man elektra-architecture (7): architecture of elektra

API

The aim of the Elektra Project is to design and implement a powerful API for configuration. When the project started, we assumed that this goal was easy to achieve, but dealing with the semantics turned out to be a difficult problem. For the implementation, an ambitious solution is required because of the necessary modularity to implement flexible backends as introduced in Elektra. But also the design of a good API has proved to be much more difficult than expected.

Changes in the APIs

From Elektra 0.7 to Elektra 0.8, we changed the API of Elektra as little as possible. It should be mentioned that KeySet is now always sorted by name. The function ksSort() is now depreciated and was removed. The handling of removed keys was modified. Additionally, the API for metadata has fundamentally changed, but the old interface still works. These changes will be described in implementation of meta data elektra-meta-data.md. However, the implementation of Elektra changed radically as discussed in algorithm elektra-algorithm.md.

API Design

API Design presents a critical craft every programmer should be aware of. We will shortly present some of the main design issues that matter and show how Elektra has solved them.

A design goal is to detect errors early. As easy as it sounds, as difficult it is to actually achieve this goal. Elektra tries to avoid the problem by checking data being inserted into Key and KeySet. Elektra catches many errors like invalid key names soon. Elektra allows plugins to check the configuration before it is written into the key database so that problematic values are never stored.

''Hard to use it wrong'' tends to be a more important design objective than ''Easy to use it right''. Searching for a stupid bug costs more time than falling into some standard traps which are explained in documentation. In Elektra, the data structures are robust and some efforts were taken to make misuse unlikely.

Another fundamental principle is that the API must hide implementation details and should not be optimised towards speed. In Elektra, the actual process of making configuration permanent is completely hidden.

''Off-by-one confusion'' is a topic of its own. The best is to stick to the conventions the programming language gives. For returning sizes of strings, it must be clear whether a terminating '\0' is included or not. All such decisions must be consistent. In Elektra the terminating null is always included in the size.

The interface must be as small as possible to tackle problems addressed by the library. Internal and external APIs must be separated. Internal APIs in libraries shall be declared as static to prevent its export. In Elektra, internal names start with elektra opposed to the external names starting with key, ks or kdb.

Elektra always passes user context pointers, but never passes or receives a full data structure by value. It is impossible to be ABI\footnote{We will read more about ABI in \secref{ABI}.} compatible otherwise. Elektra is restrictive in what it returns (strong postconditions), but as liberal as possible for what comes in (preconditions are avoided where possible). In Elektra even null pointers are accepted for any argument.

''Free everything you allocate'' is a difficult topic in some cases. If Elektra cannot free space or other resources after every call, it provides a close() function. Everything will be freed. The tool Valgrind with Memcheck helps us locate problems. The whole test suite runs without any memory problems. The user is responsible for deleting all created Key and KeySet objects and closing the KDB handle.

As a final statement, we note that the UNIX philosophy should always be considered: ''Do only one thing, but do it in the best way. Write it that way that programs work together well.''

Modules

Elektra's core can be compiled with a C compiler conforming to the ISO/IEC 9899:1999 standard:

•: One line comments,
•: inline functions,
•: snprintf()
•: inttypes.h and
•: variable declaration at any place

are used in addition to what is already defined in the standard ISO/IEC 9899:1990, called C99 in the following text. Functions not conforming to C99 are considered to be not portable enough for Elektra and are separated into plugins. But there is one notable exception: it must be the core's task to load plugins. Unfortunately, C99 does not know anything about modules. POSIX (Portable Operating System Interface) provides dlopen(), but other operating systems have dissimilar APIs for that purpose. They sometimes behave differently, use other names for the libraries and have incompatible error reporting systems. Because of these requirements Elektra provides a small internal API to load such modules independently from the operating system. This API also hides the fact that modules must be loaded dynamically if they are not available statically.

