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elektra-bootstrapping.md

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elektra-bootstrapping(7) -- default backend

One important aspect of a configuration library is the out-of-the-box experience. How does the system work before anything is configured? The optimal situation is that everything fully works, and applications, that just want to load and store configuration, do not see any difference between out-of-the-box behavior and a well-configured, fine-tuned system.

To support that experience, a so-called default backend is responsible in the case that nothing was configured so far. It must have a storage that is able to store full Elektra semantics. To avoid reimplementation of storage plugins, for default storage plugins (storage or in code KDB_STORAGE) is used. A resolver plugin (resolver or in code KDB_RESOLVER) takes care of the inevitable portability issues. The default backend stores configuration in KDB_DB_FILE. One can easily avoid the usage of the default backend by simple mounting another backend to /.

The mounting configuration (the configuration how to mount the mount points) also needs to be stored somewhere. The so called init backend is responsible for fetching configuration from system:/elektra, where the mount points are stored. Again KDB_STORAGE and KDB_RESOLVER is used, but now they write into the configuration file KDB_DB_INIT (elektra.ecf by default).

Thus for full and static build variants an exchange at run-time is not possible. Using shared libraries, however, KDB_STORAGE and KDB_RESOLVER are actually symbolic links (libelektra-resolver.so and libelektra-storage.so) to concrete plugins and thus can be changed without recompilation.

The init backend is guaranteed to stay mounted at system:/elektra where the configuration for Elektra itself is stored. After mounting all backends, Elektra checks if system:/elektra still resides at the default backend. If not, the init backend will be mounted there.

SUMMARY

To summarize, this approach delivers a good out-of-the-box experience capable of storing configuration. For special use cases, applications and administrators can mount specific backends anywhere except at, and below, system:/elektra. On kdbOpen(), the system bootstraps itself starting with the init backend.

The default backend consists of a default storage plugin and default resolver plugin. The default resolver has no specific requirements, but the default storage plugin must be able to handle full Elektra semantics. The backend is mounted to root (/), so any keys can be stored in it. The implementation of the core guarantees that user and system keys always stay separated.

TRACEABILITY

  • elektraOpenBootstrap() implements above algorithm
  • backendOpenDefault() opens the default backend
  • /src/include/kdbconfig.h.in contains above KDB_* variables
  • src/plugins/CMakeLists.txt creates the symbolic links
  • scripts/cmake/Modules/LibAddMacros.cmake create_lib_symlink function

SEE ALSO