- infos = Information about the yamlcpp plugin is in keys below
- infos/author = René Schwaiger sanssecours@me.com
- infos/licence = BSD
- infos/needs = base64 directoryvalue
- infos/provides = storage/yaml
- infos/recommends =
- infos/placements = getstorage setstorage
- infos/status = maintained unittest preview unfinished concept discouraged
- infos/metadata =
- infos/description = This storage plugin reads and writes data in the YAML format
YAML CPP
Introduction
The YAML CPP plugin reads and writes configuration data via the yaml-cpp library.
Usage
You can mount this plugin via kdb mount:
. To unmount the plugin use kdb umount:
. The following examples show how you can store and retrieve data via yamlcpp.
`` @section autotoc_md761 Mount yamlcpp plugin to cascading namespace/tests/yamlcpp` sudo kdb mount config.yaml /tests/yamlcpp yamlcpp
Manually add a mapping to the database
echo "🔑 : 🐳" > kdb file /tests/yamlcpp
Retrieve the value of the manually added key
kdb get /tests/yamlcpp/🔑 #> 🐳
Manually add syntactically incorrect data
echo "some key: @some value" >> kdb file /tests/yamlcpp kdb get "/tests/yamlcpp/some key"
STDERR: .*yaml-cpp: error at line 2, column 11: unknown token.*
ERROR: C03100
RET: 5
Overwrite incorrect data
echo "🔑: value" > kdb file /tests/yamlcpp
Add some values via <tt>kdb set</tt>
kdb set /tests/yamlcpp 🎵 kdb set /tests/yamlcpp/fleetwood mac kdb set /tests/yamlcpp/the chain
Retrieve the new values
kdb get /tests/yamlcpp #> 🎵 kdb get /tests/yamlcpp/the #> chain kdb get /tests/yamlcpp/fleetwood #> mac
Undo modifications
kdb rm -r /tests/yamlcpp sudo kdb umount /tests/yamlcpp
Mount yamlcpp plugin to <tt>user/tests/yamlcpp</tt>
sudo kdb mount config.yaml user/tests/yamlcpp yamlcpp
Manually add an array to the database
echo 'sunny:' > kdb file user/tests/yamlcpp echo ' - Charlie' >> kdb file user/tests/yamlcpp echo ' - Dee' >> kdb file user/tests/yamlcpp
List the array entries
kdb ls user/tests/yamlcpp #> user/tests/yamlcpp/sunny #> user/tests/yamlcpp/sunny/#0 #> user/tests/yamlcpp/sunny/#1
Read an array entry
kdb get user/tests/yamlcpp/sunny/#1 #> Dee
You can retrieve the last index of an array by reading the metakey <tt>array</tt>
kdb meta-get user/tests/yamlcpp/sunny array
1
Extend the array
kdb set user/tests/yamlcpp/sunny/#2 Dennis kdb set user/tests/yamlcpp/sunny/#3 Frank kdb set user/tests/yamlcpp/sunny/#4 Mac
The plugin supports empty array fields
kdb set user/tests/yamlcpp/sunny/#_10 'The Waitress' kdb meta-get user/tests/yamlcpp/sunny array #> #_10 kdb get user/tests/yamlcpp/sunny/#_9
RET: 11
Retrieve the last array entry
kdb get user/tests/yamlcpp/sunny/$(kdb meta-get user/tests/yamlcpp/sunny array) #> The Waitress
The plugin also supports empty arrays (arrays without any elements)
kdb meta-set user/tests/yamlcpp/empty array '' kdb export user/tests/yamlcpp/empty yamlcpp #> []
Arrays in Elektra always require the <tt>array</tt> metakey.
Otherwise the keys will be interpreted as normal key-value mappings.
kdb set user/tests/yamlcpp/movies kdb set user/tests/yamlcpp/movies/#0 'A Silent Voice' kdb export user/tests/yamlcpp/movies yamlcpp #> "#0": A Silent Voice kdb meta-set user/tests/yamlcpp/movies array '' kdb export user/tests/yamlcpp/movies yamlcpp #> - A Silent Voice
Undo modifications to the key database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
Mount yamlcpp plugin to <tt>user/tests/yamlcpp</tt>
sudo kdb mount config.yaml user/tests/yamlcpp yamlcpp
Add some key value pairs
kdb set user/tests/yamlcpp/key value kdb set user/tests/yamlcpp/array kdb set user/tests/yamlcpp/array/#0 scalar kdb set user/tests/yamlcpp/array/#1/key value kdb set user/tests/yamlcpp/array/#1/🔑 🙈 kdb meta-set user/tests/yamlcpp/array array '#1'
kdb ls user/tests/yamlcpp #> user/tests/yamlcpp/array #> user/tests/yamlcpp/array/#0 #> user/tests/yamlcpp/array/#1/key #> user/tests/yamlcpp/array/#1/🔑 #> user/tests/yamlcpp/key
Retrieve part of an array value
kdb get user/tests/yamlcpp/array/#1/key #> value
Since an array saves a list of values, an array parent
- which represent the array - does not store a value!
echo "user/tests/yamlcpp/array: “`kdb get user/tests/yamlcpp/array`”" #> user/tests/yamlcpp/array: “”
Remove part of an array value
kdb rm user/tests/yamlcpp/array/#1/key
kdb ls user/tests/yamlcpp #> user/tests/yamlcpp/array #> user/tests/yamlcpp/array/#0 #> user/tests/yamlcpp/array/#1/🔑 #> user/tests/yamlcpp/key
The plugin stores array keys using YAML sequences.
