| Elektra 0.11.0

Plugin: directoryvalue

- infos = Information about the directoryvalue plugin is in keys below
- infos/author = René Schwaiger sanss.nosp@m.ecou.nosp@m.rs@me.nosp@m..com
- infos/licence = BSD
- infos/needs =
- infos/provides =
- infos/recommends =
- infos/placements = postgetstorage presetstorage
- infos/status = unittest nodep preview
- infos/metadata =
- infos/description = This plugin converts directory keys to leaf keys in the set direction

# Directory Value

## Introduction

The Directory Value plugin converts

1. directory (non-leaf) keys to leaf keys in the “set” direction, and
2. converts them back to directory keys in the “get” direction.

A directory key is a key that has children. For example in the key set:

user:/grandparent = Grandparent
user:/grandparent/leaf = Leaf
user:/grandparent/parent = Parent
user:/grandparent/parent/child = Child
user:/mother = Mother
user:/mother/daughter = Daughter
user:/mother/son = Son

the keys

user:/grandparent
user:/grandparent/parent
user:/mother

represent directory keys, while the keys

user:/grandparent/leaf
user:/grandparent/parent/child
user:/mother/daughter
user:/mother/son

are leaf keys. You can easily check this by drawing the key set in the form of a rooted tree:

user
/ \
granparent mother
/ | / \
leaf parent daughter son
|
child

. The Directory Value plugin converts all directory keys to leaf keys in the “set” direction by adding new keys with the postfix `___dirdata`. Theses keys then store the old value of their parent keys

user:/grandparent =
user:/grandparent/___dirdata = Grandparent
user:/grandparent/leaf = Leaf
user:/grandparent/parent =
user:/grandparent/parent/___dirdata = Parent
user:/grandparent/parent/child = Child
user:/mother =
user:/mother/___dirdata = Mother
user:/mother/daughter = Daughter
user:/mother/son = Son

. You might ask why we need the Directory Value plugin at all. The reason why we created this plugin is that some storage plugins like `yajl` or `yamlcpp` are only able to save values inside leaf keys. By loading the Directory Value plugin these storage plugins are also able to represent directory values properly.

### Array Values

The Directory value plugin also converts array values. Let us take a look at an example key set:

user:/array = Array Value
user:/array/#0 = Firt Value
user:/array/#1 = Second Value
user:/array/#2 = Third Value

. The plugin does not convert this key set into:

user:/array =
user:/array/___dirdata = Array Value
user:/array/#0 = First Value
user:/array/#1 = Second Value
user:/array/#2 = Third Value

, since then `user:/array` would not be an array any more. Instead the plugin inserts a new element at index 0 with the value prefix `___dirdata:`:

user:/array =
user:/array/#0 = ___dirdata: Array Value
user:/array/#1 = First Value
user:/array/#2 = Second Value
user:/array/#3 = Third Value

. This way a storage plugin such as YAJL or YAML CPP are still able to store `user:/array` as an array.

#### Remarks

- The plugin only converts array parents that store string values
- The array metakey of the array parent (increased by one) will still be stored in the original parent key after conversion. This is important, since otherwise the storage plugin would lose information about which key represents an array.

## Usage

To mount the plugin use the command:

# Mount plugin at `user:/tests/directoryvalue`
sudo kdb mount config.file user:/tests/directoryvalue directoryvalue

. To unmount the plugin use the command

sudo kdb umount user:/tests/directoryvalue

## Example

# Mount plugin
sudo kdb mount config.file user:/tests/directoryvalue directoryvalue
# Add a directory value
kdb set user:/tests/directoryvalue/harold 'Father of SpongeBob SquarePants'
# Add a leaf value
kdb set user:/tests/directoryvalue/harold/spongebob 'I am ready!'
# Add an array
kdb set user:/tests/directoryvalue/patrick Star
kdb set user:/tests/directoryvalue/patrick/#0 'Being grown-up is boring. Besides, I don’t get Jazz.'
# Elektra requires that the array parent contains the metakey `array`.
# If this key is not present, then `user:/tests/directoryvalue/patrick`
# is **not an array**.
kdb meta-set user:/tests/directoryvalue/patrick array ''
# Since the plugin converts values back in the get direction
# a user of the database will not notice any changes.
kdb ls user:/tests/directoryvalue
#> user:/tests/directoryvalue/harold
#> user:/tests/directoryvalue/harold/spongebob
#> user:/tests/directoryvalue/patrick
#> user:/tests/directoryvalue/patrick/#0
kdb get user:/tests/directoryvalue/harold
#> Father of SpongeBob SquarePants
kdb get user:/tests/directoryvalue/harold/spongebob
#> I am ready!
kdb get user:/tests/directoryvalue/patrick
#> Star
kdb get user:/tests/directoryvalue/patrick/#0
#> Being grown-up is boring. Besides, I don’t get Jazz.
# Retrieve index of last element in array.
# This also works if the storage plugin does not store this index.
kdb meta-get user:/tests/directoryvalue/patrick array
#> #0
# Undo changes to the key database
kdb rm -r user:/tests/directoryvalue
sudo kdb umount user:/tests/directoryvalue

# Limitations

- Escaping is currently not possible. If you use the Directory Value plugin you can not

  - use the name `___dirdata` as the last part of a normal key,
  - use `___dirdata:` at the beginning of a normal value in the first array element

  !