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The env-switcher package provides an convenient method for users to switch between “similar” packages. System- and user-level defaults are maintained in data files and are examined at shell invocation time to determine how the user’s environment should be set up.

The canonical example of where this is helpful is using multiple implementations of the Message Passing Interface (MPI). This typically requires that the user’s “dot” files are set appropriately on each machine that is used since rsh/ssh are typically used to invoke commands on remote nodes.

The env-switcher package alleviates the need for users to manually edit their fot files, and instead gives the user commandline control to switch between multiple implementations of MPI.

While this package was specifically motivated by the use of multiple MPI implementations on OSCAR clusters, there is nothing specific to either OSCAR or MPI in env-switcher – switching between mulitple MPI implementations is only used in this description as an example. As such, it can be used in any environment for any “switching” kind of purpose.

Switcher does make sense on Debian based systems since Debian provides similar tools.

Switcher and OSCAR


Switcher is actually composed of two parts: one which is the switcher project itself (env-switcher, available on SourceForge) and the switcher integration into OSCAR (typically the switcher OPKG).

If a given OPKG is the only one to declare switcher data, the values specified by this OPKG are considered as default values. If a default value already exists, the values are stored into ODA but not used as default; to use them as default, the user will have to explicitly use switcher.

How to use switcher?

Each OPKG can “declare” switcher data via a provide tag in the config.xml file. A example of such a tag is:


Currently the storage of such data into ODA is done when executing wizard_prep, after the installation of the “api” binary packages of selected OPKGs. This is not a good solution long term (does not allow the usage of switcher in third party OPKG since switcher data won’t be saved into ODA).

Implementation details

ODA is used to save switcher data. To access (read/write) switcher data into ODA, a Perl module is available providing a simple API: SwitcherAPI.

The switcher OPKG provide some glue code: this is the package_config Perl module. This module access ODA (using SwitcherAPI) and reformat switcher data. This module should be merged with SwitcherAPI.

Future Improvements

Switcher data should be stored in ODA automatically. For that one solution is to write a ‘switcher-oscar’ tool that can store such information and modify OPKGC in order to understand this tool:

  • OPKGC includes a call to the switcher-oscar tool in all OPKG (post_install of the api package) if the config.xml file has a reference to switcher (long-term, the config.xml files should be used only by OPKGC).
  • OPKGC includes a dependency to switcher-oscar (api package).
  • Since binary packages are installed before OPKG configuration, the storage of switcher info into ODA can be automatic.