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How-To: Write a Plugin

This file serves as a tutorial on how to write a storage plugin. Storage plugins are used by Elektra in order to store data in the Elektra Key Database in an intelligent way. They act as a liaison between configuration files and the Key Database. Storage plugins are largely responsible for the functionality of Elektra and they allow many of its advanced features to work.

Basics

First, there are a few basic points to understand about Elektra plugins. This first section will explain the basic layout of a plugin and what various methods exists within one.

The Interface

All plug-ins use the same basic interface. This interface consists of five basic functions, elektraPluginOpen, elektraPluginGet, elektraPluginSet, elektraPluginError, and elektraPluginClose. The developer replaces Plugin with the name of their plugin. So in the case of my plugin, the names of these functions would be elektraLineOpen(), elektraLineGet(), elektraLineSet(), elektraLineError(), and elektraLineClose(). Additionally, there is one more function called ELEKTRA_PLUGIN_EXPORT, where once again Plugin should be replaced with the name of the plug-in, this time in lower-case. So for my line plugin this function would be ELEKTRA_PLUGIN_EXPORT(line). The developer may define elektraPluginCheckConf() if configuration validation at mount time is desired.

The KDB relies on the first five functions for interacting with configuration files stored in the key database. Calls for kdbGet() and kdbClose() will call the functions elektraPluginGet() and elektraPluginClose() respectively for the plugin that was used to mount the configuration data. kdbSet() calls elektraPluginSet() but also elektraPluginError() when an error occurs. elektraPluginOpen() is called before the first call to elektraPluginGet() or elektraPluginSet(). These functions serve different purposes that allow the plug-in to work:

  • elektraPluginOpen() is designed to allow each plug-in to do initialization if necessary.
  • elektraPluginGet() is designed to turn information from a configuration file into a usable KeySet, this is technically the only function that is REQUIRED in a plug-in.
  • elektraPluginSet() is designed to store the information from the keyset back into a configuration file.
  • elektraPluginError() is designed to allow proper rollback of operations if needed and is called if any plugin fails during the set operation. This allows exception-safety.
  • elektraPluginClose() is used to free resources that might be required for the plug-in.
  • ELEKTRA_PLUGIN_EXPORT(Plugin) simply lets Elektra know that the plug-in exists and what the name of the above functions are.

Most simply put: most plug-ins consist of five major functions, elektraPluginOpen(), elektraPluginClose(), elektraPluginGet(), elektraPluginSet(), and ELEKTRA_EXPORT_PLUGIN(Plugin).

Because remembering all these functions can be cumbersome, we provide a skeleton plugin in order to easily create a new plugin. The skeleton plugin is called "template" and a new plugin can be created by calling the copy-template script . For example for my plugin I called ../../scripts/copy-template line from within the plugins directory. Afterwards two important things are left to be done:

  • remove all functions (and their exports) from the plugin that are not needed. For example not every plugin actually makes use of the elektraPluginOpen() function.
  • provide a basic contract as described above

After these two steps your plugin is ready to be compiled, installed and mounted for the first time. Have a look at How-To: kdb mount

Contract

In Elektra, multiple plugins form a backend. If every plugin would do whatever it likes to do, there would be chaos and backends would be unpredictable.

To avoid this situation, plugins export a so called contract. In this contract the plugin states how nicely it will behave and what other plugins can depend on.

Writing a Contract

Because the contracts also contain information for humans, these parts are written in a README.md files of the plugins. To make the contracts machine-readable, the following CMake command exists:

generate_readme(pluginname)

It will generate a readme_plugginname.c (in the build-directory) out of the README.md of the plugin''s source directory.

But prefer to use

add_plugin(pluginname)

where the readme (among many other things) are already done for you. More details about how to write the CMakeLists.txt will be discussed later in the tutorial.

