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Version: March 2004 (fedora - 06/07/09)

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Section: 7 (Divers)


elektra - A framework to store configuration atoms hierarchically Library Linkage Architecture.PP The Elektra library (libelektra.so) has 2 layers: public methods and backend access, according to the following architecture (these pictures were taken from the Elektra presentation[1]):

When using local backends such as the filesys backend, all key access happens in the actual process space as bellow:

A remote daemon backend is also possible as noted bellow:

True Facts About Elektra

•It is much more an agreement then a piece of software. Relation is 99% to 1%.
•It is a simple and consistent API to help software developers programatically store and retrieve global and user-specific configuration parameters.
•All key-value pairs are stored in clear-text files, UTF-8 encoded. All old charsets are also supported, with automatic transparent conversion to and from UTF-8.
•API supports change notifications and multiple backends.
•It provides a unique namespace for all values. Anywhere, anytime, any program can preciselly access keys by their names. Security restrictions may obviously apply.
•It is designed to be secure and lightweight, to let even early boot-stage programs like /sbin/init to use it, instead of /etc/inittab file.
•It is designed to be easy to administrate with regular command line tools like cat, vi, cp, ls, ln. Its storage is 100% open.
•It tries to set distribution-independent naming standards to store things like hardware configuration, networking, user's session configuration, system's mime-types, parameters for kernel modules, etc, that are generally stored under /etc.
•It requires existing software to be changed to use its API. This will substitute hundreds of configuration-text-file parsing code, into clear Elektra's API key-value access methods.
•It is POSIX compliant. If it doesn't compile and run easily on some POSIX system, it should be easily modified to do so.
Elektra Is Not
•Is NOT something that accesses SQL/relational databases.
•Is NOT an OS service that can become unavailable and make system unusable. It is just a library to access files according to a namespace.
•Is NOT an alternative to network information systems like LDAP or NIS. These are still required for networked environments.
•Is NOT a Webmin-like or other GUI tool to be used by end users.
•Is NOT an additional software layer to edit/generate existing configuration files.
•Is NOT a "configuration system", because one can't be created by simply writing some code. A configuration system is an ecosystem, and the Elektra Project tries to help build one.
•It doesn't know a thing about the semantics of each data it stores.
Namespaces and Key Names.PP All keys are organized in a hierarchical tree with 2 Namespaces (subtrees) as showed by the picture:


Contains all subsystems and global application keys/configuration. Equivalent to files under /etc directory.


The current user's keys. Equivalent to the dotfiles in a user's $HOME directory. These keys are phisically stored under the owner user home directory. The many user:username in the picture shows the full name of those trees. Read about user domains bellow for more.
User Domains.PP Different from the system namespace, the user namespace is dynamic. For example, the key user/env/PATH may have completely different values for users luciana and valeria. In this example, if valeria wants to access this key at luciana's space, it should refer to user:luciana/env/PATH. Access permissions apply.

User domains were implemented also to address situations when different user names ($USER) have same UID. So a user key is stored in his home directory based on the user name, not the UID. Inactive Keys.PP A great thing about text configuration files is that some configuration items can be there as an example, but inactive or commented. Elektra provides a very simple way to simulate this behavior: if the key name begins with a dot (.), it is considered inactive or commented. In real world applications, the Elektra API will ignore these keys by default, but the keys are still accessible if the developer wants to.

These are some keys that have inactive subtrees:

system/sw/XFree/InputDevice/.Mouse3/Driver: All keys under .Mouse3/* subtree won't be read by default.
user:valeria/env/env2/.PATH: The $PATH environment variable won't be set when valeria login.
system/users/.louis/uid: The entire .louis/* subtree is inactive. This is the same as commenting the user entry from a configuration file.

See bellow more examples of inactive keys. Key Examples.PP Here are some valid key names, and their values:

The Elektra keys of the combined /etc/passwd and /etc/shadow entry for user 'nobody' would look like:

system/users/nobody/uid: 99
system/users/nobody/gid: 99
system/users/nobody/gecos: Nobody
system/users/nobody/home: /
system/users/nobody/shell: /sbin/nologin
system/users/nobody/password: *
system/users/nobody/passwdChangeBefore: 0
system/users/nobody/passwdChangeAfter: 99999
system/users/nobody/passwdWarnBefore: 7

The environment variables I want set, when I log in, with their full key name:

user:aviram/env/env1/JAVA_HOME: /usr/lib/jvm/java-1.4.1-ibm-
user:aviram/env/env2/PATH: $PATH:~/bin:$JAVA_HOME/bin
user:aviram/env/env2/PS1: \h:\w\$
user:aviram/env/env3/PILOTRATE: 57600

The entry in /etc/inittab that is responsible for starting X11 would look:

system/init/x/runlevels: 5
system/init/x/action: respawn
system/init/x/process: /etc/X11/prefdm -nodaemon

The users database files and /etc/inittab were Elektrified to key-value pairs using the users-convert and inittab-convert scripts included with the distribution.

