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NAME

       elektra-architecture - architecture of elektra

       In  this  document  we  start  to explain the implementation of Elektra. There are several
       follow-up documents which explain all details of:

       ·   error handling elektra-error-handling.md,

       ·   data structures elektra-data-structures.md, and

       ·   finally the core algorithm elektra-algorithm.md.

       We discuss problems and the solution space so that the reader can understand the rationale
       of how problems were solved.

       To  help  readers  to  understand  the algorithm that glues together the plugins, we first
       describe some details of the data structures elektra-data-structures.md. Full knowledge of
       the  algorithm  elektra-algorithm.md  is  not  presumed to be able to develop most plugins
       (with the exception of the resolver /src/plugins/resolver/).

       Further important concepts are explained in:

       ·   bootstrapping elektra-bootstrapping.md

       ·   granularity elektra-granularity.md

       ·   sync-flag elektra-sync-flag.md

API

       The  aim  of  the  Elektra  Project  is  to  design  and  implement  a  powerful  API  for
       configuration.  When  the  project started, we assumed that this goal was easy to achieve,
       but  dealing  with  the  semantics  turned  out  to  be  a  difficult  problem.  For   the
       implementation,  an  ambitious solution is required because of the necessary modularity to
       implement flexible backends as introduced in Elektra. But also the design of  a  good  API
       has proved to be much more difficult than expected.

   Changes in the APIs
       From  Elektra  0.7 to Elektra 0.8, we changed the API of Elektra as little as possible. It
       should be mentioned that KeySet is now always sorted by name. The function ksSort() is now
       depreciated  and was removed. The handling of removed keys was modified. Additionally, the
       API for metadata has fundamentally changed, but  the  old  interface  still  works.  These
       changes  will  be  described in implementation of meta data elektra-meta-data.md. However,
       the  implementation   of   Elektra   changed   radically   as   discussed   in   algorithm
       elektra-algorithm.md.

   API Design
       API  Design presents a critical craft every programmer should be aware of. We will shortly
       present some of the main design issues that matter and show how Elektra has solved them.

       A design goal is to detect errors early. As easy as it  sounds,  as  difficult  it  is  to
       actually  achieve  this  goal.  Elektra  tries to avoid the problem by checking data being
       inserted into Key and KeySet. Elektra catches many errors like  invalid  key  names  soon.
       Elektra  allows  plugins  to  check  the  configuration  before it is written into the key
       database so that problematic values are never stored.

       ´´Hard to use it wrong´´ tends to be a more important design objective than ´´Easy to  use
       it  right´´.  Searching  for  a stupid bug costs more time than falling into some standard
       traps which are explained in documentation. In Elektra, the data structures are robust and
       some efforts were taken to make misuse unlikely.

       Another  fundamental principle is that the API must hide implementation details and should
       not be optimised towards speed. In Elektra, the actual  process  of  making  configuration
       permanent is completely hidden.

       ´´Off-by-one  confusion´´  is  a topic of its own. The best is to stick to the conventions
       the programming language gives. For returning sizes of strings, it must be clear whether a
       terminating ´\0´ is included or not. All such decisions must be consistent. In Elektra the
       terminating null is always included in the size.

       The interface must be as small as possible to tackle problems addressed  by  the  library.
       Internal and external APIs must be separated. Internal APIs in libraries shall be declared
       as static to prevent its export. In Elektra, internal names start with elektra opposed  to
       the external names starting with key, ks or kdb.

       Elektra  always  passes  user  context  pointers, but never passes or receives a full data
       structure by value. It is impossible to be ABI\footnote{We will read  more  about  ABI  in
       \secref{ABI}.}  compatible  otherwise.  Elektra  is restrictive in what it returns (strong
       postconditions), but as liberal as possible for what comes in (preconditions  are  avoided
       where possible). In Elektra even null pointers are accepted for any argument.

       ´´Free  everything  you  allocate´´  is a difficult topic in some cases. If Elektra cannot
       free space or other resources after every call, it provides a close() function. Everything
       will  be  freed.  The tool Valgrind with Memcheck helps us locate problems. The whole test
       suite runs without any memory problems. The user is responsible for deleting  all  created
       Key and KeySet objects and closing the KDB handle.

