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Elektra
0.8.20
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author: Markus Raab elektra@markus-raab.org brief: Describes validation capabilities of Elektra. tags: validation, spec, metadata
Configuration in <abbr title="Free/Libre and Open Source Software">FLOSS</abbr> unfortunately is often stored completely without validation. Notable exceptions are sudo (visudo), or user accounts (adduser) but in most cases you only get feedback of non-validating configuration when the application fails to start.
Elektra provides a generic way to validate any configuration before it is written to disc.
Any of Elektra's user interfaces will work with the technique described in this tutorial, e.g.:
kdb qt-gui: graphical user interfacekdb editor: starts up your favourite text editor and allows you to edit configuration in any syntax. (generalization of visudo)kdb set: manipulate or add individual configuration entries. (generalization of adduser)The most direct way to validate keys is
For all other plugins (except validation) the convenience tool kdb vset is missing. Let us see what kdb vset actually did:
So it only appended some metadata (data describing the data) next to the key, which we also could do by:
The approach is not limited to validation via regular expressions, but any values-validation plugin can be used, e.g. enum. For a full list refer to the section "Value Validation" in the list of all plugins.
Note that it also easy to write your own (value validation) plugin.
The drawbacks of this approach are:
check/validation) needs to be stored next to the key which won't work with most configuration files. This is the reason why we explicitly used dump as storage in kdb mount.vset was used. In the example above we could override the cascading key /tutorial/together/test with the unvalidated key dir/tutorial/together/test.specThese issues are resolved straightforward by separating the configuration from its configuration specification (often called schemata in XML or JSON). The purpose of the spec namespace is to hold the configuration specification, i.e., the description of how to validate the keys of all other namespaces.
To make this work, we need a plugin that applies all metadata found in the spec-namespace to all other namespaces. This plugin is called spec and needs to be mounted globally (will be added by default and also with any kdb global-mount call).
We write metadata to the namespace spec and the plugin spec applies it to every cascading key:
But it also supports globbing (_ for any key, ? for any char, [] for character classes):
So let us combine this functionality with validation plugins. So we would specify:
If we now set a new key with
this key has adopted all metadata from the spec namespace:
Note that this key should not have passed the validation that we defined in the spec namespace. Nonetheless we were able to set this key, because the validation plugin was not active for this key. On that behalf we have to make sure that the validation plugin is loaded for this key with:
This mounts the backend tutorial.dump to the mountpoint user/tutorial and activates the validation plugin for the keys below the mountpoint. The validation plugin now uses the metadata of the keys below user/tutorial to validate values before storing them in tutorial.dump.
Although this is better than defining metadata in the same place as the data itself, we can still do better. The reason for that is that one of the aims of Elektra is to remove the trouble of validation and finding the files that hold your configuration from the users. At the moment a user still has to know which files should hold the configuration and which plugins must be loaded when he mounts configuration files.
This problem can be addressed by recognizing that the location of the configuration files and the plugins that must be loaded is part of the schema of our configuration and therefore should be stored in the spec namespace.
We call the files, that contain a complete schema for configuration below a specific path in form of metadata, Specfiles.
Particularly a Specfile contains metadata that defines
Let us create an example Specfile in the dump format, which supports metadata (altough the specfile is stored in the dump format, we can still create it using the human readable ni format by using kdb import):
We now have all the metadata that we need to mount and validate the data below /tutorial in one file.
Now we apply this Specfile to the key database to all keys below tutorial.
This command automatically mounts /tutorial to the backend tutorial.dump and loads the validation plugin.
Note that the backend tutorial.dump is mounted for all namespaces:
If you want to set a key for another namespace and do not want to go without validation, consider that the spec plugin works only when you use cascading keys. You can work around that by setting the keys with the -N option:
Up to now we only discussed how to reject keys that have unwanted values. Sometimes, however, applications require the presence or absence of keys. There are many ways to do so directly supported by the spec plugin. Another way is to trigger errors with the error plugin:
If we want to reject every optional key (and only want to allow required keys) we can use the plugin required as further discussed below.
Before we look further let us undo the modifications to the key database.
Sometimes we already have configuration specifications given in some other format which is more compact and more directed to the needs of an individual application. We can write a plugin that parses that format and transform the content to key-value and metadata (describing how to validate).
For example, let us assume we have enum validations in the file schema.txt:
And by convention for keys ending with *, multiple values are allowed. So we want to transform above syntax to:
Lucky, we already have a plugin which allows us to so:
We configure the plugin simplespeclang so that it conforms to our "weird" syntax. Because in schema.txt we have the line mountpoint file.txt we can also mount the schema using spec-mount.
Now we have enforced that the 3 configuration options notation tool-support* applied-to need to be present (and no other). For example we can import:
Or (afterwards) setting individual values:
Or (in visudo fashion):
1.8.13