- unstable 0.8.14-5.1
elektra-namespaces - namespaces
Every key in Elektra has a unique name. Sometimes, multiple keys denote the same configuration item from different sources, e.g.:
- by a commandline argument
- by a configuration file found below the home directory
- by a configuration file found below /etc
To allow such keys to exist in parallel, Elektra uses namespaces.
A namespace has following properties:
- in-memory Keys can start with this name
- these keys stem from a specific configuration source
- ksLookup() uses namespaces in a specific default order unless specified otherwise
Following parts of Elektra are affected by namespaces:
- the key name validation in keySetName()
- keyGetNamespace() which enumerates all namespaces
- _Backend and split.c for correct distribution to plugins (note that not all namespaces actually are distributed to configuration files)
- mount.c for cascading and root backends
- and of course many unit tests
In the rest of this document all currently available namespaces in the default order are described.
Unlike the other namespaces, the specification namespace does not contain values of the keys, but instead meta data as described in [METADATA.ini].
When a key is looked up, keys from the spec-namespace are the first to be searched. When a spec-key is found, the rest of the lookup will be done as specified, probably in a different order than the namespaces enlisted here.
Usually, the spec-keys do not directly contribute to the value, with one notable exception: the default value (meta data default, see in cascading below) might be used if every other way as specified in the spec-key failed.
Spec-keys typically include a explanation and description for the key itself (but not comments which are specific for individual keys).
The spec configuration files are below CMAKE_INSTALL_PREFIX/KDB_DB_SPEC.
spec is not part of cascading mounts, because the specifications often are written in different syntax than the configuration files.
Derived from the process (e.g. by parsing /proc/self or by arguments passed from the main method):
- program name
Keys in the namespace proc can not be stored by their nature. They might be different for every invocation of an application.
Keys from the namespace dir are derived from a directory special to the user starting/using the application, e.g.:
- the current working directory for project specific settings, e.g. .git
- the directory a server wants to present to the user, e.g. .htaccess
Note that Elektra only supports a single special directory per KDB instance. Start a new KDB instance if you need different special directories for different parts of your application. How to change the directory may be different dependent on the resolver, e.g. by using chdir or by setting the environment variable PWD
On multi-user operating systems obviously every user wants her/his own configuration. The user configuration is located in the users home directory typically below the folder KDB_DB_USER. Other pathes below the home directory are possible too (absolute path for resolver).
Note that Elektra only supports a user directory per KDB instance. Start a new KDB instance if you need different user configuration for different parts of your application. How to change the user may be different dependent on the resolver, e.g. by seteuid() or by environment variables like HOME, USER
The system configuration is the same for every chroot.
The configuration is typically located below KDB_DB_SYSTEM. Other absolut pathes, e.g. below /opt or /usr/local/etc are possible too.
Keys that are not in a namespace (i.e. start with an /) are called cascading keys. Cascading keys do not stem from a configuration source, but are used by applications to lookup a key in different namespaces. So, multiple keys can contribute to each cascading key name.
Cascading is the same as a name resolution and provides a namespace unification as described in Versatility and Unix semantics in namespace unification http://dl.acm.org/citation.cfm?id=1138045.
Keys without a namespace can not be stored by their nature. So they are transient: after a restart they are forgotten.
Keys of that namespace are only used by ksLookup when no other suitable key was found. So they have the lowest possible priority, even fallback keys are preferred.
Read more about cascading. /doc/help/elektra-cascading.md