Elektra: Cascading Lookups

Elektra 0.8.24

Cascading Lookups

This tutorial assumes that you are already familiar with namespaces. This tutorial will only explain cascading lookup.

When Elektra looks up a cascading key (i.e. key names without a namespace and a leading slash `/`, the namespaces are searched in the following order:

- spec (contains metadata, e.g. to modify elektra lookup behavior)
- proc (process-related information)
- dir (directory-related information, e.g. `.git` or `.htaccess`)
- user (user configuration)
- system (system configuration)

If a key, for example, exists in both user and system namespace, the key in the user namespace takes precedence over the one in the system namespace. If there is no such key in the user namespace the key in the system namespace acts as a fallback.

But lets demonstrate this with an example:

### Add a Key to the system Namespace

Configuration in the system namespace is the same for all users. Therefore this namespace provides a default or fallback configuration.

With the default Elektra installation only an administrator can update configuration settings within the system namespace.

# Backup-and-Restore:/tests/tutorial
# Backup old override specification
kdb set user/tests/overrides $(mktemp)
kdb export system/tests/overrides dump > $(kdb get user/tests/overrides)
kdb get /tests/tutorial/cascading/#0/current/test
# RET: 11
# STDERR: Did not find key '/tests/tutorial/cascading/#0/current/test'
# Now add the key ...
sudo kdb set system/tests/tutorial/cascading/#0/current/test "hello world"
#> Create a new key system/tests/tutorial/cascading/#0/current/test with string "hello world"
# ... and verify that it exists
kdb get /tests/tutorial/cascading/#0/current/test
#> hello world

### Add a Key to the user Namespace

A user may now want to override the configuration in system, so he/she sets a key in the user namespace:

kdb set user/tests/tutorial/cascading/#0/current/test "hello galaxy"
#> Create a new key user/tests/tutorial/cascading/#0/current/test with string "hello galaxy"
# This key masks the key in the system namespace
kdb get /tests/tutorial/cascading/#0/current/test
#> hello galaxy

Note that configuration in the user namespace only affects this user. Other users would still get the key from the system namespace.

### Add a Key to the dir Namespace

The dir namespace is associated with a directory. The configuration in the dir namespace applies to the current working directory and all its subdirectories. This is useful if you have project specific settings (e.g. your git configuration or a .htaccess file).

As dir precedes the user namespace, configuration in dir can overwrite user configuration:

# create and change to a new directory ...
mkdir kdbtutorial
cd kdbtutorial
# ... and create a key in this directories dir-namespace
# By default this data will be saved in the directory `.dir`.
kdb set dir/tests/tutorial/cascading/#0/current/test "hello universe"
#> Create a new key dir/tests/tutorial/cascading/#0/current/test with string "hello universe"
# This key masks the key in the system namespace
kdb get /tests/tutorial/cascading/#0/current/test
#> hello universe
# But is only present in the associated directory
cd ..
kdb get /tests/tutorial/cascading/#0/current/test
# hello galaxy

### Add a Key to the proc Namespace

The proc namespace is not accessible by the command line tool kdb, as it is unique for each running process using Elektra. So we have to omit an example for this namespace at this point. Elektrified applications can use this namespace to override configuration from other namespaces internally.

### Add a Key to the spec Namespace

The spec namespace is used to store metadata about keys and therefore Elektra handles the spec namespace differently to other namespaces. The following part of the tutorial is dedicated to the impact of the spec namespace on cascading lookups.

## Override Links

The `spec` namespace is special as it can completely change how the cascading lookup works.

During a cascading lookup for a specific key, the default Elektra behavior can be changed by a corresponding `spec-key`, i.e. a key in the spec namespace with the same name.

For example, the metadata `override/#0` of the respective `spec-key` can be specified to use a different key in favor of the key itself. This way, we can implement a redirect or symlink like behavior and therefore even config from current folder (`dir` namespace) can be overwritten.

The cascading lookup will consider the following metadata keys of `spec-key`:

1. `override/#n` redirect to one of the specified keys
2. `namespaces/#n` specifies which namespaces will be considered and in which order
3. `fallback/#n` if no key was found these keys will act as a fallback
4. `default` defines a default value for the key if none of the keys was found

Note: `override/#n`, `namespaces/#n` and `fallback/#n` are Elektra array keys. This means, such keys can exist several times, each with a different number for `n`, e.g. `override/#0`, `override/#1`... This way, we can define multiple values for a specific key with a defined order.

As you can see, override links are considered before everything else, which makes them really powerful.

Consider the following example:

First, we create a target key to demonstrate the override link mechanism:

sudo kdb set system/tests/overrides/test "hello override"
#> Create a new key system/tests/overrides/test with string "hello override"

Override links can be defined by adding them to the `override/#` metadata array key of the corresponding `spec-key`:

sudo kdb setmeta spec/tests/tutorial/cascading/#0/current/test override/#0 /tests/overrides/test

Now when doing a cascading lookup, we get the value of our target key instead of the specified one:

kdb get /tests/tutorial/cascading/#0/current/test
#> hello override

As we used a cascading key for our override link (`/tests/overrides/test`) we can use this to allow users to provide their own `tests/overrides/test` keys. If a user sets the `/tests/overrides/test` key, the user namespace will be used and therefore the new target for our `/tests/tutorial/cascading/#0/current/test` key will be `user/tests/overrides/test` instead `system/tests/overrides/test`.

kdb set /tests/overrides/test "hello user"
#> Using name user/tests/overrides/test
#> Create a new key user/tests/overrides/test with string "hello user"
kdb get /tests/tutorial/cascading/#0/current/test
#> hello user

Furthermore, you can specify a custom order for the namespaces, set fallback keys and more. For more information, read the `elektra-spec` help page.

## Cleanup

As last part in this tutorial we remove the modifications to the database we made previously.

kdb rm -r user/tests/tutorial/
kdb rm -r system/tests/tutorial
kdb rm -r system/tests/overrides
kdb import system/tests/overrides dump < $(kdb get user/tests/overrides)
rm $(kdb get user/tests/overrides)
kdb rm user/tests/overrides
kdb rm /tests/overrides/test
kdb rm -r spec/tests/tutorial/
rm -r .dir/
rmdir kdbtutorial