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Elektra
0.9.5
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The plugin tutorial already covers some of the most interesting parts on how to write a (storage) plugin. This text will tell you a little bit more about how a storage plugin should act. While it is usually relatively easy to create a plugin that stores basic key-value pairs, adding advanced features such as support for
takes more work. Before you continue with this text, please make sure that you read all of the linked documents above.
One common problem of storage plugins is, that they store too many keys. For example, if the user adds the keys
user:/tests/storage/root anduser:/tests/storage/root/level1/level2/level3,then your plugin should only store those two keys. Do not add the keys
user:/tests/storage/root/level1, oruser:/tests/storage/root/level1/level2to the key set. One plugin that handles this situation properly is YAML CPP, as the following Markdown Shell Recorder test shows:
. For more information on why we allow “holes” in the hierarchy, please take a look here.
Elektra supports both binary and textual values. The main difference between binary and textual data is that textual data always ends with a null byte. Therefore you are not allowed to store the code point 0 inside textual data. Binary data does not have this limitation.
The simplest textual data is the empty string ("" = 0) and has length 1, while the simplest binary data stores nothing at all and therefore has length 0. In the kdb utility you can disambiguate between these value by checking for the metakey `binary`. The following Markdown Shell Recorder test shows how a storage plugin should handle these values.
Elektra uses `0` and `1` to represent binary data. A storage plugin that uses other values (e.g. false and true) needs to convert these values to 0 and 1. The Markdown Shell Recorder test below shows that YAML CPP handles the conversion from and to YAML’s boolean type properly. In the test we also use the `type` plugin to makes sure that YAML CPP interacts correctly with this essential plugin.
Sometimes the most “natural” mapping of key-value pairs to a file format might cause a storage plugin to not be able to store values in so called directory (non-leaf) keys.
For example, in a key set that contains the keys:
, all keys at the bottom of the hierarchy:
, such as
user:/directory/leaf1user:/directory/leaf2user:/leaf3are called leaf keys, while user:/directory is a directory key. Plugins such as YAJL or YAML CPP will not be able to store data in the key with the name user:/directory directly. To work around this issue these plugin use the Directory Value plugin. In the ReadMe of the Directory Value plugin and YAML CPP you will find more information about this issue, and how to handle it.
The following Markdown Shell Recorder test shows the proper behavior:
. To make sure that your storage plugin works correctly, please just replace yamlcpp with the name of your plugin and verify that the test above still works.
You already learned about the array syntax and the mandatory array metakey in the array tutorial. Now it is time to check, if your storage plugin supports array and non-array keys properly. Let us look at a concrete example. We use a key set that contains the following keys as example:
. If we assume that only user:/tests/storage/array stores the metakey array, then the keys
user:/tests/storage/array/#0, anduser:/tests/storage/array/#1represent array elements, while
user:/tests/storage/map/#0, anduser:/tests/storage/map/#1are normal key-value pairs. The following example shows that the storage plugin YAML CPP handles this situation properly:
1.8.13