bionic (3) md_doc_help_elektra-plugins-framework.3elektra.gz

NAME

       md_doc_help_elektra-plugins-frameworkelektra-plugins-framework(7) -- Background about plugins framework
        - Many component systems pass information between the various components by calling methods of each
       other. This is not the way Elektra's plugin system works. Instead, the core passes a KeySet object in one
       direction from plugin to plugin. So they form a so called pipes-and-filter. Each of the plugins can
       modify the configuration or add any other information using metakeys. While this approach is in general
       less flexible, this information flow still allows powerful chaining. Because plugins do not have to
       bother to call other plugins, the plugin development is also easier. The ordering of plugins in backends
       is controlled using contracts.

       Every plugin should provide a full contract to give information how it will work with other plugins. Most
       parts of the contract are obligatory. Plugins cannot be loaded without this information. For example,
       plugins must provide the clause infos/version. It is vital so that the plugin loader knows which version
       of Elektra the plugin was built for.

   Conditions
       It is, however, up to the plugin not to have every clause of the contract. For example, the plugin might
       not tell what it provides or needs. It can also leave any description out. In this situation it is
       unclear what the plugin will do. Such a plugin can add or remove keys, and changes values and metadata
       regardless what other plugins expect. If only such plugins existed there would be chaos. It would be
       impossible to determine the behaviour of a backend which uses a multiple of such plugins.

       To avoid this situation, every plugin exports a contract describing how the plugin modifies the KeySet
       returned. Most often it is enough to state that it is a storage plugin or that it will filter keys.

       The data structures, however, are already responsible for most of the pre- and postconditions. Every
       condition the data structure guarantees, takes away a concern for the plugins. All the parts that are
       already guaranteed by data structures do not need to be stated in the contract.

       Plugins should not be burdened to check too many postconditions. Instead, plugins focus on their task.
       The plugin does not need to check the sync flag of keys or if the keys are below the mount point. The
       core already guarantees correct behaviour as described in algorithm.

       To sum up, contracts give the information how a plugin interacts with others. It describes if, and how,
       the KeySet returned is changed. Using contracts, we get a predictable behaviour, but still support every
       kind of plugin.

   Exporting Contracts
       As already stated, some parts of the contracts are obligatory. kdb mount needs to know which symbols the
       plugin exports. Only the elektraPluginGet() symbol is mandatory - it is used to yield this information.
       Elektra's core also uses the functions elektraPluginSet(), elektraPluginError(), elektraPluginOpen() and
       elektraPluginClose() if available. Other functions like serialise, unserialise or lookup which implement
       special features can be supported, but are ignored by the core. For the user of the library these
       functions can be very useful. These functions shall either belong to the concern of the plugin or be
       implemented within the plugin because of the dependences.

       As described in infos/provides, the plugin can also provide descriptive information, for example about
       the author and the licence. Advanced plugins can also export plugin configuration for other plugins so
       that the overall backend works properly. Last, but not least, as enumerated in infos/placement dependency
       and placement information makes the system reliable and robust. With that information, plugins can be
       placed into a backend in an automatic and secure way.

       system/elektra/modules provides for every module the information described above. The entry exists once a
       plugin of that module is loaded. For each module a special module backend is generated and mounted at
       system/elektra/modules/<pluginname>. The elektraPluginGet() function generates this described contract on
       requests.

       For example, the ccode plugin, implements:

           int elektraCcodeGet(Plugin *handle, KeySet *returned, Key *parentKey)
           {
                   if (!strcmp (keyName(parentKey), "system/elektra/modules/ccode"))
                   {
                           KeySet *contract = ksNew (30,
                                   keyNew ("system/elektra/modules/ccode",
                                           KEY_END),
                                   keyNew ("system/elektra/modules/ccode/exports",
                                           KEY_END),
                                   //...
                                   KS_END);
                           ksAppend (returned, contract);
                           ksDel (contract);
                           return 1;
                   }
                   // implementation of elektraCcodeGet

       We see in Listing above that the plugin generates and returns the contract if, and only if, the name of
       the parentKey is system/elektra/modules/ccode. The user and the contract checker can access the contract
       of ccode below the key system/elektra/modules/ccode in the same way other configuration is accessed. Note
       that we also have to return 1 at the end of the contract to not execute the regular functionality of the
       plugin.

   Changing Plugins
       This configuration is static and contains the contract information. In theory, the contract can be
       changed without any problems in ways that it provides more and obligates less. But the problem is that it
       will not be checked if this is the case because a recheck of the contracts of a backend is very
       expensive. The contract checker doing this, only runs once during mount time. Changing contracts in an
       incompatible way forces the user to remove all mount points where the plugin is and mount it again. Such
       actions are only sustainable in a development phase and not in a productive environment.

       But the plugin's implementation is allowed to change without being remounted if it is a subtype of the
       earlier version. Only in this situation it can be a drop-in placement. With a good testing framework the
       behaviour can be checked to some extent.

       We also see in Listing above that the code responsible for generating the contract and the code for the
       implementation are next to each other. Plugins need to satisfy those self-imposed obligations that are
       described in contracts. They ensure that plugins interact in predictable ways. So the process of writing
       individual plugins and composing them together can be described as Component-Based Software Engineering.

       Plugins can also be viewed as framework extensions. A component abstracts plugins. But this term is
       misleading in our case, because components usually can choose which interfaces they implement. Elektra's
       plugins, however, are restricted to implement one specific interface. Without contracts, plugins could
       not interact as described in this chapter.

   SEE ALSO
       • elektra-plugins-ordering(7)

       • elektra-contracts(7)