Plugins are usually realised with modules. Modules and libraries are technically the same in most systems. (One exception is Mac OS X.) After the module is loaded, the special function plugin factory is searched for. This function returns a new plugin. With the plugin factory the actual plugins are created.

Static loading

For the static loading of modules, the modules must be built-in. With dlopen(const char* file) POSIX provides a solution to look up such symbols by passing a null pointer for the parameter file. Non-POSIX operating systems may not support this kind of static loading. Therefore, Elektra provides a C99 conforming solution for that problem: a data structure stores the pointers to the plugin factory of every plugin. The build system generates the source file of this data structure because it depends on built-in plugins as shown in Figure~\ref{fig:architecture}.

Elektra distinguishes internally between modules and plugins. Several plugins can be created out of a single module. During the creation process of the plugin, dynamic information -- like the configuration or the data handle -- is added.

API

The API of \intro[libloader@\lstinline{libloader}]{libloader} consists of the following functions:

Interface of Module System:

elektraModulesInit (KeySet *modules, Key *error); elektraPluginFactory
elektraModulesLoad (KeySet *modules,
        const char *name, Key *error);
int elektraModulesClose (KeySet *modules, Key *error); \end{lstlisting}

elektraModulesInit() initialises the module cache and calls necessary operating system facilities if needed. \method{elektraModulesLoad()} does the main work by either returning a pointer to the plugin factory from cache or loading it from the operating system. The plugin factory creates plugins that do not have references to the module anymore. elektraModulesClose() cleans up the cache and finalises all connections with the operating system.

Not every plugin is loaded by libloader. For example, the version plugin, which exports version information, is implemented internally.

Mount Point Configuration

kdb mount creates a mount point configuration as shown in the example below. fstab is a unique name within the mount point configuration provided by the administrator.

Example for a mount point configuration:

system/elektra/mountpoints system/elektra/mountpoints/fstab
system/elektra/mountpoints/fstab/config
system/elektra/mountpoints/fstab/config/path=fstab
system/elektra/mountpoints/fstab/config/struct=list FStab
system/elektra/mountpoints/fstab/config/struct/FStab
system/elektra/mountpoints/fstab/config/struct/FStab/device
system/elektra/mountpoints/fstab/config/struct/FStab/dumpfreq
system/elektra/mountpoints/fstab/config/struct/FStab/mpoint
system/elektra/mountpoints/fstab/config/struct/FStab/options
system/elektra/mountpoints/fstab/config/struct/FStab/passno
system/elektra/mountpoints/fstab/config/struct/FStab/type
system/elektra/mountpoints/fstab/errorplugins
system/elektra/mountpoints/fstab/errorplugins/#5#resolver#resolver#
system/elektra/mountpoints/fstab/getplugins
system/elektra/mountpoints/fstab/getplugins/#0#resolver
system/elektra/mountpoints/fstab/getplugins/#5#fstab#fstab#
system/elektra/mountpoints/fstab/mountpoint /fstab
system/elektra/mountpoints/fstab/setplugins
system/elektra/mountpoints/fstab/setplugins/#0#resolver
system/elektra/mountpoints/fstab/setplugins/#1#struct#struct#
system/elektra/mountpoints/fstab/setplugins/#2#type#type#
system/elektra/mountpoints/fstab/setplugins/#3#path#path#
system/elektra/mountpoints/fstab/setplugins/#3#path#path#/config
system/elektra/mountpoints/fstab/setplugins/#3#path#path#/config/path/allow=proc tmpfs none
system/elektra/mountpoints/fstab/setplugins/#5#fstab
system/elektra/mountpoints/fstab/setplugins/#7#resolver \end{lstlisting}

Let us look at the subkeys below the key system/elektra/mountpoints/fstab:

config: Everything below \keyname{config} is the system's configuration of the backend. Every plugin within the backend will find this configuration directly below \keyname{system/} in its \intro{plugin configuration}. For example,
: system/elektra/mountpoints/fstab/config/struct/FStab/mpoint

will be translated to

system/struct/FStab/mpoint

and inserted into the plugin configuration for all plugins in the fstab backend.

