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.

                                          November 2015               ELEKTRA-PLUGINS-ORDERING(7)