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NAME

       elektra-meta-data - meta data

       metadata  is  data  about  data.  Up  to  now, there has been a limited number of metadata
       entries suited for filesys. For filesys this was efficient, but it was of limited use  for
       every other backend. This situation has now changed fundamentally by introducing arbitrary
       metadata.

       In Elektra, metakeys represent metadata. They can be  stored  in  a  key  database,  often
       within  the representation of the Key. Metakey is a Key that is part of the data structure
       Key. It can be looked up using a metaname. Metanames are unique within a Key. Metakeys can
       also  carry  a  value,  called metavalue. It is possible to iterate over all metakeys of a
       Key, but it is impossible for a metakey to hold other metakeys recursively. The purpose of
       metakeys  next  to  keys  is  to  distinguish  between configuration and information about
       settings.

Rationale

       Metadata has different purposes:

       ·   Traditionally Elektra used metadata  to  carry  file  system  semantics.  The  backend
           filesys  stores  file  metadata  (File  metadata  in POSIX is returned by stat()) in a
           struct with the same name. It contains a file type (directory,  symbolic  link,..)  as
           well  as  other  metadata like uid, gid, owner, mode, atime, mtime and ctime. into the
           Key objects. This solution, however, only makes sense when each file shelters only one
           Key object.

       ·   The  metaname  binary  shows  if a Key object contains binary data. Otherwise it has a
           null-terminated C string.

       ·   An application can set and get a flag in Key objects.

       ·   Comments and owner, together with the items above, were  the  only  metadata  possible
           before arbitrary metadata was introduced.

       ·   Further metadata can hold information on how to check and validate keys using types or
           regular expressions. Additional constraints concerning the validity of values  can  be
           convenient. Maximum length, forbidden characters and a specified range are examples of
           further constraints.

       ·   They can denote when the value has changed or  can  be  informal  comments  about  the
           content or the character set being used.

       ·   They  can express the information the user has about the key, for example, comments in
           different languages. Language specific information can be supported by simply adding a
           unique language code to the metaname.

       ·   They  can  represent  information  to  be  used by storage plugins. Information can be
           stored as syntactic, semantic or additional information rather than text  in  the  key
           database. This could be ordering or version information.

       ·   They  can  be interpreted by plugins, which is the most important purpose of metadata.
           Nearly all kinds of metadata mentioned above can belong to this category.

       ·   Metadata is used to pass error or warning information from plugins to the application.
           The  application  can  decide  to  present it to the user. The information is uniquely
           identified by numerical codes. Metadata can also embed descriptive text  specifying  a
           reason for the error.

       ·   Applications  can remember something about keys in metadata. Such metadata generalises
           the application-defined flag.

       ·   A more advanced idea is to use metadata to generate forms in a programmatic way. While
           it  is  certainly possible to store the necessary expressive metadata, it is plenty of
           work to define the semantics needed to do that.

Implementation

       In this document, we discuss  the  implementation  of  metadata.  Metakey  is  implemented
       directly  in  a  Key:  Every metakey belongs to a key inseparable. Unlike normal key names
       there is no absolute path for it in the hierarchy, but a relative one  only  valid  within
       the key.

       The  advantage  of  embedding metadata into a key is that functions can operate on a key´s
       metadata if a key is passed as a parameter. Because of this,  keyNew()  directly  supports
       adding  metadata.  A  key  with  metadata  is  self-contained. When the key is passed to a
       function, the metadata is always passed with it. Because of the tight integration  into  a
       Key, the metadata does not disturb the user.

       A disadvantage of this approach is that storage plugins are more likely to ignore metadata
       because metakeys are distinct from keys and have to  be  handled  separately.  It  is  not
       possible  to  iterate  over  all  keys and their metadata in a single loop. Instead only a
       nested loop provides full iteration over all keys and metakeys.

       The metakey itself is also represented by a Key. So  the  data  structure  Key  is  nested
       directly  into  a  Key. The reason for this is to make the concept easier for the user who
       already knows how to work with a Key. But even new users need to learn only one interface.
       During iteration the metakeys, represented through a Key object, contain both the metaname
       and the metavalue. The metaname is shorter than a key name because the name is unique only
       in the Key and not for the whole global configuration.

       The implementation adds no significant memory overhead per Key if no metadata is used. For
       embedded systems it is useful to have keys without metadata. Special plugins can help  for
       systems  that  have  very  limited memory capacity. Also for systems with enough memory we
       should consider that adding the first metadata to a key has some additional  overhead.  In
       the current implementation a new KeySet is allocated in this situation.

   Interface
       The interface to access metadata consists of the following functions:

       Interface of metadata:

           const Key *keyGetMeta(const Key *key, const char* metaName);
           ssize_t    keySetMeta(Key *key, const char* metaName,
               const char *newMetaString);

       Inside a Key, metadata with a given metaname and a metavalue can be set using keySetMeta()
       and retrieved using keyGetMeta(). Iteration over metadata is possible with:

       Interface for iterating metadata:

           int keyRewindMeta(Key *key);
           const Key *keyNextMeta(Key *key);
           const Key *keyCurrentMeta(const Key *key);

       Rewinding and forwarding to the next key works as for  the  KeySet.  Programmers  used  to
       Elektra  will  immediately  be  familiar  with the interface. Tiny wrapper functions still
       support the old metadata interface.

   Sharing of Metakey
       Usually substantial amounts of metadata are shared between keys. For  example,  many  keys
       have  the  type  int.  To  avoid  the  problem  that every key with this metadata occupies
       additional space, keyCopyMeta() was invented. It copies metadata from one key to  another.
       Only  one  metakey  resides  in  memory  as  long  as  the  metadata  is  not changed with
       keySetMeta(). To copy metadata, the following functions can be used:

           int keyCopyMeta(Key *dest, const Key *source, const char *metaName);
           int keyCopyAllMeta(Key *dest, const Key *source);

       The name copy is used because the information is copied from one key to  another.  It  has
       the  same meaning as in ksCopy(). In both cases it is a flat copy. keyCopyAllMeta() copies
       all metadata from one key to another. It is more efficient  than  a  loop  with  the  same
       effect.

       keyDup()  copies  all  metadata as expected. Sharing metadata makes no difference from the
       user´s point of view. Whenever a metavalue is changed a new metakey is generated. It  does
       not matter if the old metakey was shared or not. This is the reason why a const pointer is
       always passed back. The metakey must not be changed because it can be used within  another
       key.

                                          November 2015                      ELEKTRA-META-DATA(7)