Elektra: elektra-plugins(7) -- plugins overview

Elektra 0.8.14

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elektra-plugins(7) -- plugins overview

Plugins can be mounted into the KDB and can access or manipulate the KeySet on every access.

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

# Introduction

Elektra already 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

For background information see elektra-plugins-framework(7).

## 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()`.

## 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).

## See also

For an easy introduction, see this tutorial how to write a storage plugin. For more background information of the plugin framework, continue here. Otherwise, you can visit the the API documentation.

# Plugins

## Resolver

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

- resolver uses POSIX APIs to handle conflicts gracefully
- wresolver minimalistic resolver for non-POSIX systems
- noresolver does not resolve, but can act as one

and afterwards the configuration file must be synced with harddisc (recommended to add at every kdb mount):

- sync uses POSIX APIs to sync configuration file with harddisc

## 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

Read (and write) standard config files of /etc:

- augeas parses and generates many different configuration files using the augeas library
- hosts read/write hosts files
- line reads any file line by line

Using semi-structured data for config files:

- tcl-like config files (including meta data).
- ni parses INI files based on ni.
- ini parses INI files based on inih.
- xmltool uses XML.
- yajl uses JSON.

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

- fstab reads fstab files.
- regexstore
- simpleini is ini without sections
- csvstorage for csv files

## System Information

Information compiled in Elektra:

- version is a build-in plugin directly within the core so that it cannot give wrong version information
- constants various constants, including version information

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.

### Encoding

Rewrite unwanted characters with different techniques:

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

Transformations:

- keytometa transforms keys to metadata
- rename renames keys according to different rules

Doing other stuff:

- crypto encrypts / decrypts confidential values
- iconv make 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
- journald
- syslog
- logchange prints the change of every key on the console

## Debug

Trace everything that happens within KDB:

- timeofday print timestamps
- tracer
- counter count and print how often plugin is used

## Checker

Copies meta data to keys:

- glob using globbing techniques
- struct using a defined structure (may also reject configuration not conforming to that structure)

Plugins that check if values are valid based on meta data (typically copied by another plugin just before):

- validation by using regex
- network by using network APIs
- path by checking files on filesystem
- type using runtime type checking (CORBA types/)
- enum compares the keyvalue against a list of valid values
- mathcheck by mathematical expressions using keysvalues as operants
- conditionals by using if-then-else like statements

## Interpreter

These plugins start an interpreter and allow you to use a bindings.

- jni java plugins started by jni, works with jna plugins
- python Python 3 plugins (technical preview/)
- python2 Python 2 plugins (technical preview, deprecated/)
- lua Lua plugins (technical preview/)

## 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
- lineendings tests file for consistent line endings
- list loads other plugins
- filecheck does sanity checks on a file

To add a new plugin you can copy the template plugin. Please make sure to add your plugin:

- to this list, at least in Others
- to cmake, at least in ALL