NAME

       doc_decisions_capabilities_md - Capabilities

   Issue
       Every backend, except filesys, was unable to represent full file system semantics. Capabilities described
       the differences between filesys and another backend. (For POSIX file systems a similar technique is
       pathconf(). It allows the user to query the capabilities of a specific mounted file system given by
       path.) Capabilities made it possible to implement a backend different from the way filesys works and let
       the backend still have predictable behaviour. The user could query a backend if it was capable of a
       specific detail of file system semantics.

       Capabilities were initially introduced to make backend development easier, because they also expressed
       the disabilities of a backend. For example, it was possible for a backend to claim that it is not aware
       of comments.

       Getting a single key works well for filesys. However, for configuration files the whole content must be
       parsed even for a single key. Capabilities were able to describe that the backend will always retrieve
       and store all keys and will not be able to retrieve and store individual keys. This restriction
       simplified the implementation of such backends considerably. Capabilities allowed us to implement fstab,
       passwd and hosts backends.

       But we soon found the limits of capabilities. Capabilities were unable to describe:

       • that some key names are not allowed.

       • that not every structure of configuration is allowed.

       • that some characters are not allowed.

       • that only specific key values are allowed.

   Constraints
   Assumptions
   Considered Alternatives
   Decision
       No capabilities are used, but instead other plugins implement the limitations other plugins have.

   Argument
       The  main  problem  was  that  the complexity was just moved to the applications. In the end, application
       developers had to understand what the backend can write out. They  no  longer  had  confidence  that  the
       storage  was  able  to  understand  the  full  semantics.  Elektra's  core  was unable to hide that fact.
       Capabilities did not turn out to  make  backend  development  much  easier.  Instead,  Elektra  was  more
       difficult to use with capabilities. Capabilities are no longer part of Elektra for these reasons.

   Implications
   Related decisions
   Notes
Version 0.8.14                                   Tue Dec 15 2015         doc_decisions_capabilities_md(3elektra)