29 April 2020
Dark launching a feature means taking a new or changed back-end behavior and calling it from existing users without the users being able to tell it's being called. It's done to assess the additional load and performance impacts upon the system before making a public announcement of the new capability.
An example of this might be adding cross-selling recommendations to a retail site's checkout flow. (The kind of thing that says if I buy a toaster, I clearly want to also buy a new set of steak knives.) Figuring out recomendations will obviously add some time and load to the system, slowing down the checkout process and potentially the whole site. Dark launching gives us a way of assessing this impact before we release the feature to the public. We begin by using an Keystone Interface to build the new feature, integrating into production releases of the code, but with no user-interface so the users cannot tell it's there. But before we add the keystone, we modify the checkout flow to call the recommendation engine just as we would do in production, but not showing the results in the UI. This way the recommendation does all the work it would do when it's released, but nobody can see that it's doing it. If we use a Feature Flag we can switch the recommendation on and off easily in production, so if we do see an worrying impact on performance we switch it off before anyone really notices. We can then continue to tune the recommendation engine until its performance is acceptable, at which point we can finally add the keystone and reveal the feature to the world
Dark launching can also enable parallel running of a re-implemented feature. The old and new code can both be called and their results checked to see if there are changes with the new algorithm, but only one answer returned to the interface.
Dark launching works best when it's a process that enhances existing user interactions and isn't something users choose to do. To test something that depends on a user's choice, Canary Release is the way to go.
Since the term first appeared, however, its usage has been subject to Semantic Diffusion. So I've heard people use "dark launch" to mean canary releases, or other variations on partial release strategies.