The README.md will be used by:

  • the build system (-DPLUGINS=), e.g. to exclude experimental plugins (infos/status)
  • the mount tool, e.g. to correctly place and order plugins
  • to know dependencies between plugin and what metadata they process

Content of README.md

The first lines must look like:

  • infos = Information about YAIL plugin is in keys below
  • infos/author = Markus Raab [email protected]
  • infos/licence = BSD
  • infos/needs =
  • infos/provides = storage
  • infos/placements = getstorage setstorage
  • infos/recommends = rebase directoryvalue comment type
  • infos/description = JSON using YAIL

The information of these parts are limited to a single line. Only for the description an unlimited amount of lines can be used (until the end of the file).

For the meaning (semantics) of those entries, please refer to contract specification.

The already said generate_readme will produce a list of Keys using the information in README.md. It would look like (for the third key):

	keyNew ("system/elektra/modules/yajl/infos/licence",
		KEY_VALUE, "BSD", KEY_END),

Including readme_pluginname.c

In your plugin, specifically in your elektraPluginGet() implementation, you have to return the contract whenever configuration below system/elektra/modules/plugin is requested:

if (!strcmp (keyName(parentKey), "system/elektra/modules/plugin"))
{
	KeySet *moduleConfig = elektraPluginContract();
	ksAppend(returned, moduleConfig);
	ksDel(moduleConfig);
	return 1;
}

The elektraPluginContract() is a method implemented by the plug-in developer containing the parts of the contract not specified in README.md. An example of this function (taken from the yajl plugin):

static inline KeySet *elektraYajlContract()
{
	return ksNew (30,
	keyNew ("system/elektra/modules/yajl",
		KEY_VALUE, "yajl plugin waits for your orders", KEY_END),
	keyNew ("system/elektra/modules/yajl/exports", KEY_END),
	keyNew ("system/elektra/modules/yajl/exports/get",
		KEY_FUNC, elektraYajlGet,
		KEY_END),
	keyNew ("system/elektra/modules/yajl/exports/set",
		KEY_FUNC, elektraYajlSet,
		KEY_END),
#include "readme_yourplugin.c"
	keyNew ("system/elektra/modules/yajl/infos/version",
		KEY_VALUE, PLUGINVERSION, KEY_END),
	keyNew ("system/elektra/modules/yajl/config", KEY_END),
	keyNew ("system/elektra/modules/yajl/config/",
		KEY_VALUE, "system",
		KEY_END),
	keyNew ("system/elektra/modules/yajl/config/below",
		KEY_VALUE, "user",
		KEY_END),
	KS_END);
}

It basically only contains the symbols to be exported (that are dependent on your functions to be available) and the plugin version information that is always defined to the macro PLUGINVERSION.

As already said, readme_yourplugin.c is generated in the binary directory, so make sure that your CMakeLists.txt contains (prefer to use add_plugin where this is already done correctly):

include_directories (${CMAKE_CURRENT_BINARY_DIR})

CMake

For every plugin you have to write a CMakeLists.txt. If your plugin has no dependencies, you can jump this section. The full documentation of add_plugin is available here.

In order to understand how to write the CMakeLists.txt, you need to know that the same file is included multiple times for different reasons.

  1. The first time, only the name of plugins and directories are enquired. In this phase, only the add_plugin should be executed.
  2. The second time (if the plugin is actually requested), the CMakeLists.txt is used to detect if all dependencies are actually available.

This means that in the first time, only the add_plugin should be executed and in the second time the detection code together with add_plugin.

