6.17 Layered Installations
A typical Racket configuration includes two layers: an installation layer and a user layer. The intent is that the installation layer is read-only to all users of a system, while the user layer allows each individual user to install additional packages that extend the installation layer. The installation layer is intended not only to be read-only, but to not change after users start installing in their own layers.
In an environment where Racket itself is under development, the
installation layer will change. In that setting, if the
user layer is used at all, care must be taken to not create
conflicts for the user layer when modifying the installation
By default, raco setup updates both layers whenever it is run; if a user does not have write permission the installation, raco setup with no arguments is all but certain to report permission errors. The actions of raco setup can be constrained to the user layer by supplying the --avoid-main argument, or raco setup can be constrained to the installation layer by using the --no-user argument. When raco pkg performs setup actions, it effectively supplies one of the other of those based on the package’s scope (and raco pkg refuses to operate on both scopes/layers at once).
The user layer is always both user- and version-specific. More precisely, it is specific to the user and an installation’s name, where the installation’s name is typically its version number. However, the name of an installation can be changed through the 'installation setting in "config.rktd" (see Installation Configuration and Search Paths). Setting an installation name changes the directory where packages and executables reside within (find-system-path 'addon-dir). The result of (find-system-path 'addon-dir) itself can be changed through 'addon-dir in "config.rktd".
each of 'lib-dir, 'share-dir, 'links-file, 'pkgs-dir, 'bin-dir, 'gui-bin-dir, 'apps-dir, 'doc-dir, and 'man-dir is a new directory or file; and
the corresponding search lists 'lib-search-dirs, 'share-search-dirs, 'links-search-files, 'pkgs-search-dirs, 'bin-search-dirs, 'gui-bin-search-dirs, (no 'apps-dir search needed), 'doc-search-dirs, and 'man-search-dirs each add the old directory or file to the search list just after #f; note that the default for each search list is (list #f).
The default path to "config.rktd" is hardwired within a racket executable. In some cases, it can make sense for the innermost layer’s configuration to point to another layer, perhaps because the filesystem provides an indirection. For example, on Unix, a Racket installation in "/usr" might reasonably configure the installation layer’s directories to be in "/usr/local" with "/usr" directories included in the search lists.
To use racket with a new "config.rktd", you can supply the --config or --G flag to racket or set the PLTCONFIGDIR environment variable to point to the directory containing "config.rktd". Alternatively, you can create a tethered layer that creates replacement executables like racket that are hardwired to the layer’s configuration directory.