Inheritance and variables

This page explains how component inheritance and facets impact the definition of variables. As a reminder, variables are exchanged by and among Roboconf agents to update configuration files dynamically.

Back to basics


MyComponent {
	exports: ip, property = value;
}

This (incomplete) graph portion indicates MyComponent will share MyComponent.ip and MyComponent.property with other application parts.

Component Extensions

Let’s now consider the following example.


MySQL {
	exports: ip, port = 3306;
	installer: puppet;
}

My-Client-Database {
	extends: MySQL;
	exports: my-own-variable = something;
}

We have two components, one extending the other.
In the example above, MySQL exports…

MySQL.ip
MySQL.port

And My-Client-Database exports…

MySQL.ip and MySQL.port (since My-Client-Database is a MySQL).
My-Client-Database.ip and My-Client-Database.port (inherited variables, replicated with its own prefix).
My-Client-Database.my-own-variable (new variable added by the extending component).

All the inherited variables are replicated in the component’s scope.
This way, those who import MySQL.* will receive notifications from MySQL and My-Client-Database. And those who import My-Client-Database will only receive notifications of My-Client-Database only, but with all the available information.

Facets can define variables too.
When a component is associated with a facet, it proceeds the same way than for the components it exports. It inherits the variables with their original prefix, and replicates them with its own prefix.

Overriding

Components can override variables they inherit.
And instances can override variables they receive from their component.

Let’s take back the example above.


My-Client-Database {
	extends: MySQL;
	exports: my-own-variable = something;
}

Here are the default values of the variables.

MySQL.ip (set at runtime by Roboconf)
MySQL.port (3306, defined in the graph)
My-Client-Database.ip (set at runtime by Roboconf)
My-Client-Database.port (3306, inherited from MySQL)
My-Client-Database.my-own-variable (something, defined in the graph)

Now, how can we override the port value in My-Client-Database?
There are two ways. This will override both port variables.


My-Client-Database {
	extends: MySQL;
	exports: port = 3307;
}

Or we can override the MySQL.port value directly.
And by inheritance, we will get it.


My-Client-Database {
	extends: MySQL;
	exports: MySQL.port = 3307;
}

If for some reason, we do not want to expose the same property for the ports, we can proceed this way.


My-Client-Database {
	extends: MySQL;
	exports: MySQL.port = 3307, My-Client-Database.port = 3308;
}

Setting the prefix defines a scope.
When there is no prefix, then all the variables are updated.