bionic (3) md_doc_help_elektra-contracts.3elektra.gz

NAME

       md_doc_help_elektra-contractselektra-contracts(7) -- contracts for plugins
        - Each plugin in a backend can cause run time errors. Additionally, the chaining of the plugins can
       introduce further run time errors. For example, a plugin can modify keys so that the next plugin cannot
       process these keys anymore. Or a plugin can omit changes to the keys that are required by the next
       plugin. To deal with such situations in a controlled way, each plugin exports a contract that describes
       the interaction with other parts of the backend. A KeySet contains the description.

       Mounting of backends actually takes place at run time, we will refer to it as mount time. The time when
       applications access the key database, however, will be called run time.

       The contract checker revises contracts of plugins during the mount time. Afterwards, at run time, no such
       type errors can occur. kdb mount implements the contract checker. It can refuse to add a plugin to the
       backend because of a conflict or constraint. As long as not all contracts are satisfied kdb mount waits
       for more plugins to be attached.

       The content of the contract is well-specified in CONTRACT.ini.

       Plugins are the only place to export contracts, but they are not the only party. It is also possible that
       the administrator extends the requirements to every backend, for example, if notification is required.
       This implies that every backend has to provide a specific additional service that will be checked using
       contracts. Plugins are also involved in contracts with Elektra's core. This topic will be discussed in
       elektra-algorithm(7) in which the algorithm used by Elektra's core is explained.

       Contracts as introduced by Meyer define preconditions and postconditions on routines. Assertions handle
       these conditions by checking them before entering or after leaving a routine. Because most of this can
       happen only at run time, it often leaves the user alone with an exception.

       Elektra goes a step further. Instead of exiting the normal control flow when a precondition is not met,
       it is the responsibility of a special plugin to handle the situation and make sure that the precondition
       is met afterwards. Sometimes this is not possible. In these cases, the plugins check the necessary
       conditions and return with error code when they are not met. As we will see, these situations do not
       occur very often.

       Because of the modular approach, we can have several checkers, and correcting and checking can be
       combined. Plugins can work together to reach a certain goal.

       KeySet and Key already handle most parts of checking pre- and postconditions imposed on data structures.
       Elektra's core provides preconditions and weak postconditions for the plugins.

   SEE ALSO
       • elektra-ordering(7)