Elektra  0.9.8
Understanding Namespaces

Structure of the Key Database

The key database of Elektra is hierarchically structured. This means that keys are organized similar to directories in a file system.

Let us add some keys to the database. To add a key we can use kdb, the key database access tool:

kdb set <key> <value>

Now add the key **/a** with the Value Value 1 and the key **/b/c** with the Value Value 2:

kdb set /a 'Value 1'
#> Using name user:/a
#> Create a new key user:/a with string "Value 1"
kdb set /b/c 'Value 2'
#> Using name user:/b/c
#> Create a new key user:/b/c with string "Value 2"

Hierarchical structure of key database

Here you see the internal structure of the database after adding the keys **/a** and **/b/c**. For instance the key **/b/c** has the path **/** -> b -> c.

Note how the name of the key determines the path to its value.

You can use the file system analogy as a mnemonic to remember these commands (like the file system commands in your favorite operating system):

  • kdb ls <path> lists keys below path
  • kdb rm <key> removes a key
  • kdb cp <source> <dest> copies a key to another path
  • kdb get <key> gets the value of key

For example kdb get /b/c should return Value 2 now, if you set the values before.


Now we abandon the file system analogy and introduce the concept of namespaces.

Every key in Elektra belongs to one of these namespaces:

  • spec for specification of other keys
  • proc for in-memory keys (e.g. command-line)
  • dir for dir keys in current working directory
  • user for user keys in home directory
  • system for system keys in /etc or /

All namespaces save their keys in a separate hierarchical structure from the other namespaces.

But when we set the keys **/a** and **/b/c** before we didn't provide a namespace. So I hear you asking, "if every key has to belong to a namespace, where are the keys?" They are in the user namespace, as you can verify with:

kdb ls user:/ | grep -E '(/a|/b/c)'
#> user:/a
#> user:/b/c

When we don't provide a namespace Elektra assumes a default namespace, which should be user for non-root users. So if you are a normal user the command ‘kdb set /b/c 'Value 2’was synonymous tokdb set user:/b/c 'Value 2'`.

At this point the key database should have this structure: Elektra’s namespaces

Cascading Keys

Another question you may ask yourself now is, what happens if we look up a key without providing a namespace. So let us retrieve the key **/b/c** with the -v flag in order to make kdb more talkative.

kdb get -v /b/c
# STDOUT-REGEX: got \d+ keys
#> searching spec:/b/c, found: <nothing>, options: KDB_O_CALLBACK
#> searching proc:/b/c, found: <nothing>, options:
#> searching dir:/b/c, found: <nothing>, options:
#> searching user:/b/c, found: user:/b/c, options:
#> The resulting key name is user:/b/c
#> Value 2

Here you see how Elektra searches all namespaces for matching keys in this order: spec, proc, dir, user and finally system

If a key is found in a namespace, it masks the key in all subsequent namespaces, which is the reason why the system namespace isn't searched. Finally, the virtual key **/b/c** gets resolved to the real key user:/b/c. Because of the way a key without a namespace is retrieved, we call keys, that start with '**/**', cascading keys. You can find out more about cascading lookups in the cascading tutorial.

Having namespaces enables both admins and users to set specific parts of the application's configuration, as you will see in the following example.

How it Works on the Command Line (kdb)

We will provide an example of how you can configure elektrified applications.

Our exemplary application will be the key database access tool kdb as this should already be installed on your system.

kdb can be configured by the following configuration data:

  • _/sw/elektra/kdb/#**X**/**PROFILE**/verbose_ - sets the verbosity of kdb
  • _/sw/elektra/kdb/#**X**/**PROFILE**/quiet_ - if kdb should suppress non-error messages
  • _/sw/elektra/kdb/#**X**/**PROFILE**/namespace_ - specifies the default namespace used, when setting a cascading name

X is a placeholder for the major version number and PROFILE stands for the name of a profile to which this configuration applies. If we want to set configuration for the default profile we can set PROFILE to %. The name of the key follows the convention described here.

Say we want to set kdb to be more verbose when it is used in the current directory. In this case we have to set verbose to 1 in the dir namespace of the current directory.

kdb set "dir:/sw/elektra/kdb/#0/%/verbose" 1
#> Create a new key dir:/sw/elektra/kdb/#0/%/verbose with string "1"

The configuration for a directory is actually stored in this directory. By default the configuration is contained in a folder named .dir, as you can verify with kdb file dir:/ (kdb file tells you the file where a key is stored in).

For the purpose of demonstration we chose to only manipulate the verbosity of kdb. Note that setting dir:/sw/elektra/kdb/#0/%/namespace to dir can be handy if you want to work with configuration of an application in a certain directory.

If we now search for some key, kdb will behave just as if we have called it with the -v option.

kdb get /some/key
# STDOUT-REGEX: got \d+ keys
#> searching spec:/some/key, found: <nothing>, options: KDB_O_CALLBACK
#> searching proc:/some/key, found: <nothing>, options:
#> searching dir:/some/key, found: <nothing>, options:
#> searching user:/some/key, found: <nothing>, options:
#> searching system:/some/key, found: <nothing>, options:
#> searching default of spec:/some/key, found: <nothing>, options: KDB_O_NOCASCADING
#> Did not find key

Verbosity is not always useful because it distracts from the essential. So we may decide that we want kdb to be only verbose if we are debugging it. So let us move the default configuration to another profile:

kdb mv -r "dir:/sw/elektra/kdb/#0/%" "dir:/sw/elektra/kdb/#0/debug"
#> using common basename: dir:/sw/elektra/kdb/#0
#> key: dir:/sw/elektra/kdb/#0/%/verbose will be renamed to: dir:/sw/elektra/kdb/#0/debug/verbose
#> Will write out:
#> dir:/sw/elektra/kdb/#0/debug/verbose

If we now call kdb get /some/key it will behave non-verbose, but if we call it with the debug profile kdb get -p debug /some/key the configuration under **/sw/elektra/kdb/#0/debug** applies.

We configured kdb only for the current directory. If we like this configuration we could move it to the system namespace, so that every user can enjoy a preconfigured debug profile.

sudo kdb mv -r "dir:/sw/elektra/kdb" "system:/sw/elektra/kdb"
#> using common basename: /sw/elektra/kdb
#> key: dir:/sw/elektra/kdb/#0/%/verbose will be renamed to: system:/sw/elektra/kdb/#0/%/verbose
#> Will write out:
#> system:/sw/elektra/kdb/#0/%/verbose

Now every user can use the debug profile with kdb.

Cascading keys are keys that start with **/** and are a way of making key lookups much easier. Let's say you want to see the configuration from the example above. You do not need to search every namespace by yourself. Just make a lookup for **/sw/elektra/kdb/#0/debug/verbose**, like this:

kdb get "/sw/elektra/kdb/#0/debug/verbose"
#> 1

When using cascading key the best key will be searched at run-time. If you are only interested in the system key, you would use:

kdb get "system:/sw/elektra/kdb/#0/debug/verbose"
#> 1

Because of cascading keys a user can override the behavior of the debug profile by setting the corresponding keys in his user namespace (as we discussed before). If a user sets verbose in his user namespace to 0 she overrides the default behavior from the system namespace.

kdb set "user:/sw/elektra/kdb/#0/debug/verbose" 0
#> Create a new key user:/sw/elektra/kdb/#0/debug/verbose with string "0"
kdb get "/sw/elektra/kdb/#0/debug/verbose"
#> 0

Now kdb get -p debug /some/key is not verbose anymore for this user.