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Elektra 0.11.0
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Elektra 0.11.0
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In this tutorial you will learn how to interactively use the SpecElektra specification language and kdb to write a configuration specification for dockerd.
kdbkdb to create and mount a specification for dockerdIn this tutorial we will introduce a possible specification for dockerd. Using kdb we will configure the specification.
NOTE: As the specification for dockerd is quite big we will only present a sample for the above mentioned metakeys and link to a full specification dockerd-full-spec.
Before we start just an overview of the structure:
/docker/daemon.jsonspec:/sw/dockerd/dockerd/#0/currentElektra supports multiple types which leads to a more flexible specification. See type plugin for information about all the types that are supported.
We will be mounting an existing example dockerd-spec.
First you need to mount a specification file, in this case dockerd.ini to the spec:/ namespace. You can define the path inside the spec:/ namespace as /sw/docker/dockerd/#0/current, refer to the documentation to find out more about constructing the name.
NOTE: If you encounter any error saying that you have already mounted some specification with the same name you can run
sudo kdb umount spec:/sw/docker/dockerd/#0/currentand rerun the above command.
Note: ni is the format which is used for the specification in the file. You can also choose to use json, then you need use yajl instead of ni.
Next you can define, that this specification uses a specific mountpoint for a concrete application configuration. So you can say the concrete configuration should be written to dockerd.ini.
Your dockerd.ini file should now contain the mountpoint metakey:
NOTE: Excerpt of
cat $(kdb file spec:/sw/docker/dockerd/#0/current).
Next we will define that our configuration should be written json.
We can do this by running:
This specification mount makes sure that the paths where the concrete configuration should be (daemon.json) are ready to fulfill our specification (dockerd.ini). Be aware that different files get mounted for different namespaces. You've a specification file (dockerd.ini) for the spec-namespace and three files (daemon.json) on different locations for the dir- user- and system-namespaces.
You can see the files by providing the namespace as prefix to the kdb file command (each shows a different path):
NOTE: The $PWD should equal the PWD where you run
sudo kdb mount "$PWD/examples/spec/dockerd.ini" spec:/sw/docker/dockerd/#0/current ni.
**_Note_**: The files only exist, when configuration values are stored there, i.e. they are created on the first
kdb setand removed with the lastkdb rm.
For more information about namespaces in Elektra please see here, a tutorial about the topic is available here.
NOTE: All output we display for
cat $(kdb file spec:/sw/docker/dockerd/#0/current)is an excerpt of the whole file output.
In this example for dockerd we will be using 3 types of specifications:
As the dockerd specification is big we will just present one of each of the above mentioned specification types.
NOTE: In Elektra we use
/instead of-to seperate key names. This results in a hierarchical structure of key names. This commands will automatically store the specification in thedockerd-daemon.nispecification file.
In order to get the above specification we will need the following commands:
In case no data/root key gets configured the value /var/lib/docker is used.
Let us verify that the metakeys have been set correctly:
NOTE: Excerpt of
cat $(kdb file spec:/sw/docker/dockerd/#0/current).
In order to get the above specification we will need the following commands:
With this configuration we have managed to allow two possible values for default/cgroupns/mode. The values are host and private. The default value if we do not set any configuration is private.
Let us verify that the metakeys have been set correctly:
NOTE: Excerpt of
cat $(kdb file spec:/sw/docker/dockerd/#0/current).
For this specification we want to allow an arbitrary number of default ulimits. The name of the ulimits does not matter but all should have the same metakeys.
In order to get the above specification we will need following commands:
The above specification will allow us to create any name below the default/ulimits key. The value needs to be a string. The sample value is 64000.
Let us verify that the metakeys have been set correctly:
NOTE: Excerpt of
cat $(kdb file spec:/sw/docker/dockerd/#0/current).
The specification above shows the use of an array specification with the # character. We define the array to have a minimum value of 0 and arbitrary max length. We use type, description and example as metakeys on the keys beneath each array element.
This configuration above assures that we can configure pools with base and size.
It prevents a configuration like:
It will fail as default/address/pools/#/size is required to be of type short when set.
In order to get the above specification we will need following commands:
The above specification defines that we can create array elements and each can have base or size.
Your specification should be complete now! After adding all the keys that are necessary for our application, you can verify that all specification keys are contained by running:
NOTE: We want display the output because it is too long to display (~ 400-500 lines).
The above tutorial has given a good overview of how to write a specification. You might want to add a full example specification for dockerd using kdb import. To do so, follow the next steps.
To make sure we don't run into errors we will clean up everything we have done by now.
sudo kdb rm -r spec:/sw/docker/dockerd/#0/currentsudo kdb umount spec:/sw/docker/dockerd/#0/currentrm -rf $PWD/dockerd (make sure that you are in the same PWD as when you run the sudo kdb mount)Now we are going to add an example of dockerd-full-spec.
Make sure you are in the root of the cloned libelektra repository:
sudo kdb mount "$PWD/dockerd/dockerd-daemon.ni" spec:/sw/docker/dockerd/#0/current nikdb meta-set spec:/sw/dockerd/dockerd/#0/current mountpoint /dockerd/daemon.nikdb meta-set spec:/sw/dockerd/dockerd/#0/current infos/plugin "yajl"sudo kdb spec-mount "/sw/docker/dockerd/#0/current"sudo kdb import spec:/sw/docker/dockerd/#0/current ni < ./examples/spec/dockerd.iniTo verify that everything was created successfully, run:
NOTE: We want display the output because it is too long to display (~ 400-500 lines).
The full specification can be viewed at dockerd-full-spec.
1.9.1