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Validation

Introduction

Configuration in FLOSS 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.

User Interfaces

Any of Elektra's user interfaces will work with the technique described in this tutorial, e.g.:

  1. kdb qt-gui: graphical user interface

  2. kdb editor: starts up your favorite text editor and allows you to edit configuration in any syntax. (generalization of visudo)

  3. kdb set: manipulate or add individual configuration entries. (generalization of adduser)

  4. Any other tool using Elektra to store configuration (e.g. if the application itself has capabilities to modify its configuration)

Metadata Together With Keys

The most direct way to validate keys is

kdb meta-set user:/tests/together/test check/validation "[1-9][0-9]*"
kdb meta-set user:/tests/together/test check/validation/match LINE
kdb meta-set user:/tests/together/test check/validation/message "Not a number"
kdb set user:/tests/together/test 123
#> Set string to "123"

# Undo modifications
kdb rm -r user:/tests/together

The approach is not limited to validation via regular expressions, but any values-validation plugin can be used, e.g. type. 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:

  • The administrator needs to take care that the validation plugins are available where needed.
  • Some metadata (in this case 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.
  • After the key is removed, the validation information is gone, too.
  • You cannot validate structure of which keys must be present or absent.

Get Started with spec

These 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).

Before we start, let us make a backup of the current data in the spec and user namespace:

kdb set system:/tests/specbackup $(mktemp)
kdb set system:/tests/userbackup $(mktemp)
kdb export spec:/ dump > $(kdb get system:/tests/specbackup)
kdb export user:/ dump > $(kdb get system:/tests/userbackup)

We write metadata to the namespace spec and the plugin spec applies it to every cascading key:

kdb meta-set spec:/tests/spec/test hello world
kdb set /tests/spec/test value
# STDOUT-REGEX: Using name (user|system):/tests/spec/test⏎Create a new key (user|system):/tests/spec/test with string "value"
kdb meta-ls spec:/tests/spec/test | grep -v '^internal/ini'
#> hello
kdb meta-ls /tests/spec/test | grep -v '^internal/ini'
#> hello
kdb meta-get /tests/spec/test hello
#> world

# The default namespace for a non-root user is `user`, while
# for root users a cascading key usually refers to the `system` namespace.
kdb meta-get user:/tests/spec/test hello || kdb meta-get system:/tests/spec/test hello
#> world

But it also supports globbing (_ for any key, ? for any char, [] for character classes):

kdb meta-set "spec:/tests/spec/_" new metaval
kdb set /tests/spec/test value
# STDOUT-REGEX: Using name (user|system):/tests/spec/test⏎Set string to "value"
kdb meta-ls /tests/spec/test | grep -v '^internal/ini'
#> hello
#> new

# Remove keys and metadata from the commands above
kdb rm -r spec:/tests/spec
kdb rm -r user:/tests/spec || kdb rm -r system:/tests/spec

So let us combine this functionality with validation plugins. So we would specify:

kdb meta-set spec:/tests/spec/test check/validation "[1-9][0-9]*"
kdb meta-set spec:/tests/spec/test check/validation/match LINE
kdb meta-set spec:/tests/spec/test check/validation/message "Not a number"

If we now set a new key with

kdb set /tests/spec/test "not a number"
# STDOUT-REGEX: Using name [a-z]+:/tests/spec/test⏎Create a new key [a-z]+:/tests/spec/test with string "not a number"

this key has adopted all metadata from the spec namespace:

kdb meta-ls /tests/spec/test | grep -v '^internal/ini'
#> check/validation
#> check/validation/match
#> check/validation/message

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:

kdb mount tutorial.dump /tests/spec dump validation

This mounts the backend tutorial.dump to the mount point /tests/spec and activates the validation plugin for the keys below the mount point. The validation plugin now uses the metadata of the keys below /tests/spec 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.

# Undo modifications
kdb rm -r spec:/tests/spec/test
kdb rm -r user:/tests/spec || kdb rm -r system:/tests/spec

Specfiles

We call the files, that contain a complete schema for configuration below a specific path in form of metadata, Specfiles.

A Specfile contains metadata, among others, that defines how the configuration settings should be validated.

Let us create an example Specfile in the dump format, which supports metadata (although the specfile is stored in the dump format, we can still create it using the human readable ni format by using kdb import):

sudo kdb mount tutorial.dump spec:/tests/tutorial dump
cat << HERE | kdb import spec:/tests/tutorial ni  \
[]                                         \
 mountpoint=tutorial.dump                \
 infos/plugins=dump validation           \
                                           \
[/links/_]                                 \
check/validation=https?://.*\..*         \
check/validation/match=LINE              \
check/validation/message=not a valid URL \
description=A link to some website       \
HERE
kdb meta-ls spec:/tests/tutorial
#> infos/plugins
#> mountpoint

We now have all the metadata that we need to mount and validate the data below /tutorial in one file.

For a description which metadata is available, have a look in METADATA.ini.

Now we apply this Specfile to the key database to all keys below tests/tutorial.

kdb spec-mount /tests/tutorial

This command automatically mounts /tests/tutorial to the backend tutorial.dump. Furthermore it adds all plugins necessary for all metadata within the specification. So in this example the validation plugin will be loaded automatically for us. spec-mount basically does a normal mount except that it automatically selects plugins. As a result there is no spec-umount command since the normal umount is sufficient.

Please be aware that if you require many plugins for the same mount point, you can run into this error.

kdb set /tests/tutorial/links/url "invalid url"
# STDOUT-REGEX: Using name (user|system):/tests/tutorial/links/url
# STDERR: .*Validation Syntactic.*not a valid URL.*
# ERROR:  C03100
# RET:    5

Note that the backend tutorial.dump is mounted for all namespaces:

kdb file user:/tests/tutorial
# STDOUT-REGEX: /.*/tutorial\.dump
kdb file system:/tests/tutorial
# STDOUT-REGEX: /.*/tutorial\.dump
kdb file dir:/tests/tutorial
# STDOUT-REGEX: /.*/tutorial\.dump

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:

kdb set -N system /tests/tutorial/links/elektra https://www.libelektra.org
#> Using name system:/tests/tutorial/links/elektra
#> Create a new key system:/tests/tutorial/links/elektra with string "https://www.libelektra.org"

Rejecting Configuration Keys

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:

kdb meta-set /tests/tutorial/spec/should_not_be_here trigger/error C03200
#> Using keyname spec:/tests/tutorial/spec/should_not_be_here
kdb spec-mount /tests/tutorial
kdb set /tests/tutorial/spec/should_not_be_here abc
# STDOUT-REGEX: Using name (user|system):/tests/tutorial/spec/should_not_be_here
# RET:    5
# ERROR:C03200
kdb get /tests/tutorial/spec/should_not_be_here
# RET: 11
# STDERR: Did not find key '/tests/tutorial/spec/should_not_be_here'

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.

kdb rm -r spec:/tests/tutorial
kdb rm -r system:/tests/tutorial
kdb rm -rf user:/tests/tutorial
kdb umount spec:/tests/tutorial
kdb umount /tests/tutorial
kdb rm -rf spec:/
kdb rm -rf user:/
kdb import spec:/ dump < $(kdb get system:/tests/specbackup)
kdb import user:/ dump < $(kdb get system:/tests/userbackup)
rm $(kdb get system:/tests/specbackup)
rm $(kdb get system:/tests/userbackup)
kdb rm system:/tests/specbackup
kdb rm system:/tests/userbackup