Server-side testing is a method of A/B testing that allows you to leverage back-end technologies to handle processing before anything is sent to the browser (i.e.: in the backend technology stack via some sort of server-side SDK type approach.
Optimizely Full Stack, LaunchDarkly, VWO Full Stack and SiteSpect are just some of the server-side testing tools that are available on the market, but there are many many more.
Most vendors have SDKS for a plethora of programming languages including Java, C#, Go, Objective-C, PHP, Python, React, Ruby, Node and Swift.
If you are using something a bit more obscure or low-level, it's likely not that difficult to come up with a new SDK that does not currently exist to support it. Most SDKs are fairly small and easier on the portability scale.
They're less well known or agreed upon in the experimentation community right now. In the old days, cloaking was a black-hat SEO technique in which the content presented to the search engine spider is different from that presented to the user's browser.
Theoretically, if you are not careful about the way your server-side tooling tracks and measures (or more accurately, should exclude entirely) robots and crawlers -- your server-side experiments (if large enough in nature which they often are) might unintentionally cause your actions to be misinterpreted as cloaking by major search engines.
Unfortunately, it is generally not very straight forward to re-use the same existing goals or events that may exist within your web tracking tooling. If you have a well constructed data layer (client-side), this may not be easily accessible from the server-side so thinking about the overall application architecture is important.
If you'd like to explore some integration POCs, let us know.
It depends upon the architecture and structure of your application (and what language it's written in). Generally, to make a server-side testing engagement go smoothly requires some degree of ensuring alignment between marketing team and product/development teams. If you'd like to understand more about how we may be able to assist, get in touch.