  Multiscale Finite Element Methods (MsFEMs) are now well-established finite
element type approaches dedicated to multiscale problems. They first compute
local, oscillatory, problem-dependent basis functions that generate a suitable
discretization space, and next perform a Galerkin approximation of the problem
on that space. We investigate here how these approaches can be implemented in a
non-intrusive way, in order to facilitate their dissemination within industrial
codes or non-academic environments. We develop an abstract framework that
covers a wide variety of MsFEMs for linear second-order partial differential
equations. Non-intrusive MsFEM approaches are developed within the full
generality of this framework, which may moreover be beneficial to steering
software development and improving the theoretical understanding and analysis
of MsFEMs.
