Arrays

Key-Value Pairs

The main building block of Elektra’s database are hierarchical key-value pairs. You can create such a pair using `kdb set`:

kdb set user:/tests/parent value
#> Create a new key user:/tests/parent with string "value"

. The command above created a key user:/tests/parent with the value value. Since Elektra uses a hierarchical database we can also create keys below user:/tests/parent:

# We can create keys with an empty value by passing an empty string.
kdb set user:/tests/parent/son ""
#> Create a new key user:/tests/parent/son with string ""
 
kdb set user:/tests/parent/daughter ""
#> Create a new key user:/tests/parent/daughter with string ""
 
kdb set user:/tests/parent/daughter/grandchild ""
#> Create a new key user:/tests/parent/daughter/grandchild with string ""

. We can check the hierarchy of the keys using kdb ls and retrieve data using kdb get:

# Elektra sorts keys alphabetically
kdb ls user:/tests/parent
#> user:/tests/parent
#> user:/tests/parent/daughter
#> user:/tests/parent/daughter/grandchild
#> user:/tests/parent/son
 
kdb get user:/tests/parent
#> value
kdb get user:/tests/parent/daughter
#>

Array Keys

Since Elektra sorts keys alphabetically we can use the key-value pair structure described above to store sequences of values (aka. arrays).

Empty Arrays

For an empty array ([]) we just add the metakey array:

# Create an empty array with the name `user:/tests/sequence`
kdb meta-set user:/tests/sequence array ''

Array Elements

To create an array element we start the basename of a key with the # character and add the index of the array element afterwards. For example, the commands below adds three elements to our array user:/tests/sequence:

kdb set user:/tests/sequence/#0 'First Element'
kdb set user:/tests/sequence/#1 'Second Element'
# Arrays do not need to be contiguous
kdb set user:/tests/sequence/#3 'Fourth Element'

. As you can see above arrays can contain “empty fields”: The key user:/tests/sequence/#2 is missing.

For array elements with an index larger than 9 we must add underscores (_) to the basename, so we do not destroy the alphabetic order of the array. For example, to add a eleventh element to our array we use the following command:

kdb set user:/tests/sequence/#_10 'Eleventh Element'

. The order of the array sequence is still correct afterwards, as the following command shows:

# List all array elements
kdb ls user:/tests/sequence/
#> user:/tests/sequence/#0
#> user:/tests/sequence/#1
#> user:/tests/sequence/#3
#> user:/tests/sequence/#_10

. For larger indices we add one underscore less, than the number of digits of the index. For example, to add a element with index 1337 (4 digits) we use the basename #___1337. We can also generate the basename programmatically:

ruby -e 'print("#", "_" * (ARGV[0].length - 1), ARGV[0])' 12345
#> #____12345
 
ruby -e 'print("#", "_" * (ARGV[0].length - 1), ARGV[0])' 0
#> #0
 
ruby -e 'print("#", "_" * (ARGV[0].length - 1), ARGV[0])' 42
#> #_42

Metadata

Elektra’s arrays require that you always add the metakey array to the array parent. Otherwise the values below the parent will not be interpreted as array elements, but rather as normal key-value pairs. To make the array metakey more useful storage plugins should save the basename of the last key in the array parent. This of course works only, if the plugins already stores this information in the config file, either

• implicitly (e.g. the data below the array parent is stored as array/list/sequence in the config file), or
• explicitly (e.g. the plugin stores the metakey array directly in the config file)

. Either way, in some situations you might have to add this value manually via kdb meta-set:

# Add array elements
kdb set user:/tests/favorites/superheros/#0 'One-Punch Man'
kdb set user:/tests/favorites/superheros/#1 'Mermaid Man and Barnacle Boy'
kdb set user:/tests/favorites/superheros/#____99999 'The guy with the bow and arrow'
 
# The metakey `array` should save the basename of the last element.
kdb meta-set user:/tests/favorites/superheros array '#____99999'

. This way you can always retrieve the last element of an array easily:

kdb get user:/tests/favorites/superheros/`kdb meta-get user:/tests/favorites/superheros array`
#> The guy with the bow and arrow

Closing Remarks

We close this tutorial by removing the data created by the commands above:

kdb rm -r user:/tests