Plugins

Elektra plugin framework. More...

Macros
#define	ELEKTRA_PLUGIN_FUNCTION(module, function)   libelektra_##module##_LTX_elektraPlugin##function
	Declare a plugin's function name suitable for compilation variants (see doc/tutorials).
#define	ELEKTRA_README(module)   ELEKTRA_README2(module)
	The filename for inclusion of the readme for compilation variants (see doc/tutorials).
#define	ELEKTRA_SET_ERROR(number, key, text)
	Sets the error in the keys metadata.
#define	ELEKTRA_SET_ERRORF(number, key, formatstring,...)
	Sets the error in the keys metadata.
#define	ELEKTRA_ADD_WARNINGF(number, key, formatstring,...)
	Adds an warning in the keys metadata.
#define	ELEKTRA_ADD_WARNING(number, key, text)
	Adds an warning in the keys metadata.

Functions
Plugin *	elektraPluginExport (const char *pluginName,...)
	Allows to Export Methods for a Plugin.
KeySet *	elektraPluginGetConfig (Plugin *handle)
	Returns the configuration of that plugin.
void	elektraPluginSetData (Plugin *plugin, void *data)
	Store a pointer to any plugin related data.
void *	elektraPluginGetData (Plugin *plugin)
	Get a pointer to any plugin related data stored before.
int	elektraDocOpen (Plugin *handle, Key *warningsKey)
	Initialize data for the plugin.
int	elektraDocClose (Plugin *handle, Key *warningsKey)
	Finalize the plugin.
int	elektraDocGet (Plugin *handle, KeySet *returned, Key *parentKey)
	Get data from storage to application.
int	elektraDocSet (Plugin *handle, KeySet *returned, Key *parentKey)
	Set data from application to storage.
int	elektraDocError (Plugin *handle, KeySet *returned, Key *parentKey)
	Rollback in case of errors.

Detailed Description

Elektra plugin framework.

Since

version 0.4.9, Elektra can dynamically load different key storage plugins.

version 0.7.0 Elektra can have multiple backends, mounted at any place in the key database.

version 0.8.0 Elektra backends are composed out of multiple plugins.

To get started with writing plugins, first read our plugin tutorial in doc/tutorials!

A plugin can implement any functionality related to configuration. There are 6 possible entry points for a plugin.

• elektraDocGet() will be called when configuration or the plugin's contract is retrieved from the key database
• elektraDocSet() will be called when configuration is written to the key database
• elektraDocOpen() will be called before any other method of the plugin is called
• elektraDocClose() will be called as last method
• elektraDocError() will be called when kdbSet() failed (to give the plugin a chance to recover/undo its actions)
• elektraPluginExport() exports all methods for the plugin.

The names described here contain "Doc" within the method's name just because the plugin described in this document is called doc (the doxygen source was generated from src/plugins/doc/doc.h). Always replace Doc with the name of the plugin you are going to implement or use ELEKTRA_PLUGIN_FUNCTION.

Overview

There are different types of plugins for different concerns. They all only have the entry points as defined above. The types of plugins handled in this document:

• A storage plugin gets an empty keyset in elektraDocGet() and constructs the information out from a file. In elektraDocSet() the keyset is written to a file.
  Other persistent storage then a file is not handled within this document because it involves many other issues. For files the resolver plugin already takes care for transactions and rollback. So the storage plugin is the source and dump as known from pipes and filters.
• A filter plugin is a plugin which operates on existing keys. It may process or change the keyset. Or it may reject specific keysets which do not meet some criteria.

Use following include to have the functions that are not implement by you available:

#include <kdbplugin.h>

Error and Warnings

In case of trouble, in some methods you can use the macro ELEKTRA_SET_ERROR (in other methods it is not allowed). You might add warnings with the macro ELEKTRA_ADD_WARNING. Read the documentation of the individual methods to decide what you should do.

Use following include to have the macros for setting the error and adding the warnings available:

#include <kdberrors.h>

See Also

http://www.libelektra.org/ftp/elektra/thesis.pdf for an detailed explanation and description of other types of plugins. Do not hesitate to ask at the mailinglist if anything is unclear.

Macro Definition Documentation

#define ELEKTRA_ADD_WARNING	(		number,
			key,
			text
	)

Adds an warning in the keys metadata.

