NAME

       doc_decisions_capabilities_mdCapabilities
        -

   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