       As  a  final statement, we note that the UNIX philosophy should always be considered: ´´Do
       only one thing, but do it in the best way. Write it that way that programs  work  together
       well.´´

Modules

       Elektra´s  core  can  be  compiled  with  a C compiler conforming to the ISO/IEC 9899:1999
       standard:

       ·   One line comments,

       ·   inline functions,

       ·   snprintf()

       ·   inttypes.h and

       ·   variable declaration at any place

       are used in addition to what is already defined in the standard ISO/IEC 9899:1990,  called
       C99  in  the  following  text.  Functions  not  conforming to C99 are considered to be not
       portable enough for Elektra and are separated into  plugins.  But  there  is  one  notable
       exception:  it  must  be the core´s task to load plugins. Unfortunately, C99 does not know
       anything about modules. POSIX (Portable Operating System Interface) provides dlopen(), but
       other  operating  systems  have  dissimilar  APIs  for that purpose. They sometimes behave
       differently, use other names for the  libraries  and  have  incompatible  error  reporting
       systems.  Because of these requirements Elektra provides a small internal API to load such
       modules independently from the operating system. This API also hides the fact that modules
       must be loaded dynamically if they are not available statically.

       Plugins  are usually realised with modules. Modules and libraries are technically the same
       in most systems. (One exception is Mac OS X.) After the  module  is  loaded,  the  special
       function  plugin  factory  is  searched  for. This function returns a new plugin. With the
       plugin factory the actual plugins are created.

   Static loading
       For the static loading of modules, the modules must be built-in. With  dlopen(const  char*
       file)  POSIX provides a solution to look up such symbols by passing a null pointer for the
       parameter file. Non-POSIX operating systems may not support this kind of  static  loading.
       Therefore,  Elektra  provides a C99 conforming solution for that problem: a data structure
       stores the pointers to the plugin factory of every plugin. The build system generates  the
       source  file  of  this  data  structure because it depends on built-in plugins as shown in
       Figure~\ref{fig:architecture}.

       Elektra distinguishes internally between modules  and  plugins.  Several  plugins  can  be
       created  out  of  a  single  module.  During  the  creation process of the plugin, dynamic
       information -- like the configuration or the data handle -- is added.

   API
       The API of \intro[libloader@\lstinline{libloader}]{libloader} consists  of  the  following
       functions:

       Interface of Module System:

           elektraModulesInit (KeySet *modules, Key *error); elektraPluginFactory
           elektraModulesLoad (KeySet *modules,
                   const char *name, Key *error);
           int elektraModulesClose (KeySet *modules, Key *error); \end{lstlisting}

       elektraModulesInit()  initialises  the  module  cache and calls necessary operating system
       facilities if needed. \method{elektraModulesLoad()} does the main work by either returning
       a  pointer  to  the plugin factory from cache or loading it from the operating system. The
       plugin factory creates plugins  that  do  not  have  references  to  the  module  anymore.
       elektraModulesClose() cleans up the cache and finalises all connections with the operating
       system.

       Not every plugin is loaded by libloader. For example, the version  plugin,  which  exports
       version information, is implemented internally.

Mount Point Configuration

       kdb  mount  creates  a mount point configuration as shown in the example below. fstab is a
       unique name within the mount point configuration provided by the administrator.

       Example for a mount point configuration:

           system/elektra/mountpoints system/elektra/mountpoints/fstab
           system/elektra/mountpoints/fstab/config
           system/elektra/mountpoints/fstab/config/path=fstab
           system/elektra/mountpoints/fstab/config/struct=list FStab
           system/elektra/mountpoints/fstab/config/struct/FStab
           system/elektra/mountpoints/fstab/config/struct/FStab/device
           system/elektra/mountpoints/fstab/config/struct/FStab/dumpfreq
           system/elektra/mountpoints/fstab/config/struct/FStab/mpoint
           system/elektra/mountpoints/fstab/config/struct/FStab/options
           system/elektra/mountpoints/fstab/config/struct/FStab/passno
           system/elektra/mountpoints/fstab/config/struct/FStab/type
           system/elektra/mountpoints/fstab/errorplugins
           system/elektra/mountpoints/fstab/errorplugins/#5#resolver#resolver#
           system/elektra/mountpoints/fstab/getplugins
           system/elektra/mountpoints/fstab/getplugins/#0#resolver
           system/elektra/mountpoints/fstab/getplugins/#5#fstab#fstab#
           system/elektra/mountpoints/fstab/mountpoint /fstab
           system/elektra/mountpoints/fstab/setplugins
           system/elektra/mountpoints/fstab/setplugins/#0#resolver
           system/elektra/mountpoints/fstab/setplugins/#1#struct#struct#
           system/elektra/mountpoints/fstab/setplugins/#2#type#type#
           system/elektra/mountpoints/fstab/setplugins/#3#path#path#
           system/elektra/mountpoints/fstab/setplugins/#3#path#path#/config
           system/elektra/mountpoints/fstab/setplugins/#3#path#path#/config/path/allow=proc tmpfs none
           system/elektra/mountpoints/fstab/setplugins/#5#fstab
           system/elektra/mountpoints/fstab/setplugins/#7#resolver \end{lstlisting}

       Let us look at the subkeys below the key system/elektra/mountpoints/fstab:

       config Everything below \keyname{config} is the system´s  configuration  of  the  backend.
              Every  plugin  within  the  backend  will  find  this  configuration directly below
              \keyname{system/} in its \intro{plugin configuration}. For example,

              system/elektra/mountpoints/fstab/config/struct/FStab/mpoint

       will be translated to

           system/struct/FStab/mpoint

       and inserted into the plugin configuration for all plugins in the fstab backend.