Include kdberrors.h to make it work:

#include <kdberrors.h>

Parameters

number	the warning number from src/liberror/specification
key	to write the error to
text	additional text for the user

#define ELEKTRA_ADD_WARNINGF	(		number,
			key,
			formatstring,
			...
	)

Adds an warning in the keys metadata.

Include kdberrors.h to make it work:

#include <kdberrors.h>

Parameters

number	the warning number from src/liberror/specification
key	to write the error to
formatstring	a format string as in printf
...	further arguments as in printf

#define ELEKTRA_PLUGIN_FUNCTION	(		module,
			function
	)		libelektra_##module##_LTX_elektraPlugin##function

Declare a plugin's function name suitable for compilation variants (see doc/tutorials).

It can be used in the same way as elektraPluginExport().

See Also

ELEKTRA_PLUGIN_EXPORT

Parameters

plugin	the name of the plugin
which	which function it is (open, close, get, set, error)

#define ELEKTRA_README

(

module

)

ELEKTRA_README2(module)

The filename for inclusion of the readme for compilation variants (see doc/tutorials).

Parameters

plugin

the name of the plugin

#define ELEKTRA_SET_ERROR	(		number,
			key,
			text
	)

Sets the error in the keys metadata.

Include kdberrors.h to make it work:

#include <kdberrors.h>

Parameters

number	the error number from src/liberror/specification
key	to write the error to
text	additional text for the user

#define ELEKTRA_SET_ERRORF	(		number,
			key,
			formatstring,
			...
	)

Sets the error in the keys metadata.

Include kdberrors.h to make it work:

#include <kdberrors.h>

Parameters

number	the error number from src/liberror/specification
key	to write the error to
formatstring	a format string as in printf
...	further arguments as in printf

Function Documentation

int elektraDocClose	(	Plugin *	handle,
		Key *	warningsKey
	)

Finalize the plugin.

Called prior to unloading the plugin dynamic module. After this function is called, it is ensured that no functions from your plugin will ever be accessed again.

Make sure to free all memory that your plugin requested at runtime. Also make sure to free what you stored by elektraPluginSetData() before.

So for the Doc plugin we need to:

int elektraDocClose(Plugin *handle, Key *warningsKey ELEKTRA_UNUSED)
{
        free (elektraPluginGetData(handle));

        return 0; /* success */
}

After this call, libelektra.so will unload the plugin library, so this is the point to shutdown any affairs with the storage.

Parameters

handle	contains internal information of the plugin
warningsKey	can be used to to add warnings using ELEKTRA_ADD_WARNING (Do not add errors!)

Return values

0	on success (no other return value currently allowed)
-1	on problems (only use ELEKTRA_ADD_WARNING, but never set an error).

See Also

kdbClose()

elektraPluginGetData(), elektraPluginSetData() and elektraPluginGetConfig()

int elektraDocError	(	Plugin *	handle,
		KeySet *	returned,
		Key *	parentKey
	)

Rollback in case of errors.

First for all plugins elektraDocSet() will be called. If any plugin had problems before the commit (done by the resolver plugin), we can safely rollback our changes.

This method is rarely used by plugins, it is mainly used for resolvers (to implement rollback) or by logging plugins. It is not needed for storage plugins, because they only operate on temporary files created by the resolver.

Parameters

handle	contains internal information of the plugin
returned	contains a keyset with relevant keys
parentKey	contains the information where to set the keys. can be used to add warnings with the macro ELEKTRA_ADD_WARNING, but do not add errors!

Return values

1	on success
0	on success with no action
-1	on failure (you can add warnings, but we are already in an error state, so do not set the error).

int elektraDocGet	(	Plugin *	handle,
		KeySet *	returned,
		Key *	parentKey
	)

Get data from storage to application.

Retrieve information from a permanent storage to construct a keyset.

.

The elektraDocGet() function handle everything related to receiving keys.

Contract Handling

The contract is a keyset that needs to be returned if the parentKey is system/elektra/modules/yourpluginname.