Since yaml-cpp stores keys in arbitrary order -
either <tt>key</tt> or <tt>array</tt> could be the “first” key -
we remove <tt>key</tt> before we retrieve the data. This way
we make sure that the output below will always look
the same.
kdb rm user/tests/yamlcpp/key kdb file user/tests/yamlcpp | xargs cat #> array: #> - scalar #> - 🔑: 🙈
Undo modifications to the key database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
Mount yamlcpp plugin
sudo kdb mount config.yaml user/tests/yamlcpp yamlcpp
kdb set user/tests/yamlcpp/#0/map/#1/#0 value kdb set user/tests/yamlcpp kdb meta-set user/tests/yamlcpp array '#0' kdb set user/tests/yamlcpp/#0/map kdb meta-set user/tests/yamlcpp/#0/map array '#1' kdb set user/tests/yamlcpp/#0/map/#1 kdb meta-set user/tests/yamlcpp/#0/map/#1 array '#0' kdb file user/tests/yamlcpp | xargs cat #> - map: #> - ~ #> - #> - value
The plugin adds the missing array parents to the key set
kdb ls user/tests/yamlcpp #> user/tests/yamlcpp #> user/tests/yamlcpp/#0/map #> user/tests/yamlcpp/#0/map/#0 #> user/tests/yamlcpp/#0/map/#1 #> user/tests/yamlcpp/#0/map/#1/#0
Undo modifications to the key database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
. As we can see above the value containing metadata is marked by the tag handle !elektra/meta. The data type contains a list with two elements. The first element of this list specifies the value of the key, while the second element contains a map saving the metadata for the key. The data above represents the following key set in Elektra if we mount the file directly to the namespace user:
| Name | Value | Metaname | Metavalue |
|---|---|---|---|
| user/key without metadata | value1 | — | — |
| user/key with metadata | value2 | metakey | metavalue |
| empty metakey | — | ||
| another metakey | another metavalue |
. The example below shows how we can read and write metadata using the yamlcpp plugin via kdb.
`` @section autotoc_md802 Mount yamlcpp plugin touser/tests/yamlcpp` sudo kdb mount config.yaml user/tests/yamlcpp yamlcpp
Manually add a key including metadata to the database
echo "🔑: !elektra/meta [🦄, {comment: Unicorn}]" > kdb file user/tests/yamlcpp kdb meta-ls user/tests/yamlcpp/🔑 #> comment kdb meta-get user/tests/yamlcpp/🔑 comment #> Unicorn
Add a new key and add some metadata to the new key
kdb set user/tests/yamlcpp/brand new kdb meta-set user/tests/yamlcpp/brand comment "The Devil And God Are Raging Inside Me" kdb meta-set user/tests/yamlcpp/brand rationale "Because I Love It"
Retrieve metadata
kdb meta-ls user/tests/yamlcpp/brand #> comment #> rationale kdb meta-get user/tests/yamlcpp/brand rationale #> Because I Love It
Undo modifications to the key database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
sudo kdb mount config.yaml user/tests/yamlcpp yamlcpp type kdb set user/tests/yamlcpp/typetest/number 21 kdb meta-set user/tests/yamlcpp/typetest/number check/type short
kdb set user/tests/yamlcpp/typetest/number "One"
RET: 5
STDERR: .*Validation Semantic.*
ERROR: C03200
kdb get user/tests/yamlcpp/typetest/number #> 21
Undo modifications to the key database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
Mount YAML CPP plugin at <tt>user/tests/binary</tt>
sudo kdb mount test.yaml user/tests/binary yamlcpp
Manually add binary data
echo 'bin: !!binary aGk=' > kdb file user/tests/binary
Base 64 decodes the data <tt>aGk=</tt> to <tt>hi</tt> and stores the value in binary form.