An example of an elektrified /etc/X11/xorg.conf or /etc/X11/XF86Config:

system/sw/xorg/current/Layouts/Default Layout/Inputs/Keyboard0/CoreKeyboard:
system/sw/xorg/current/Layouts/Default Layout/Inputs/Mouse0/CorePointer:
system/sw/xorg/current/Layouts/Default Layout/Screens/Screen0/Absolute.x: 0
system/sw/xorg/current/Layouts/Default Layout/Screens/Screen0/Absolute.y: 0
system/sw/xorg/current/Layouts/Default Layout/Screens/Screen0/ScreenNumber: 0
system/sw/xorg/current/Files/FontPath: unix/:7100
system/sw/xorg/current/Files/RgbPath: /usr/X11R6/lib/X11/rgb
system/sw/xorg/current/Devices/Videocard0/BoardName: Intel 740 (generic)
system/sw/xorg/current/Devices/Videocard0/Driver: i740
system/sw/xorg/current/Devices/Videocard0/VendorName: Videocard vendor
system/sw/xorg/current/InputDevices/Keyboard0/Driver: keyboard
system/sw/xorg/current/InputDevices/Keyboard0/Options/XkbLayout: us_intl
system/sw/xorg/current/InputDevices/Keyboard0/Options/XkbModel: pc105
system/sw/xorg/current/InputDevices/Mouse0/Driver: mouse
system/sw/xorg/current/InputDevices/Mouse0/Options/Device: /dev/input/mice
system/sw/xorg/current/InputDevices/Mouse0/Options/Emulate3Buttons: yes
system/sw/xorg/current/InputDevices/Mouse0/Options/Protocol: IMPS/2
system/sw/xorg/current/InputDevices/Mouse0/Options/ZAxisMapping: 4 5
system/sw/xorg/current/Monitors/Monitor0/DisplaySize.height: 230
system/sw/xorg/current/Monitors/Monitor0/DisplaySize.width: 300
system/sw/xorg/current/Monitors/Monitor0/HorizSync: 30.0 - 61.0
system/sw/xorg/current/Monitors/Monitor0/ModelName: SyncMaster
system/sw/xorg/current/Monitors/Monitor0/VendorName: Monitor Vendor
system/sw/xorg/current/Monitors/Monitor0/VertRefresh: 56.0 - 75.0
system/sw/xorg/current/Monitors/.Monitor1/HorizSync: 30.0 - 61.0
system/sw/xorg/current/Monitors/.Monitor1/ModelName: Impression
system/sw/xorg/current/Monitors/.Monitor1/VendorName: Monitor Vendor
system/sw/xorg/current/Monitors/.Monitor1/VertRefresh: 56.0 - 75.0
system/sw/xorg/current/Screens/Screen0/DefaultDepth: 16
system/sw/xorg/current/Screens/Screen0/Device: Videocard0
system/sw/xorg/current/Screens/Screen0/Displays/00/Depth: 16
system/sw/xorg/current/Screens/Screen0/Displays/00/Modes: 1024x768,800x600,640x480
system/sw/xorg/current/Screens/Screen0/Displays/00/Viewport.x: 0
system/sw/xorg/current/Screens/Screen0/Displays/00/Viewport.y: 0
system/sw/xorg/current/Screens/Screen0/Monitor: Monitor0
system/sw/xorg/current/DRI/Group: 0
system/sw/xorg/current/DRI/Mode: 0666

Pay attention that the keys bellow system/sw/XFree/current/Monitor/.Monitor1 are inactive because we have .Monitor1 as their parent. So unless special options are used when calling the API, these keys will not be retrieved from the database.

Throughout this text you will see other examples of key names. Key Data Types.PP There are only two types of data that can be stored:


Handled as pure text. Regardeless of the charset being used, these values are always stored as UTF-8. This ensures very strong internationalization and migration capabilities, while keeping simplicity. If you don't want the Elektra framework to convert your non-ASCII text to UTF-8 (not recomended), you should use the Binary data format.


A stream of bytes, not necessarily text. It is recommended that you avoid using binary values because UNIX system administrators tend to consider them as unmanageable blackboxes. Anyway, the value will be encoded into pure text format based on hexadecimal digits, for openness and ease of administration. This data type should also be avoided because it is less efficient.