Which keys and their meaning is specified in doc/CONTRACT.ini

Here is an example for our doc plugin:

int elektraDocGet(Plugin *plugin ELEKTRA_UNUSED, KeySet *returned, Key *parentKey)
{
        if (!strcmp(keyName(parentKey), "system/elektra/modules/doc"))
        {
                KeySet *contract = ksNew (30,
                keyNew ("system/elektra/modules/doc",
                        KEY_VALUE, "doc plugin waits for your orders", KEY_END),
                keyNew ("system/elektra/modules/doc/exports", KEY_END),
                keyNew ("system/elektra/modules/doc/exports/open",
                        KEY_FUNC, elektraDocOpen, KEY_END),
                keyNew ("system/elektra/modules/doc/exports/close",
                        KEY_FUNC, elektraDocClose, KEY_END),
                keyNew ("system/elektra/modules/doc/exports/get",
                        KEY_FUNC, elektraDocGet, KEY_END),
                keyNew ("system/elektra/modules/doc/exports/set",
                        KEY_FUNC, elektraDocSet, KEY_END),
                keyNew ("system/elektra/modules/doc/exports/error",
                        KEY_FUNC, elektraDocError, KEY_END),
#include ELEKTRA_README(doc)
                keyNew ("system/elektra/modules/doc/infos/version",
                        KEY_VALUE, PLUGINVERSION, KEY_END),
                KS_END);
                ksAppend (returned, contract);
                ksDel (contract);

                return 1; /* success */
        }

Some clauses of the contract, especially the description of the plugin can be done more conveniently directly in a README.md that is included by ELEKTRA_README.

Storage Plugins

For storage plugins the filename is written in the value of the parentKey. So the first task of the plugin is to open that file. Then it should parse its content and construct a keyset with all information of that file.

You need to be able to reconstruct the same file with the information of the keyset. So be sure to copy all comments, whitespaces and so on into some metadata of the keys. Otherwise the information is lost after writing the file the next time.

Now lets look at an example how the typical elektraDocGet() might be implemented. To explain we introduce some pseudo functions which do all the work with the storage (which is of course 90% of the work for a real plugin):

• parse_key will parse a key and a value from an open file handle

The typical loop for a storage plugin will be like:

        FILE *fp = fopen (keyString(parentKey), "r");
        char *key;
        char *value;

        while (parseKey(fp, &key, &value) >= 1)
        {
                Key *read = keyNew(0);
                if (keySetName(read, key) == -1)
                {
                        fclose (fp);
                        keyDel (read);
                        ELEKTRA_SET_ERROR(59, parentKey, key);
                        return -1;
                }
                keySetString(read, value);

                ksAppendKey (returned, read);
                free (key);
                free (value);
        }

        if (feof(fp) == 0)
        {
                fclose (fp);
                ELEKTRA_SET_ERROR(60, parentKey, "not at the end of file");
                return -1;
        }

        fclose (fp);

Filter Plugins

For filter plugins the actual task is rather unspecified. You basically can do anything with the keyset. To get roundtrip properties you might want to undo any changes you did in elektraDocSet().

The pseudo functions (which do the real work) are:

• do_action() which processes every key in this filter

        Key *k;
        ksRewind (returned);
        while ((k = ksNext (returned)) != 0)
        {
                doAction(k);
        }

        return 1; // success
}

Precondition

The caller kdbGet() will make sure before you are called that the parentKey:

• is a valid key (means that it is a system or user key).
• is your mountpoint and that your plugin is responsible for it.

and that the returned:

• is a valid keyset.
• your plugin is only called when needed (e.g. only if file was modified)
• has all keys related to your plugin.
• contains only valid keys below (see keyIsBelow()) your parentKey.
• is in a sorted order (given implicit by KeySet)

and that the handle:

• is valid for your plugin.
• that elektraPluginGetData() contains the same handle for lifetime of your plugin.

The caller kdbGet() will make sure that afterwards you were called, that:

• other plugins below your plugin will be called again recursively.
• that all keys are merged to one keyset the user gets
• that all keys (that should not be removed) are passed to kdbSet() if writing to disc is needed.

Invariant

There are no global variables and elektraPluginSetData() stores all information. The handle is to be guaranteed to be the same if it is the same plugin.

Postcondition

The keyset returned has the parentKey and all keys below (keyIsBelow()) with all information from the storage. Make sure to return all keys, all directories and also all hidden keys. If some of them are not wished, the caller kdbGet() will drop these keys with additional plugins.