So that you can distinguish the first and second phase, the variable DEPENDENCY_PHASE is set to ON iff you should find for all needed cmake packages. You should avoid to search for packages otherwise, because this would:

  • clutter the output
  • introduce more variables into the CMakeCache which are irrelevant for the user
  • maybe even find libraries in wrong versions which are incompatible to what other plugins need

So usually you would have:

if (DEPENDENCY_PHASE)
	find_package (LibXml2)
	if (LIBXML2_FOUND)
		# add testdata, testcases...
	else ()
		remove_plugin (xmltool "libxml2 not found")
	endif ()
endif ()

So if you are in the second phase (DEPENDENCY_PHASE), you will search for all dependencies, in this case LibXml2. If all dependencies are satisfied, you add everything needed for the plugin, except the plugin itself. This happens after endif ():

add_plugin (xmltool
	SOURCES
		...
	LINK_LIBRARIES
		${LIBXML2_LIBRARIES}
	DEPENDENCIES
		${LIBXML2_FOUND}
	)

Important is that you pass the information which packages are found as boolean. The plugin will actually be added iff all of the DEPENDENCIES are true.

Note that no code should be outside of if (DEPENDENCY_PHASE) thus it would be executed twice otherwise. The only exception is add_plugin which must be called twice to successfully add a plugin.

If your plugin makes use of compilation variants you should also read the information there.

Coding

This section will focus on an overview of the kind of code you would use to develop a plugin. It gives examples from real plugins and should serve as a rough guide of how to write a storage plugin that can read and write configuration data into the Elektra KeySet.

elektraPluginGet

elektraPluginGet is the function responsible for turning information from a file into a usable KeySet. This function usually differs pretty greatly between each plug-in. This function should be of type int, it returns 0 on success or another number on an error. The function will take in a Key, usually called parentKey which contains a string containing the path to the file that is mounted. For instance, if you run the command kdb mount /etc/linetest system/linetest line then keyString(parentKey) should be equal to /etc/linetest. At this point, you generally want to open the file so you can begin saving it into keys. Here is the trickier part to explain. Basically, at this point you will want to iterate through the file and create keys and store string values inside of them according to what your plug-in is supposed to do. I will give a few examples of different plug-ins to better explain.

The line plug-in was written to read files into a KeySet line by line using the newline character as a delimiter and naming the keys by their line number such as #1, #2, .. #_22 for a file with 22 lines. So once I open the file given by parentKey, every time as I read a line I create a new key, let's call it new_key using dupKey(parentKey). Then I set new_key's name to lineNN (where NN is the line number) using keyAddBaseName and store the string value of the line into the key using keySetString. Once the key is initialized, I append it to the KeySet that was passed into the elektraPluginGet function, let's call it returned for now, using ksAppendKey(return, new_key). Now the KeySet will contain new_key with the name #N properly saved where it should be according to the kdb mount command (in this case, system/linetest/#N), and a string value equal to the contents of that line in the file. The line plug-in repeats these steps as long as it hasn't reached end of file, thus saving the whole file into a KeySet line by line.

The simpleini plug-in works similarly, but it parses for ini files instead of just line-by-line. At their most simple level, ini files are in the format of name=value with each pair taking one line. So for this plug-in, it makes a lot of sense to name each Key in the KeySet by the string to the left of the = sign and store the value into each key as a string. For instance, the name of the key would be name and keyGetString(name) would return value.

As you may have noticed, simpleini and line plug-ins work very similarly. However, they just parse the files differently. The simpleini plug-in parses the file in a way that is more natural to ini file (setting the key's name to the left side of the equals sign and the value to the right side of the equals sign). The elektraPluginGet function is the heart of a storage plug-in, its what allows Elektra to store configurations in it's database. This function isn't just run when a file is first mounted, but whenever a file gets updated, this function is run to update the Elektra Key Database to match.

elektraPluginSet

We also gave a brief overview of elektraPluginSet function. This function is basically the opposite of elektraPluginGet. Where elektraPluginGet reads information from a file into the Elektra Key Database, elektraPluginSet writes information from the database back into the mounted file.

