As of 5.0+, Alba only supports .Net 5.0 or greater. You can still use older versions of Alba to test previous versions of ASP.Net Core.
What is Alba?
Alba is a class library that you use in combination with unit testing tools like xUnit.Net or NUnit to author integration tests against ASP.NET Core HTTP endpoints. Alba scenarios actually exercise the full ASP.Net Core application by running HTTP requests through your ASP.NET system in memory using the built in ASP.Net Core TestServer.
You can certainly write integration tests by hand using the lower level
HttpClient, but you'll write much less code with Alba to author integration tests against your ASP.Net Core web services. Moreover, Alba scenarios were meant to be declarative to maximize the readability of the integration tests, making those tests much more valuable as living technical documentation.
What is now Alba was originally created in the early 2010's as a mechanism to test the content negotiation features in FubuMVC, an alternative web application framework that predates ASP.Net Core. We needed to run the entire web handling stack including all the middleware and HTTP endpoints just as the application would be configured. The early scenario support in FubuMVC was a way to run HTTP requests end to end completely in memory and make declarative checks on expected HTTP behavior.
As FubuMVC wound down as a project, the scenario testing code was first extracted out into its own library called Alba, then ported to depend on an OWIN-based execution pipeline. And then again with the advent of ASP.Net Core and the deprecation of OWIN, Alba was again re-wired to use the newer HTTP abstractions from the ASP.Net Core HttpContext type. At this point, Alba is a value added wrapper around the ASP.Net Core TestServer.
The scenario testing in Alba was inspired by the testing support in the Scala PlayFramework.