It is the place where configuration can be provided for every plugin of a backend. The contract checker deduces this configuration to satisfy the contract for a plugin. Fstab, for example, claims in a contract that it needs ''struct''. But the struct plugin needs a configuration to work properly. Fstab will provide this configuration. The \empha{contract checker} writes out the configuration looking like the one in this example.

config/path: is a common setting needed by the resolver plugin. It is the relative path to a filename that is used by this backend. On UNIX systems, the resolver would determine the name /etc/fstab for system configuration.
mountpoint: is a key that represents the mount point. Its value is the location where the backend is mounted. If a mount point has an entry for both the user and the system hierarchy, it is called \intro{cascading mount point}. A cascading mount point differs from two separate mount points because internally only one backend is created. In the example, the mount point /fstab means that the backend handles both user/fstab and system/fstab. If the mount point is /, the backend will be mounted to all namespaces except spec, including both user and system.
errorplugins: presents a list of all plugins to be executed in the error case of kdbSet() which will be explained in \secref{error situation}.
getplugins: is a list of all plugins used when reading the configuration from the key database. They are executed in kdbGet().
setplugins: contains a list of all plugins used when storing configuration. They are executed in kdbSet().

Each of the plugins inside the three lists may have the subkey config. The configuration below this subkey provides plugin specific configuration. This configuration appears in the user's configuration of the plugin. Configuration is renamed properly. For example, the key

system/elektra/mountpoints/fstab/setplugins/#3#path#path#/config/path/allow

is transformed to

user/path/allow

and appears in the plugin configuration of the path plugin inside the fstab backend.

Referencing

The same plugin often must occur in more than one place within a backend. The most common use case is a plugin that has to be executed for both kdbGet() and kdbSet(). It must be the same plugin if it preserves state between the executions.

Other plugins additionally have to handle error or success situations. One example of exceptional intensive use is the resolver plugin. It is executed twice in kdbSet(). In kdbGet() it is also used as shown in Listing~\ref{lst:mount point configuration}.

\lstinline[language=]{#nname} introduces a new plugin from the module name which cannot be referenced later. The cypher n appoints the actual placement of the plugin. \lstinline[language=]{#n#name#

Changing Mount Point Configuration

When the user changes the mount point configuration, without countermeasures, applications already started will continue to run with the old configuration. This could lead to a problem if backends in use are changed or removed. It is necessary to restart all such programs. Notification is the best way to deal with the situation. Changes of the mount point configuration, however, do not occur often. For some systems, the manual restart may also be appropriate.

In this situation, applications can receive warning or error information if the configuration files are moved or removed. The most adverse situation occurs if the sequence of locking multiple files produces a \empha{dead lock}. Under normal circumstances, the sequence of locking the files is deterministic, so either all locks can be requested or another program will be served first. But several programs with different mount point configurations running at the same time can cause a disaster. The problem gets even worse, because kdb mount is unable to detect such situations. Every specific mount point configuration for itself is trouble-free.

But still a dead lock can arise when multiple programs run with different mount point configurations. Suppose we have a program A which uses the backends B1 and B2 that requests locks for the files F1 and F2. Then the mount point configuration is changed. The user removes B1 and introduces B3. B3 is in a different path mounted after B2, but also accesses the same file F1. The program B starts after the mount point configuration is changed. So it uses the backends B2 and B3. If the scheduler decides that first A and then B both successfully lock the files F1 and F2, a dead lock situation happens because in the afterwards the applications A and B try to lock F2 and F1.

A manual solution for this problem is to enable kdb to output a list of processes that still use old mount point configuration. The administrator can restart these processes. The preferred solution is to use notification for mount point configuration changes or simply to use a lock-free resolver.

Continue reading with the data structures elektra-data-structures.md.

API