The command <tt>kdb get</tt> prints the data as hexadecimal byte values.
kdb get user/tests/binary/bin #> \x68\x69
Add a string value to the database
kdb set user/tests/binary/text mate
Base 64 does not modify textual values
kdb get user/tests/binary/text #> mate
The Base 64 plugin re-encodes binary data before YAML CPP stores the key set. Hence the
configuration file contains the value <tt>aGk=</tt> even after YAML CPP wrote a new configuration.
grep -q 'bin: !.* aGk=' kdb file user/tests/binary
RET: 0
Undo modifications to the database
kdb rm -r user/tests/binary sudo kdb umount user/tests/binary
Mount YAML CPP plugin at <tt>user/tests/yamlcpp</tt>
sudo kdb mount test.yaml user/tests/yamlcpp yamlcpp
Check if the plugin saves null keys correctly
kdb set user/tests/yamlcpp/null kdb set user/tests/yamlcpp/null/level1/level2 kdb meta-set user/tests/yamlcpp/null/level1/level2 comment 'Null key'
kdb ls user/tests/yamlcpp/null #> user/tests/yamlcpp/null #> user/tests/yamlcpp/null/level1/level2 kdb get -v user/tests/yamlcpp/null | grep -q 'The key is null.' kdb get -v user/tests/yamlcpp/null/level1/level2 | grep -q 'The key is null.'
Check if the plugin saves empty keys correctly
kdb set user/tests/yamlcpp/empty "" kdb set user/tests/yamlcpp/empty/level1/level2
kdb ls user/tests/yamlcpp/empty #> user/tests/yamlcpp/empty #> user/tests/yamlcpp/empty/level1/level2 kdb get -v user/tests/yamlcpp/empty | grep -vq 'The key is null.'
Undo modifications to the database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
Mount YAML CPP plugin at <tt>user/tests/yamlcpp</tt>
sudo kdb mount config.yaml user/tests/yamlcpp yamlcpp
Manually add boolean key
echo 'truth: true' > kdb file user/tests/yamlcpp
kdb get user/tests/yamlcpp/truth #> 1
A boolean in Elektra has the type <tt>boolean</tt>
kdb meta-get user/tests/yamlcpp/truth type #> boolean
Add another boolean value
kdb set user/tests/yamlcpp/success 0 kdb meta-set user/tests/yamlcpp/success type boolean kdb get user/tests/yamlcpp/success #> 0 kdb export user/tests/yamlcpp/success yamlcpp #> false
Undo modifications to the database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
stores all of the values (🍂, leaf and 🍁) in the leaves of the mapping. The drawing below makes this situation a little bit clearer.
The key set that this plugin creates using the data above looks like this (assuming we mount the plugin to user/tests/yamlcpp):
| Name | Value |
|---|---|
| user/tests/yamlcpp/level | |
| user/tests/yamlcpp/level 1/level 2 | |
| user/tests/yamlcpp/level 1/level 2/level 3 | 🍁 |
| user/tests/yamlcpp/root | |
| user/tests/yamlcpp/root/below root | leaf |
| user/tests/yamlcpp/root/subtree | 🍂 |
. Now why is this plugin unable to store values outside leaf nodes? For example, why can we not store a value inside user/tests/yamlcpp/level 1/level 2? To answer this question we need to look at the YAML representation:
. In a naive approach we might just try to add a value e.g. 🙈 right next to level 2:
. This however would be not correct, since then the YAML node level 2 would contain both a scalar value (🙈) and a mapping ({ level 3: 🍁 }). We could solve this dilemma using a list:
. However, if we use this approach we are not able to support Elektra’s array type properly.
Directory Values
To overcome the limitation described above, the YAML CPP plugin requires the Directory Value plugin. This plugin converts the value of a non-leaf node to a leaf node with the name ___dirdata. For example, let us assume we have the following key set:
. The Directory Value plugin will convert the key set in the set (write) direction to
. Consequently the YAML plugin will store the key set as
. A user of the YAML plugin will not notice this feature unless he edits the configuration file by hand, as the following example shows:
`` @section autotoc_md831 Mount YAML CPP plugin atuser/tests/yamlcpp` sudo kdb mount test.yaml user/tests/yamlcpp yamlcpp
kdb set user/tests/yamlcpp/directory 'Directory Data' kdb meta-set user/tests/yamlcpp/directory comment 'Directory Metadata' kdb set user/tests/yamlcpp/directory/file 'Leaf Data'
kdb ls user/tests/yamlcpp/directory #> user/tests/yamlcpp/directory #> user/tests/yamlcpp/directory/file
kdb get user/tests/yamlcpp/directory #> Directory Data kdb meta-get user/tests/yamlcpp/directory comment #> Directory Metadata kdb get user/tests/yamlcpp/directory/file #> Leaf Data
Undo modifications to the database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
Mount plugin
sudo kdb mount test.yaml user/tests/yamlcpp yamlcpp
kdb set user/tests/yamlcpp/boolean true
The following command should print a quoted YAML scalar
(e.g. <tt>"true"</tt> or ‘'true’`).
kdb export user/tests/yamlcpp/boolean yamlcpp #> true
Since the value is not quoted the YAML CPP plugin will
correctly convert the YAML data into one of Elektra’s
boolean values (<tt>0</tt> or <tt>1</tt>).
kdb get user/tests/yamlcpp/boolean #> 1
Undo modifications to the database
kdb rm -r user/tests/yamlcpp sudo kdb umount user/tests/yamlcpp
kdb mount default.yaml / yamlcpp ```
all tests should work correctly. The problem here is that Elektra does not load additional required plugins (infos/needs) for a default storage plugin.
