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NAME

       md_src_plugins_keytometa_READMEREADME
        -

       · infos = Information about keytometa plugin is in keys below

       · infos/author = Felix Berlakovich elektra@berlakovich.net

       · infos/licence = BSD

       · infos/needs =

       · infos/provides = conversion

       · infos/placements = presetstorage postgetstorage

       · infos/description = conversion of keys to meta keys and vice versa

       This plugin converts keys into metakeys of other keys. The keys to be converted are tagged
       with special metadata. Converting keys into metakeys basically raises two questions:

       · which keys are to be converted

       · which key to append the resulting metakeys to

       The keys to be converted are identified by metakeys below 'convert' (e.g.
       'convert/append'). The keys receiving the resulting meta data are identified by append
       strategies. The plugin currently supports the following metakeys for controlling the
       conversion:

       · convert/metaname: specifies the name of the resulting metakey. For example tagging the
         key user/config/key1 with 'convert/metaname = comment' means that the key will be
         converted to a metakey with the name 'comment'.

       · convert/append: specifies the append strategy (see below)

       · convert/append/samelevel: specifies that the key should only be written to the metadata
         of a key with the same hiearchy level (see below).

       The keys converted to metadata are restored as soon as the keyset is written back.
       However, the plugin is stateful. This means that a keyset must be read and keys must be
       converted by the plugin in order to undo this conversion in the set direction.
       Modifications to the metadata which resulted from converted keys are propagated back to
       the corresponding key (see merging for more details).

       The keys are ordered by the 'order' metadata. If two keys are equal according to the order
       metadata, they are ordered by name instead.

   APPEND STRATEGIES
       The append strategy specifies which key will receive the resulting metadata. Currently the
       plugin supports the following strategies:

   PARENT STRATEGY
       The metadata is added to the first existing parent of the converted key. This does not
       neccessarily have to be the parent of the keyset. If no such key is found, the first key
       in a sorted keyset will receive the metadata (this is usually the parent key of the
       keyset). For example consider the following keyset:

                                   user/config/key1
                                   user/config/key1/child1
                                   user/config/key2
                                   user/config/key2/deeper/child2
                                   user/config/child3

       If child1, child2 and child3 were tagged with 'convert/append = parent', key1 would
       receive the metadata from child1 and child3. Key2 would receive the metadata from child2.

   NEXT STRATEGY
       The metadata is added to the key following the converted key in a sorted keyset. If no
       such key is found (for example because the key to be converted is the last one), the
       strategy is reverted to parent. For example consider the following keyset:

                                   user/config/deeper/key1
                                   user/config/key2
                                   user/config/key3
                                   user/config/key4

       If key1 and key3 were tagged with 'convert/append = next', key2 would receive the metadata
       resulting from key1 and key4 would receive the metadata resulting from key3.

   PREVIOUS STRATEGY
       The metadata is added to the key preceding the converted key in a sorted keyset. If no
       such key is found (for example because the key to be converted is the first one), the
       strategy is reverted to parent. For example consider the following keyset:

                                   user/config/key1
                                   user/config/deeper/key2
                                   user/config/key3
                                   user/config/key4

       If key2 and key4 were tagged with 'convert/append = previous', key1 would receive the
       metadata resulting from key2 and key3 would receive the metadata resulting from key4.

   MERGING
       The metadata resulting from a converted key is never appended to another key which is
       going to be converted. This prevents that the data of converted keys is invisible after
       the conversion. Instead the metadata resulting from different converted keys with the same
       append strategy is merged together (separated by a newline). Keys with different append
       strategies are skipped, until either a key with the same strategy is found (which is
       simply merged as decribed above) or the target key is found. The keys are always processed
       in the order of an ordered keyset. For example consider the following keyset:

                                   user/conifg/key0
                                   user/config/key1 = value1
                                   user/config/key2 = value2
                                   user/config/key3 = value3
                                   user/config/key4 = value4
                                   user/config/key5

       If key1 and key2 were tagged with 'convert/append = next' and key3 and key4 were tagged
       with 'convert/append = previous' the following would happen:

       · the resulting metadata of key0 would contain 'value3\nvalue4' (the values of key3 and
         key4 are merged together and key1 and key2 are skipped, as they have differnt append
         strategy)

       · the resulting metadata of key5 would contain 'value1\nvalue2' (the values of key1 and
         key2 are merged together and key3 and key4 are skipped, as they have different append
         strategy)

   SAMELEVEL APPENDING
       The option 'convert/append/samelevel' can be used to force that the metadata is only
       appended to a key on the same hierarchy level. If no such key is found, the strategy is
       reverted to parent. Note, that the value of the samelevel key does not matter. Only its
       existence is relevant. For example consider the following keyset:

                                   user/config/key0
                                   user/config/key1/child1
                                   user/config/key2
                                   user/config/key3/child2
                                   user/config/key4
                                   user/config/key5
                                   user/config/key6

       If child1, child2 and key4 were each tagged with 'convert/append = next' and child2 and
       key4 were tagged with 'convert/append/samelevel', key2 would receive the metadata
       resulting from child1. key0 would receive the metadata resulting from child2 (strategy
       reverted to parent, as the samelevel request cannot be fulfilled) key5 would receive the
       metadata resulting from key4

   REAL WORLD EXAMPLE
       The keytometa plugin was initially developed to aid the integration of the Augeas plugin.
       The Augeas plugin represents comments in configuration files as keys. However, in Elektra
       comments are usually represented within comment meta keys. Therefore it would be desirable
       to convert all comment keys to comment meta keys. This is achieved by adding the following
       to the Augeas plugin contract.

                                   keyNew ("system/elektra/modules/augeas/config/needs/glob/get/#1",
                                           KEY_VALUE, "*#comment*",
                                           KEY_META, "convert/metaname", "comment",
                                           KEY_META, "convert/append", "next",
                                           KEY_END),
                                   keyNew ("system/elektra/modules/augeas/config/needs/glob/get/#1/flags",
                                           KEY_VALUE, "0", /* disable the path matching mode */
                                           KEY_END)

       Tagging the keys to be converted to comment meta keys happens via the glob plugin. The
       meta data set on the key glob/get/#1 is copied to each key that matches the pattern
       '*#comment*', i.e. each comment key generated by the Augeas plugin. 'convert/metaname' =
       'comment' because we want the comment keys to be converted to the comment metadata. 'next'
       is chosen as append strategy because usually comments occur before the key they describe.