NAME

       elektra-plugins-ordering - ordering of plugins

       You should first read elektra-plugins(7) /src/plugins/ to get an idea about plugins.

       This  document  describes  how  elektra-plugins(7)  /src/plugins/  are ordered with an elektra-backend(7)
       elektra-backend.md.

       Multiple plugins open up many ways in which they can be arranged. A simple way is to have one array  with
       pointers  to  plugins.  To  store a KeySet, the first plugin starts off with the KeySet passed to it. The
       resulting KeySet is given to the next plugin. This is repeated for every plugin in the array.

       To obtain a KeySet, the array of plugins is processed the other way round. An empty KeySet is  passed  to
       the  last  plugin.  The  resulting  KeySet  is  handed over to the previous one until the first plugin is
       executed.

       This approach has shown not to be powerful enough to express all use cases by  counter-evidence.  Logging
       should take place after the storage plugin performs its actions in both directions. It is not possible to
       do this with a single array processed in a way as described above.

Separated lists

       Two  individual  lists  (get+set  list)  of plugins solve the described problem. Instead of bidirectional
       processing of a single list two separate arrays are used. It turns out that  error  scenarios  also  make
       this  approach  unsuccessful.  When  a plugin fails, no other plugin must be executed later on because it
       might depend on the previous plugin to work correctly. In kdbGet(), this works well -- the update process
       will be stopped. But during kdbSet(), changes to the file system must be reverted. Plugins  can  leave  a
       lock, temporary file or journal. These resources need to be cleaned up properly.

Error List

       So  every  backend  additionally needs a third array that is executed in error scenarios during kdbSet().
       Plugins responsible for the cleanup, rollback or error notification are inserted into it.

       The resolver plugin requires this error list to do a proper rollback. Another use case is logging after a
       failure has happened.

Placements

       The  ordering  of  plugins  inside   these   three   arrays   is   controlled   by   elektra-contracts(7)
       elektra-contracts.  Each  of  the three arrays has ten slots. These slots have names to be referred to in
       the contract.

       Here you see a table that contains all names:

           0     prerollback       getresolver         setresolver
           1     prerollback       pregetstorage      presetstorage
           2     prerollback       pregetstorage      presetstorage
           3     prerollback       pregetstorage      presetstorage
           4     prerollback       pregetstorage      presetstorage
           5      rollback            getstorage         setstorage
           6    postrollback      postgetstorage      precommit
           7    postrollback      postgetstorage         commit
           8    postrollback      postgetstorage     postcommit
           9    postrollback      postgetstorage     postcommit

       how the placement is influenced using infos/placement, infos/ordering and infos/stacking as described  in
       CONTRACT.ini /doc/CONTRACT.ini.

                                                    July 2017                        ELEKTRA-PLUGINS-ORDERING(7)