There are very good reasons why types like Integer, Time, Font, List, etc were not implemented: Elektra was designed to be usefull for any type of program, so having more specific data types implicates in the definition of value limits, separators in the storage format, etc, that may be good for some application and bad for other. So the semantics of the data is handled by the application. A program or framework may define its own special data handling methods using these essential basic types. See the keyGetType() and keySetType() methods documentation in the kdb(3) man page to understand how to set keys with your own data types.

There are more two types of keys:


It can't store a value, but, as a directory in a filesystem, it serves as a way to group correlated keys.


It is a link to another key. They work as symbolic links in the filesystem: when trying to access them, you will actually access the key they point to. The API also provides ways to access these special keys without dereferencing them.
Key Meta Data.PP Besides the key name and the value, each key has other attributes:

Owner's User and Group

This is a system's UID and GID equal to the ones found in regular files' attributes.

Access Permissions

Filesystem-like access permissions for user, group and others.

Modification, Access and Stat Times

Last time a key was modified, readed and stated (listed), respectively.

Key Comment

Pretty much as a configuration file comment. Not intended to be used in GUI applications, because it isn't internationalizable.
Fine Grained Security Example.PP To show this metadata in action, this screen shows the kdb command listing keys and their attributes related to user nobody.
 bash$ kdb ls -Rlv system/users/nobody
 -rw-r--r--   root  root    17 Mar 31 09:07 system/users/nobody/uid=99
 -rw-r--r--   root  root    17 Mar 31 09:07 system/users/nobody/gid=99
 -rw-r--r--   root  root    21 Mar 31 09:07 system/users/nobody/gecos=Nobody
 -rw-r--r--   root  root    16 Mar 31 09:07 system/users/nobody/home=/
 -rw-r--r--   root  root    28 Mar 31 09:07 system/users/nobody/shell=/sbin/nologin
 -rw-------   root  root    16 Mar 31 09:07 system/users/nobody/password
 -rw-------   root  root    16 Mar 31 09:07 system/users/nobody/passwdChangeBefore
 -rw-------   root  root    20 Mar 31 09:07 system/users/nobody/passwdChangeAfter
 -rw-------   root  root    16 Mar 31 09:07 system/users/nobody/passwdWarnBefore
 -rw-------   root  root    15 Mar 31 09:07 system/users/nobody/passwdDisableAfter
 -rw-------   root  root    15 Mar 31 09:07 system/users/nobody/passwdDisabledSince
 -rw-------   root  root    15 Mar 31 09:07 system/users/nobody/passwdReserved

We ran the kdb command without super-user credentials, asking for long (-l), recursive (-R) listing, and to show each key value (-v). But (since we are) regular user, we don't have permission to see the values of the system/users/nobody/passwd* fields.

The users database files were elektrified to key-value pairs using the users-convert script included with the distribution. ExamplesSetting Keys.PP bash$kdb set -c "My first key" user/example/key "Some nice value"

bash$kdb set user:luciana/example/key -- "Some - nice - value with dashes"

bash#KDB_ROOT=user:http/sw/httpd kdb set -u nobody -g http key "Some value"

bash$kdb set -b image.png -t bin user/example/binaryKey

bash$kdb set -b file.txt user/example/regularKey

bash#kdb set -t link system/sw/XFree/current system/sw/XFree/handmade Getting Keys.PP bash$KDB_ROOT=user/example kdb get some/key/name

bash$eval `kdb get -s user/env/env1/PS1`

bash$KDB_BACKEND=gconf kdb get user/sw/gnome-terminal/global/active_encodings Listing.PP bash$kdb ls -laR user:valeria

bash$kdb ls -lR system/sw/xorg/current

bash$KDB_ROOT=system/sw kdb ls -lR xorg

bash$KDB_BACKEND=fstab kdb ls -Rv system/filesystems

bash$eval `kdb ls -Rvs user/env/env2` Miscelaneous.PP bash#kdb ln system/sw/xorg/handmade system/sw/xorg/current

bash#kdb mv system/sw/xorg/current system/sw/xorg/old

bash#kdb rm system/inittab/rc4

bash$KDB_BACKEND=gconf kdb rm user/gconfKey XML Import and Export.PP bash#kdb export user/sw/app | sed -e 's|/app/|/app2/|g' | kdb import

bash#KDB_ROOT=system/sw kdb export myapp > myappconf.xml

bash#kdb import myappconf.xml

bash$KDB_BACKEND=gconf kdb export user/sw


kdb(1), elektra(5)


Avi Alkalay <avi at unix.sh>
Linux Market Developer, Senior IT and Software Architect, IBM Linux Impact Team :: ibm.com/linux

Copyright © 2004 Avi Alkalay


Elektra presentation