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Definitions allow you to create new Resources by stringing together existing resources. I think an example will explain best: apache_site Definition This definition file creates a new resource apache_site. We utilize it by placing: apache_site resource |
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How definitions work
Definitions have three components.
- The resouce name
- The Prototype Parameters
- The Params hash
You start with define, followed by the name of the new resource (apache_site). What follows is the Prototype Arguments for the definition - these are used to set default values for parameters within the definition. Those parameters are then accessed within your definition via the params hash. Since this is a hash, each parameter must have a value. If no default value is specified, use nil.
 | Hey, CGI programmers... The params hash should look familiar to you - it's basically the exact same model we've been following ever since the CGI.pm days. |
When a definition resource is used in a recipe, such as:
We take the attributes of our new apache_site resource, and make them accessible via the params hash. This will be true of any attribute we set - you do not have to prototype all your parameters.
Within the context of the Chef run, this definition will be replaced by all the resources you specify within the definition - in this case, it is expanded to:
You can have as many resources in a definition as you want.
Definitions aren't resources
They are replaced by the resources they contain. This is important. You cannot notify a definition to take an action - you can only notify the resources it creates.
When to use Definitions
You want a definition if:
- You are repeating a pattern of resources
- You do not want to send actions directly to this resource - i.e., you never need to notify it
If you do need to send actions to the resource, you should look in to creating a new Provider.
Comments (1)
Mar 18
Zachary Belzer says:
It's worth mentioning that this page doesn't note that you are currently (0.8.x)...It's worth mentioning that this page doesn't note that you are currently (0.8.x) required to have one definition per file. There's a ticket for this behavior: http://tickets.opscode.com/browse/CHEF-135