computers had test batches run on the same hardware as the real batches. Later for OLTP/continuous bank (ATM) systems, the big

(fuzzy...) Mainframes and mini-computers had test batches run on the same hardware as the real batches. Later for OLTP/continuous bank (ATM) systems, the big variable was storage: when to attach a disk you've been hoarding since it could take a quarter or more to get another one, but you're flirting with capacity issues. So there was some relatively ad-hoc reconfiguration of production machines. eBay was probably the first internet company to really scale to national/international always-on services, first with perl then java. They had (have?) two-week release "trains", with hundreds of branches being merged in a topological order (in parallel: merge processes happening for multiple trains). What train you were on determined what services and resources were available. Only the best of the best actually did the merges. Google watched eBay tangled in IBM ClearCase and decided: one repo, no branches. The now-comment 3-stage environment (test, staging, production) is relatively simple, depending on your data pipelines.