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NAME
md_doc_help_elektra-meta-dataelektra-meta-data(7) -- 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.
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.
Version 0.8.14 Mon Jul 24 2017 md_doc_help_elektra-meta-data(3elektra)