xenial (7) elektra-semantics.7.gz

NAME

       elektra-semantics - Semantics of KDB

       The  use of arbitrary metadata has extensive effects in Elektra´s semantics. They become simpler and more
       suited to carry key value pairs. The semantics now gives us independence of the underlying  file  system.
       So  none  of  the  file  system´s  restrictions apply anymore. No constraints on the length of a key name
       disturbs the user any more. Additionally, key names can be arbitrarily deep nested. Depth is  the  number
       of unescaped / in the key name.

       The  directory  concept  is  enforced  by default. Keys can be created everywhere. Keys always can have a
       value. The  only  constraint  is  that  key  names  are  unique  and  occur  in  one  of  the  namespaces
       elektra-namespaces.md. Every Key has an absolute name. There is no concept of relative names in Elektra´s
       Keys except for meta keys belonging to a key. Every other Key is independent of each other.  We  just  do
       not care if there is another key below or above the accessed one in the storage or not.

       Some  applications  need  specific structure in the keys. Plugins can introduce and enforce relationships
       between keys. They can implement a type system, check if holes are present and check  the  structure  and
       interrelations.  They  may propagate the metadata and introduce inheritance. We see that plugins are able
       to add more semantics to Elektra.

       There are no symbolic links, hard links, device files or anything else different from  key  value  pairs.
       Again, most of these behaviours can be mimicked using metadata. Especially, links are available using the
       metadata override and fallback.

       Hidden keys are not useful for Elektra. Instead comments or other metadata contain information about keys
       that  is  not considered to belong to the configuration. If hidden keys are desired, we can still write a
       plugin to filter specific keys out.

Problems

       This section explains why using file system semantics for configuration is not a good idea.

   filesys
       filesys was the first backend. It implemented the principle that every key is  represented  by  a  single
       file.  The  key name was actually mapped to a file name and the value and the comment was written to that
       file.

       If the backend filesys was the ideal solution, Elektra´s API (application programming interface) would be
       of  limited  use.  E.g.cascading, type checking and optional cross-cutting features would be missing. The
       storage problem itself  and  the  location  of  a  key  in  a  key  database  would  be  solved.  because
       well-established APIs for accessing files are available in every applicable programming language.

       Elektra  0.7  already  supported more than one backend, but filesys was the only backend implementing the
       full semantics.

   Limitations of File Systems
       Here we will discuss, why the file system´s semantics are not well suited for configuration at all.

       One file per key turned out to be inefficient because of the file system´s practical limitations. In most
       file  systems,  a  file needs about four kilobytes (Depends on the block size, four kilobytes is a common
       value often used as default.) of space, no matter how little content is  in  it.  Thus  the  file  system
       wastes 99.9% of the space if keys have a payload of four bytes. Additionally, every file allocates a file
       node, which might be limited, too. We can argue, however, that we can use a file system  which  does  not
       have these problem.

       Many  additional  restrictions  occur  for  portable  access. The file name length in POSIX is limited to
       fourteen characters. Additionally, issues with case sensitivity are likely. The  common  denominator  for
       all  file systems is a surprisingly small one. If, for example, the traditional FAT file system should be
       supported, file names are limited to eight characters and case insensitivity.

       On the one hand, there are many file system features that are not needed for configuration. File  systems
       have  a  strict hierarchy. It is not possible to create a file in a non-existing directory. We will refer
       to such a missing object as hole. File systems do not support such holes.

       A single root directory is not a useful concept for configuration. Instead, the system configuration  and
       each user configuration has its own root. These root keys themselves are typically not needed.

       There  is  additional  metadata  of  files which is typically not needed for configuration: atime, mtime,
       ctime, uid, gid and mode just to name a few. Additional file types, for  example,  device  files,  links,
       fifos  and  sockets,  are  not  needed  either.  Features  like sparse files are ridiculous for the small
       strings, that key values typically are.

       On the other hand, there are many features missing in  file  systems  that  we  need  in  a  serious  key
       database.  Creating  a  whole hierarchy of files at once atomically is not possible. Ways to achieve this
       are currently academic and not portable. Directories cannot have any content next  to  the  files  below.
       Swap semantics are missing: it is not possible to rename a file without removing the target first.

       To sum up, file systems are not suitable to hold configuration with one entry per file. Instead, they are
       perfectly suitable to hold larger pieces of information like configuration files.

                                                  November 2015                             ELEKTRA-SEMANTICS(7)