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NAME
md_doc_help_elektra-architecture - elektra-architecture
• infos = Information about the conditionals plugin is in keys below
• infos/author = Thomas Waser thomas.waser@libelektra.org
• infos/licence = BSD
• infos/needs =
• infos/provides =
• infos/placements = presetstorage
• infos/description =
This plugin uses if-then-else like conditions stored in the metakey check/condition to validate data.
Syntax
(IF-condition) ? (THEN-condition) : (ELSE-condition) where the ELSE-condition is optional
Condition: Key Operation `('String' | '1234.56' | Key | '')`
Operations: !=, ==, <, <=, =>, >, :=, where:
• := is used to set a key value
• others are for comparison as in C
Example
`(this/key != 'value') ? (then/key == some/other/key) : (or/key <= '125')`
Meaning: IF this/key NOT EQUAL TO 'value' THEN then/key MUST EQUAL some/other/key ELSE or/key MUST BE
LESS THAN 125
Another full example:
kdb mount conditionals.dump /tmount/conditionals conditionals dump
kdb set user/tmount/conditionals/fkey 3.0
kdb set user/tmount/conditionals/hkey hello
kdb setmeta user/tmount/conditionals/key check/condition "(hkey == 'hello') ? (fkey == '3.0')" # success
kdb setmeta user/tmount/conditionals/key check/condition "(hkey == 'hello') ? (fkey == '5.0')" # fail
int 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 \nd{lstlisting}
Let us look at the subkeys below the key system/elektra/mountpoints/fstab:
• config: Everything below {config} is the system's configuration of the backend. Every plugin within the
backend will find this configuration directly below {system/} in its {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 {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
{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 {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~lst:mount point configuration}.
[language=]{#n<name>} introduces a new plugin from the module name which cannot be referenced later. The
cypher n appoints the actual placement of the plugin. [language=]{#n#<name>#<label>#} also introduces a
new plugin from the module name and gives it the name label. The last [language=]{#} shows that a new
name is being introduced. [language=]{#n#<ref>} references back to a label which was introduced before.
This configuration does not create a new plugin. kdb mount already implements the generation of these
names as described above.
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
{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.
Version 0.8.14 Tue Dec 15 2015 md_doc_help_elektra-architecture(3elektra)