Software developement in scientific computing is often based on old fashioned architectures inherited from the 80s (typically monolithic codes with static libraries in C or Fortran). These architectures are clearly not well suited for large scale collaborative development, which is now necessary to face current challenges in multiphysic simulation. Num3Sis is a modular platform developed at INRIA and is based on two new concepts : the concept of "node" which represents a visual wrapper of fundamental concepts in simulation, such as data (grid, field), process (mesher, field generator) or view. For a given abstract node, several implementations can be found, each of them being embedded in a plugin system managed by a factory. The second concept is the "composition", which represents the algorithmic pipeline used to link the nodes together, using a visual programing interface. It provides a highly flexible, re-usable and efficient approach to develop new computational scenarii.