Updating

To get all keys out of the storage over and over again can be very inefficient. You might know a more efficient method to know if the key needs update or not, e.g. by stating it or by an external time stamp info. For file storage plugins this is automatically done for you by the resolver. For other types (e.g. databases) you need to implement your own resolver doing this.

See Also

kdbGet() for caller.

Parameters

handle	contains internal information of opened key database
returned	contains a keyset where the function need to append the keys got from the storage. There might be also some keys inside it, see conditions. You may use them to support efficient updating of keys, see updating.
parentKey	contains the information below which key the keys should be gotten.

Returns

1 on success

0 when nothing was to do

-1 on failure, the current key in returned shows the position. use ELEKTRA_SET_ERROR of kdberrors.h to define the error code. You additionally can add as many warnings as you would like to add.

int elektraDocOpen	(	Plugin *	handle,
		Key *	warningsKey
	)

Initialize data for the plugin.

This is the first method called after dynamically loading this plugin. It is guaranteed, that this method will be called before any other method.

This method is responsible for:

• plugin's specific configuration gathering
• initialization of all plugin's internal structs
• initial setup of all I/O details such as opening a file, connecting to a database, setup connection to a server, iff this cannot be done per invocation in elektraDocGet() and elektraDocSet().

You may also read the configuration you can get with elektraPluginGetConfig() and transform it into other structures used by your plugin.

Note

The plugin must not have any global variables. If you have one Elektra will not be threadsafe. Do not assume that your plugin will be opened only once or will not be reopened at a later time.

Instead of global variables the methods elektraPluginGetData() and elektraPluginSetData() exist to store and get any information related to your plugin.

The correct substitute for global variables will be:

typedef struct { int global; } GlobalData;

and then initialize it using:

int elektraDocOpen(Plugin *handle, Key *warningsKey ELEKTRA_UNUSED)
{
        GlobalData *data;
        KeySet *config = elektraPluginGetConfig(handle);
        Key * kg = ksLookupByName(config, "/global", 0);

        data=malloc(sizeof(GlobalData));
        data->global = 0;
        if (kg) data->global = atoi(keyString(kg));
        elektraPluginSetData(handle,data);

Make sure to free everything you allocate within elektraDocClose().

If your plugin has no useful way to startup without config, the module loader would not be able to load the module. To solve that problem the module loader adds the configuration key /module. Even if your plugin is basically not able to startup successfully, it should still provide a fallback when /module is present, so that elektraDocGet() on system/elektra/modules can be called successfully later on.

        if (ksLookupByName(config, "/module", 0))
        {
                return 0;
        }
        // do some setup that will fail without configuration

Return values

-1	on error, your plugin will be removed then and the missing plugin added instead. Use ELEKTRA_ADD_WARNING to indicate the problem. The system will automatically add the information that the plugin was removed, so you do not need the user give that information.
0	on success

Parameters

handle	contains internal information of the plugin
warningsKey	can be used to add warnings with the macro ELEKTRA_ADD_WARNING (Do not add errors!)

See Also

elektraPluginGetData(), elektraPluginSetData() and elektraPluginGetConfig()

elektraDocClose()

int elektraDocSet	(	Plugin *	handle,
		KeySet *	returned,
		Key *	parentKey
	)

Set data from application to storage.

This function does everything related to set and remove keys in a plugin. There is only one function for that purpose to make implementation of file based plugins much easier.

The keyset returned was filled in with information from the application using elektra and the task of this function is to store it in a permanent way so that a subsequent call of elektraPluginGet() can rebuild the keyset as it was before. See the live cycle to understand:

static void usercode (KDB *handle, KeySet *keyset, Key *key)
{
        // some more user code
        keySetString (key, "mycomment"); // the user changes the key
        ksAppendKey(keyset, key); // append the key to the keyset
        kdbSet (handle, keyset, 0); // and syncs it to disc
}

// so now kdbSet is called
int elektraKdbSet(KDB *handle, KeySet *keyset, Key *parentKey)
{
        int ret = 0;
        // find appropriate plugin and then call it:
        Plugin *plugin = findPlugin(handle);
        ret = elektraDocSet (plugin, keyset, parentKey);
        // the keyset with the key (and others for this plugin)
        // will be passed to this function
        return ret;
}

// so now elektraPluginSet(), which is the function described here,
// is called:
int elektraPluginSet(Plugin *plugin ELEKTRA_UNUSED, KeySet *returned, Key *parentKey ELEKTRA_UNUSED)
{
        // the task of elektraPluginSet is now to store the keys
        Key *k;
        ksRewind (returned);
        while ((k = ksNext (returned)) != 0)
        {
                saveToDisc (k);
        }

        return 1; /* success */
}

Of course all information of every key in the keyset returned need to be stored permanently. So this specification needs to give an exhaustive list of information present in a key.