       It is the place where configuration can be provided for every plugin  of  a  backend.  The
       contract  checker  deduces this configuration to satisfy the contract for a plugin. Fstab,
       for example, claims in a contract that it needs ´´struct´´. But the struct plugin needs  a
       configuration to work properly. Fstab will provide this configuration. The \empha{contract
       checker} writes out the configuration looking like the one in this example.

       config/path
              is a common setting needed by the resolver plugin. It is the  relative  path  to  a
              filename  that  is  used  by  this  backend.  On  UNIX  systems, the resolver would
              determine the name /etc/fstab for system configuration.

       mountpoint
              is a key that represents the mount point. Its  value  is  the  location  where  the
              backend  is mounted. If a mount point has an entry for both the user and the system
              hierarchy, it is called \intro{cascading mount  point}.  A  cascading  mount  point
              differs  from  two  separate  mount  points  because internally only one backend is
              created. In the example, the mount point /fstab means that the backend handles both
              user/fstab  and  system/fstab. If the mount point is /, the backend will be mounted
              to all namespaces except spec, including both user and system.

       errorplugins
              presents a list of all plugins to be executed in the error case of  kdbSet()  which
              will be explained in \secref{error situation}.

       getplugins
              is a list of all plugins used when reading the configuration from the key database.
              They are executed in kdbGet().

       setplugins
              contains a list of all plugins used when storing configuration. They  are  executed
              in kdbSet().

       Each  of  the plugins inside the three lists may have the subkey config. The configuration
       below this subkey provides plugin specific configuration. This  configuration  appears  in
       the  user´s  configuration  of the plugin. Configuration is renamed properly. For example,
       the key

           system/elektra/mountpoints/fstab/setplugins/#3#path#path#/config/path/allow

       is transformed to

           user/path/allow

       and appears in the plugin configuration of the path plugin inside the fstab backend.

   Referencing
       The same plugin often must occur in more than one place within a backend. The most  common
       use  case  is  a plugin that has to be executed for both kdbGet() and kdbSet(). It must be
       the same plugin if it preserves state between the executions.

       Other plugins additionally have to handle error or  success  situations.  One  example  of
       exceptional  intensive  use  is  the resolver plugin. It is executed twice in kdbSet(). In
       kdbGet() it is also used as shown in Listing~\ref{lst:mount point configuration}.

       \lstinline[language=]{#nname} introduces a new plugin from the module name which cannot be
       referenced   later.   The   cypher   n  appoints  the  actual  placement  of  the  plugin.
       \lstinline[language=]{#n#name#

   Changing Mount Point Configuration
       When the user changes the mount point configuration, without countermeasures, applications
       already  started  will  continue  to  run with the old configuration. This could lead to a
       problem if backends in use are changed or removed. It is necessary  to  restart  all  such
       programs.  Notification  is  the best way to deal with the situation. Changes of the mount
       point configuration, however, do not occur often. For some systems, the manual restart may
       also be appropriate.

       In  this  situation,  applications  can  receive  warning  or  error  information  if  the
       configuration files are moved or  removed.  The  most  adverse  situation  occurs  if  the
       sequence   of   locking   multiple  files  produces  a  \empha{dead  lock}.  Under  normal
       circumstances, the sequence of locking the files is deterministic, so either all locks can
       be  requested or another program will be served first. But several programs with different
       mount point configurations running at the same time can cause a disaster. The problem gets
       even  worse,  because  kdb mount is unable to detect such situations. Every specific mount
       point configuration for itself is trouble-free.

       But still a dead lock can arise when multiple programs  run  with  different  mount  point
       configurations.  Suppose  we  have  a  program  A  which  uses the backends B1 and B2 that
       requests locks for the files F1 and F2. Then the mount point configuration is changed. The
       user  removes  B1  and introduces B3. B3 is in a different path mounted after B2, but also
       accesses the same file F1. The program B starts after the  mount  point  configuration  is
       changed. So it uses the backends B2 and B3. If the scheduler decides that first A and then
       B both successfully lock the files F1 and F2, a dead lock situation happens because in the
       afterwards the applications A and B try to lock F2 and F1.

       A  manual  solution  for  this problem is to enable kdb to output a list of processes that
       still use old mount point configuration. The administrator can  restart  these  processes.
       The  preferred  solution  is  to use notification for mount point configuration changes or
       simply to use a lock-free resolver.

       Continue reading with the data structures elektra-data-structures.md.

                                          November 2015                   ELEKTRA-ARCHITECTURE(7)