Multiple plugins can be mounted into the key database (KDB). On every access to the key data base they are executed and thus can change the functionality and behavior.

Unlike Elektra's core the plugins have all kinds of dependencies. It is the responsibility of the plugin to find and check its dependencies using CMake. If a dependency cannot be found, the plugin will automatically disable itself.

Description

Elektra has a wide range of different plugins. The plugin folders should contain a README.md with further information. (Or follow links below.) The plugins are:

C-Interface

All plugins implement the same interface:

kdbOpen() calls elektraPluginOpen() of every plugin to let them do their initialisation.
kdbGet() requests elektraPluginGet() of every plugin in the queried backends to return a key set.
kdbSet() usually calls elektraPluginSet() of every plugin in the queried backends to store the configuration.
kdbSet() also calls elektraPluginError() for every plugin when an error happens. Because of elektraPluginError(), plugins are guaranteed to have their chance for necessary cleanups.
kdbClose() makes sure that plugins can finally free their own resources in elektraPluginClose().

Furthermore, plugins might export symbols:

checkconf can be called during mounting to ensure a plugin has valid configuration.
genconf can be called to produce all valid configurations of a plugin.

KDB-Interface

To list all plugins use kdb-plugin-list(1).
To check a plugin use kdb-plugin-check(1).
For information on a plugin use kdb-plugin-info(1).
For mount plugin(s) use kdb-mount(1).

Installation

See INSTALL. Many plugins are already part of the core package libelektra5. The package that includes a plugin which does not belong to the libelektra5 package can be found in it's README.md.

Plugins

Backends

Backends provide access to different data sources (e.g. files, databases, network resources)

backend is the default plugin implementing backend functionality for configuration files
backend_odbc provides access to ODBC data sources for storing configuration data (keys, values, metadata)

Resolver

Before configuration is actually written, the file name needs to be determined (resolvers will be automatically added by kdb mount):

resolver uses advanced POSIX APIs to handle conflicts gracefully
noresolver does not resolve as no file name is needed (for non-file storage plugins)
wresolver minimalistic resolver for non-POSIX systems

Furthermore, there are following experimental resolvers:

blockresolver resolves tagged blocks inside config files
curlget fetches configuration file from a remote host
gitresolver checks out and commits files to a local Git repository

Storage

Are responsible for reading writing the configuration to configuration files.

Read and write everything a KeySet might contain:

dump makes a dump of a KeySet in an Elektra-specific format
quickdump uses binary portable format based on dump, but more efficient
mmapstorage uses binary, not portable memory mapped file for a high performance storage

Read (and write) standard config files:

toml reads and writes data using a parser generated by Flex and Bison
hosts reads/writes hosts files
kconfig reads/writes KConfig ini files
line reads/writes any file line by line
yajl reads/writes JSON.
augeas reads/writes many different configuration files using the Augeas library
xfconf reads/writes to arbitrary xfconf channels

Using semi-structured data for config files, mainly suitable for spec-namespace (put a focus on having nice syntax for metadata):

ni parses INI files based on (including metadata)

Only suited for import/export:

mini dependency free, line based key-value storage plugin.
simpleini line-based key-value pairs with configurable format (without sections). Only works on glibc systems.
xerces uses XML (without a specific schema).
xmltool uses XML in the deprecated Elektra XML schema for importing configuration from Elektra 0.7.
c writes Elektra C-structures (ksNew(.. keyNew(...)
ansible writes the given KeySet as Ansible Playbook that uses the ansible-libelektra module

Plugins that just show some functionality, (currently) not intended for productive use:

csvstorage for csv files
dpkg reads /var/lib/dpkg/{available,status}
file reads and writes a file from/to a single key
fstab for fstab files.
mozprefs for Mozilla preference files
passwd for passwd files
specload calls an external application to request its specification, depends on quickdump
yamlcpp reads and writes data in the YAML format using yaml-cpp

System Information

Information compiled in Elektra:

version is a built-in plugin directly within the core so that it cannot give wrong version information
constants various constants fixed when Elektra was compiled
desktop contains information which desktop is currently running

Providing information found on the system not available in persistent files:

uname information from the uname syscall.

Filter

Filter plugins process keys and their values in both directions. In one direction they undo what they do in the other direction. Most filter plugins available now encode and decode values. Storage plugins that use characters to separate key names, values or metadata will not work without them.

Rewrite unwanted characters within strings (code-plugins):

ccode using the technique from arrays in the programming language C
hexcode using hex codes

Rewrite unwanted characters within binary data (binary-plugins):

base64 using the Base64 encoding scheme (RFC4648)

Other filters:

crypto encrypts / decrypts confidential values
fcrypt encrypts / decrypts entire files
gpgme encrypts / decrypts confidential values (with GPGME)
iconv makes sure the configuration will have correct character encoding

Experimental transformations (are not recommended to be used in production):

directoryvalue converts directory values to leaf values
hexnumber converts between hexadecimal and decimal
keytometa transforms keys to metadata
rename renames keys according to different rules
profile renames keys according to current profile

Notification and Logging

Log/Send out all changes to configuration to:

dbus sends notifications for every change via dbus notification
journald logs key database changes to journald
syslog logs key database changes to syslog
zeromqsend sends notifications for every change via ZeroMQ sockets notification

Notification of key changes:

internalnotification get updates automatically when registered keys were changed
dbusrecv receives notifications via dbus notification
zeromqrecv receives notifications via ZeroMQ sockets notification

Debug

Trace everything that happens within KDB:

counter count and print how often a plugin is used
timeofday prints timestamps
tracer traces all calls
iterate iterate over all keys and run exported functions on tagged keys
logchange prints the change of every key on the console

Checker

Copies metadata to keys:

glob using globbing techniques (needed by some plugins)
spec copies metadata from spec namespace (the standard way)

Plugins that check if values are valid based on metadata (typically copied by the spec plugin just before) to validate values:

type type checking (CORBA types) with enum functionality
ipaddr checks IP addresses using regular expressions
email checks email addresses using regular expressions
network by using network APIs
path by checking files on file system
unit validates and normalizes units of memory (e.g. 20KB to 20000 Bytes)
blacklist blacklist and reject values
length validates that string length is less or equal to given value

The same but experimental:

conditionals by using if-then-else like statements
date validates date and time data
mathcheck by mathematical expressions using key values as operands
macaddr checks if MAC addresses are valid and normalizes them
range checks if a value is within a given range
reference checks if a value is a valid reference to another key
rgbcolor validates and normalizes hexcolors
validation by using regex

Other validation mechanisms not based on metadata:

filecheck does sanity checks on a file
lineendings tests file for consistent line endings

Interpreter

These plugins start an interpreter and allow you to execute a script in an interpreted language whenever Elektra’s key database gets accessed. Note that they depend on the presence of the respective binding during run-time:

jni java plugins started by jni, works with jna plugins
lua Lua plugins
python Python 3 plugins
ruby Ruby plugins
shell executes shell commandos

Other Important Plugins

cache caches keysets from previous kdbGet() calls
sync uses POSIX APIs to sync configuration files with the hard disk
gopts global plugin to automatically call elektraGetOpts
process proxy plugin that uses separate executables as plugin implementations
recorder is the plugin used to implement session recording

Plugins for Development

template to be copied for new plugins
cpptemplate a template for C++ based plugins
doc contains the documentation of the plugin interface
error yields errors as described in metadata (handy for test purposes)

Internal Plugins

Internally used and hard coded into libelektra-kdb. Don't try to use manually.

missing
modules
version

Deprecated Plugins

Please avoid, if possible, to use following plugins:

(currently none 🎉)