First have a look at the signature of elektraLineSet:

elektraLineSet(Plugin *handle ELEKTRA_UNUSED, KeySet *toWrite, Key *parentKey)

Lets start with the most important parameters, the KeySet and the parentKey. The KeySet supplied is the KeySet that is going to be persisted in the file. In our case it would contain the Keys representing the lines. The parentKey is the topmost Key of the KeySet and serves several purposes. First, it contains the filename of the destination file as its value. Second, errors and warnings can be emitted via the parentKey. We will discuss error handling in more detail later. The Plugin handle can be used to persist state information in a threadsafe way with elektraPluginSetData. As our plugin is not stateful and therefore does not use the handle, it is marked as unused in order to suppress compiler warnings.

Basically the implementation of elektraLineSet can be described with the following pseudocode:

open the file
if (error)
{
	ELEKTRA_SET_ERROR(74, parentKey, keyString(parentKey));
}
for each key
{
	write the key value together with a newline
}
close the file

The full-blown code can be found at line plugin

As you can see, all elektraLineSet does is open a file, take each Key from the KeySet (remember they are named #1, #2 ... #_22) in order, and write each key as it's own line in the file. Since we don't care about the name of the Key in this case (other than for order), we just write the value of keyString for each Key as a new line in the file. That's it. Now, each time the mounted KeySet is modified, elektraPluginSet will be called and the mounted file will be updated.

ELEKTRA_SET_ERROR

We haven't discussed ELEKTRA_SET_ERROR yet. Because Elektra is a library, printing errors to stderr wouldn't be a good idea. Instead, errors and warnings can be appended to a key in the form of metadata. This is what ELEKTRA_SET_ERROR does. Because the parentKey always exists even if a critical error occurs, we append the error to the parentKey. The first parameter is an id specifying the general error that occurred. A listing of existing errors together with a short description and a categorization can be found at error specification. The third parameter can be used to provide additional information about the error. In our case we simply supply the filename of the file that caused the error. The kdb tools will interpret this error and print it in a pretty way. Notice that this can be used in any plugin function where the parentKey is available.

elektraPluginOpen and elektraPluginClose

The elektraPluginOpen and elektraPluginClose functions are not commonly used for storage plug-ins, but they can be useful and are worth reviewing. elektraPluginOpen function runs before elektraPluginGet and is useful to do initialization if necessary for the plug-in. On the other hand elektraPluginClose is run after other functions of the plug-in and can be useful for freeing up resources.

elektraPluginCheckConf

The elektraPluginCheckConf function may be used for validation of the plugin configuration during mount time. The signature of the function is:

int elektraLineCheckConfig (Key * errorKey, KeySet * conf)

The configuration of the plugin is provided as conf. The function may report an error or warnings using the errorKey and the return value.

The following convention was established for the return value of elektraPluginCheckConf:

  • 0: The configuration was OK and has not been changed
  • 1: The configuration has been changed and now it is OK
  • -1: The configuration was not OK and could not be fixed. An error has to be set to errorKey.

The following example demonstrates how to limit the length of the values within the plugin configuration to 3 charaters.

int elektraLineCheckConfig (Key * errorKey, KeySet * conf)
{
	Key * cur;
	ksRewind (conf);
	while ((cur = ksNext (conf)) != 0)
	{
		const char * value = keyString (cur);
		if (strlen (value) > 3)
		{
			ELEKTRA_SET_ERRORF (ELEKTRA_ERROR_VALUE_LENGTH, errorKey, "value %s is more than 3 characters long", value);
			return -1; // The configuration was not OK and could not be fixed
		}
	}
	return 0; // The configuration was OK and has not been changed
}

ELEKTRA_PLUGIN_EXPORT

The last function, one that is always needed in a plug-in, is ELEKTRA_PLUGIN_EXPORT. This functions is responsible for letting Elektra know that the plug-in exists and which methods it implements. The code from the line plugin is a good example and pretty self-explanatory:

Plugin *ELEKTRA_PLUGIN_EXPORT(line)
{
	return elektraPluginExport("line",
	ELEKTRA_PLUGIN_GET, &elektraLineGet,
	ELEKTRA_PLUGIN_SET, &elektraLineSet,
	ELEKTRA_PLUGIN_END);
}

For further information see the API documentation.