Once upon a time, our software was simple. All it took was a database, a user interface, and some glue code between them, running behind a web server farm, to go online. Most of the business-critical data stayed in the database. People rented space in colocation centers and bought servers to run their databases and code. Those colocation centers were not reliable, and your business could go down due to power or cooling failures. The servers were also not reliable, and you could lose data due to disk failures. A nasty cable damage somewhere, and the network could be out for hours or days. To get around such booboos, IT teams rented space in secondary colocation centers, bought servers and expensive replication systems to replicate data to those secondary sites, and took steps to bring up secondary sites in the event of disasters.