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.

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:

Overview Plugins

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().
kdbCheckConfig() can be called manually to ensure a plugin is configured properly.
kdbGenConfig() can be called to produce all valid configurations of a plugin.

KDB-Interface

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

Plugins

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
wresolver minimalistic resolver for non-POSIX systems
noresolver does not resolve, but can act as one
gitresolver checks out and commits files to a local git repository and afterwards the configuration file must be synced with harddisc (recommended to add at every kdb mount):
curlget fetches configuration file from a remote host
blockresolver resolves tagged blocks inside config files
multifile
sync uses POSIX APIs to sync configuration files with the hard disk

Storage

Are responsible for reading writing the configuration to configuration files.

Read and write everything a KeySet might contain:

dini uses by default the ini plugin but has legacy support for dump
ini supports a range of INI file formats.
dump makes a dump of a KeySet in an Elektra-specific format

Read (and write) standard config files:

augeas reads/writes many different configuration files using the augeas library
hosts reads/writes hosts files
line reads/writes any file line by line
yajl reads/writes JSON.

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) ni.
tcl-like config files (including metadata).

Only suited for import/export:

xerces uses XML (without a specific schema).
xmltool uses XML (in Elektra's XML schema).
simpleini line-based key-value pairs with configurable format (without sections)
mini dependency free, line based key-value storage plugin.

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

fstab for fstab files.
regexstore
csvstorage for csv files
passwd for passwd files
dpkg reads /var/lib/dpkg/{available,status}
mozprefs for Mozilla preference files
c writes Elektra C-structures (ksNew(.. keyNew(...)
file reads and writes a file from/to a single key
camel reads and writes a very limited subset of YAML
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, including version information
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.

cachefilter stores filtered keys internally so that they do not get accidentally lost and can be written to the storage again without the user having to remember including them in the writeout

Encoding

Rewrite unwanted characters with different techniques:

ccode using the technique from arrays in the programming language C
hexcode using hex codes
base64 using the Base64 encoding scheme (RFC4648)

Transformations:

directoryvalue converts directory values to leaf values
keytometa transforms keys to metadata
rename renames keys according to different rules
boolean canonicalizes boolean keys

Doing other stuff:

crypto encrypts / decrypts confidential values
fcrypt encrypts / decrypts entire backend files
iconv makes sure the configuration will have correct character encoding
hidden hides keys whose names start with a ..
null takes care of null values and other binary specialities

Notification and Logging

Log/Send out all changes to configuration to:

dbus sends notifications for every change via dbus
syslog logs key database changes to syslog
journald logs key database changes to journald
logchange prints the change of every key on the console

Debug

Trace everything that happens within KDB:

timeofday prints timestamps
tracer traces all calls
counter count and print how often a plugin is used

Checker

Copies metadata to keys:

spec copies metadata from spec namespace (the standard way)
glob using globbing techniques (needed by some plugins)
struct using a defined structure (may also reject configuration not conforming to that structure)

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

Value Validation

validation by using regex
network by using network APIs
ipaddr checks IP addresses using regular expressions
path by checking files on file system
type using run-time type checking (CORBA types/)
enum compares the keyvalue against a list of valid values
mathcheck by mathematical expressions using key values as operands
conditionals by using if-then-else like statements
required rejects non-required keys
date validates date and time data
range checks if a value is within a given range

Other Validation

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
python Python 3 plugins
python2 Python 2 plugins (deprecated)
ruby Ruby plugins
lua Lua plugins
shell executes shell commandos
haskell used for linking haskell plugins and is a small example for such plugins itself

Others

doc contains the documentation of the plugin interface
error yields errors as described in metadata (handy for test purposes)
template to be copied for new plugins
list loads other plugins
iterate iterate over all keys and run exported functions on tagged keys
semlock a semaphore based global locking logic
profile links profile keys
simplespeclang simple configuration specification language