.TH "md_doc_help_elektra-contracts" 3elektra "Sun May 29 2016" "Version 0.8.14" "Elektra" \" -*- nroff -*-
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.SH NAME
md_doc_help_elektra-contracts \- elektra-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 \fCKeySet\fP contains the description\&.
.PP
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\&.
.PP
The contract checker revises contracts of plugins during the mount time\&. Afterwards, at run time, no such type errors can occur\&. \fCkdb mount\fP 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 \fCkdb mount\fP waits for more plugins to be attached\&.
.PP
The content of the contract is well-specified in \fBCONTRACT\&.ini\fP\&.
.PP
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 \fBelektra-algorithm(7)\fP in which the algorithm used by Elektra's core is explained\&.
.PP
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\&.
.PP
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\&.
.PP
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\&.
.PP
\fCKeySet\fP and \fCKey\fP already handle most parts of checking pre- and postconditions imposed on data structures\&. Elektra's core provides preconditions and weak postconditions for the plugins\&.
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.SS "SEE ALSO"
.PP
.IP "\(bu" 2
\fBelektra-ordering(7)\fP 
.PP