Post-processing in MARS and dissemination is performed by MIR, a library of routines for Meteorological Interpolation and Regridding (see ECMWF newsletter no.152 for a description).
Post-processing in MARS is performed by the client (only available on ECMWF computers). When using the Web API (or dissemination) the data is first processed at ECMWF prior to its transmission to the user.