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“Wanted to add one additional note from the perspective of someone who was intimately involved in developing this system.

When Eric writes, "By the time we started doing continuous integration, we had tens of thousands of lines of code, all not under test coverage." he is substantially understating how far we were from the eventual system.

We had closer to several hundreds of thousands of lines of code. Most of this code was from a variety of open source PHP projects that were glued together with the shortest path to goal possible. Most of this was code that not scalable, not secure, and not particularly extensible. Most of the code had an internal structure that would cause reasonable architect to conclude that the best thing to do was start a re-write project.

Nonetheless through a consistent application of root cause analysis over we migrated to a highly scalable and highly available system.” source...