Precondition

The keyset returned holds all keys which must be saved permanently for this keyset. The keyset is sorted and rewinded.

The parentKey is the key which is the ancestor for all other keys in the keyset. The first key of the keyset returned has the same keyname. The name of the parentKey marks the mountpoint. The string of the parentKey is the filename to write to.

Make sure to set all keys, all directories and also all hidden keys. If some of them are not wished, the caller kdbSet() and plugins will sort them out.

Invariant

There are no global variables, but instead elektraPluginGetData() will be used. The handle is the same when it is the same plugin.

Postcondition

The information of the keyset returned is stored permanently.

See Also

kdbSet() for caller.

Parameters

handle	contains internal information of the plugin
returned	contains a keyset with relevant keys
parentKey	contains the information where to set the keys (name is mountpoint your plugin is mounted, string is the file to write to)

Returns

When everything works gracefully return the number of keys you set. The cursor position and the keys remaining in the keyset are not important.

Return values

1	on success
0	on success with no changed key in database
-1	on failure. The cause of the error needs to be added in parentKey You also have to make sure that ksGetCursor() shows to the position where the error appeared. Set an error using ELEKTRA_SET_ERROR to inform the user what went wrong. Additionally you can add any number of warnings with ELEKTRA_ADD_WARNING.

Plugin* elektraPluginExport	(	const char *	pluginName,
			...
	)

Allows to Export Methods for a Plugin.

This function must be called within ELEKTRA_PLUGIN_EXPORT. It define the plugin's methods that will be exported.

All KDB methods implemented by the plugin basically could have random names (convention is elektraName*), except ELEKTRA_PLUGIN_EXPORT.

This is the single symbol that will be looked up when loading the plugin, and the first method of the backend implementation that will be called.

You need to use a macro so that both dynamic and static loading of the plugin works. For example for the doc plugin:

Plugin *ELEKTRA_PLUGIN_EXPORT(doc)
{
        return elektraPluginExport(DOC_PLUGIN_NAME,
                ELEKTRA_PLUGIN_OPEN,    &elektraDocOpen,
                ELEKTRA_PLUGIN_CLOSE,   &elektraDocClose,
                ELEKTRA_PLUGIN_GET,     &elektraDocGet,
                ELEKTRA_PLUGIN_SET,     &elektraDocSet,
                ELEKTRA_PLUGIN_ERROR,   &elektraDocError,
                ELEKTRA_PLUGIN_END);
}

The first parameter is the name of the plugin. Then every plugin should have: ELEKTRA_PLUGIN_OPEN, ELEKTRA_PLUGIN_CLOSE, ELEKTRA_PLUGIN_GET, ELEKTRA_PLUGIN_SET and optionally ELEKTRA_PLUGIN_ERROR.

The list is terminated with ELEKTRA_PLUGIN_END.

You must use static "char arrays" in a read only segment. Don't allocate storage, it won't be freed.

Parameters

pluginName

the name of this plugin

Returns

an object that contains all plugin informations needed by libelektra.so

KeySet* elektraPluginGetConfig

(

Plugin *

handle

)

Returns the configuration of that plugin.

• The user/ config holds plugin specific configuration
• The system/ config holds backend specific configuration

So prefer cascading lookups to honor both.

Parameters

handle

a pointer to the plugin

Returns

keyset to the configuration for that plugin

void* elektraPluginGetData

(

Plugin *

plugin

)

Get a pointer to any plugin related data stored before.

Parameters

plugin

a pointer to the plugin

Returns

a pointer to the data

void elektraPluginSetData	(	Plugin *	plugin,
		void *	data
	)

Store a pointer to any plugin related data.

Parameters

plugin	a pointer to the plugin
data	the pointer to the data