Introduction
When one has multiple programs serving the same purpose (for instance SMTP servers such as sendmail, exim and postfix; or print servers such as lprng and cups), it is usual to wrap these using alternatives. Alternatives provides a clean way to have many types of software serving the same purpose installed at the same time and have the commands such as mail and lpr point to the wanted versions.
However, when there are multiple variants that each serve the needs of some user and thus must be available simultaneously by users, the alternatives system simply isn't enough since it is system-wide. This has been reality on supercomputers and clusters for eons, and multiple implementations of a solution has been developed: environment modules and Lmod. Fedora currently makes use of this primarily for handling switching between differen MPI implementations.
Environment modules are also useful in situations where a package wants to install binaries that use common names and might conflict file in or otherwise pollute /usr/bin. Use must then load an environment module before being able to make use of those programs.
Using environment modules
To see what modules are available, run $ module avail.
To load a module run e.g. $ module load mpi/openmpi-x86_64.
To unload a module, run e.g. $module unload mpi/openmpi-x86_64.
The upstream documentation for the module command is available here or with man module.
Creating environment modules
When creating RPMs containing module files, be sure to specify
Requires: environment(modules) for directory ownership and usability, and BuildRequires: environment(modules) for the %{_modulesdir} macro.To install an environment module, place a module file into %{_modulesdir}, which should evaluate to /usr/share/modulefiles. This macro is available in Fedora 21+ and EPEL 7+. The directory /usr/share/Modules/modulefiles is to be used only for internal modules of environment-modules. /etc/modulefiles is available to local system administrator use.
Suffix the module file name with the architecture to prevent clashes from multilib/multiarch packages.
The module files are plain text with optional tcl syntax, for instance an environment module for 64-bit OpenMPI mpi/openmpi-x86_64:
#%Module 1.0 # # OpenMPI module for use with 'environment-modules' package: # conflict mpi prepend-path PATH /usr/lib64/openmpi/bin prepend-path LD_LIBRARY_PATH /usr/lib64/openmpi/lib prepend-path PYTHONPATH /usr/lib64/python2.7/site-packages/openmpi prepend-path MANPATH /usr/share/man/openmpi-x86_64 setenv MPI_BIN /usr/lib64/openmpi/bin setenv MPI_SYSCONFIG /etc/openmpi-x86_64 setenv MPI_FORTRAN_MOD_DIR /usr/lib64/gfortran/modules/openmpi-x86_64 setenv MPI_INCLUDE /usr/include/openmpi-x86_64 setenv MPI_LIB /usr/lib64/openmpi/lib setenv MPI_MAN /usr/share/man/openmpi-x86_64 setenv MPI_PYTHON_SITEARCH /usr/lib64/python2.7/site-packages/openmpi setenv MPI_COMPILER openmpi-x86_64 setenv MPI_SUFFIX _openmpi setenv MPI_HOME /usr/lib64/openmpi
The module file begins with the magic cookie #%Module <version>, where <version> is the version of the module file used. The current version is 1.0.
The above commands prepends the path with the bindir of the 64-bit OpenMPI (compiled with GCC) and adds the relevant library path. Then it sets various environment variables.
It is also possible to set CFLAGS and LDFLAGS with the above manner, but in the case of MPI compilers it is not necessary since the compilers are invoked with the mpicc, mpicxx, mpif77 and mpif90 wrappers that already contain the necessary include and library paths. Also, in the case of development packages an override of CFLAGS and/or LDFLAGS is not sane, as it may cause trouble in building RPMs as it overrides %{optflags}.
The upstream documentation for module files is available here or with man modulefile.
Switching between module implementations
Switching between the environment-modules and Lmod implementations is done via alternatives. The shell init scripts /etc/profile.d/modules.{csh,sh} are links to /etc/alternatives/modules.{csh.sh} and can be manipulated with the alternatives command.
Lmod
Lmod is an environment modules implementation written in Lua, and can make use of module files written in Lua as well as Tcl. Such files have a ".lua" extensions. However, such files must not be installed /usr/share/modulefiles so as to not cause issues when the environment-modules package is in use. Instead install into %{_datadir}/lmod/lmod/modulefiles/Core.