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I'm using a chef community cookbook that downloads, installs, a configures an SDK. (Let's call it the blah-sdk.) You just include_recipe 'blah-sdk' and viola, it's installed. It has an attribute specifying the version of the blah-sdk it will install. This version attribute in turn is used to form the value of a corresponding 'download_url' attribute. In theory I should be able to set the value of version attribute to something else in the cookbook where I include the blah-sdk. But there is a problem. The download_url attribute gets set (using the default version specified in the blah-sdk cookbook) before my override version attribute does. So the wrong url is used to retrieve the default version rather than the version I want. I could set the download_url in my cookbook as well, but that breaks the encapsulation of the 'blah-sdk' cookbook. I also might end up playing whack-a-mole experimentally with some long stream of attributes before getting it to work. There has got to be a better way. What is it?

cookbooks/blah-sdk/attributes/default.rb:

default['blah']['version'] = '24.4'
default['blah']['download_url'] = "http://dl.company.com/blah/blah-sdk_r#{node['blah']['version']}-linux.tgz"

cookbooks/blah-sdk/recipes/default.rb:

...
print("blah version: #{node['blah']['version']}")
print("blah download_url: #{node['blah']['download_url']}")
...

cookbooks/my_cookbook/attributes/default.rb:

normal['blah']['version'] = '24.4.1'

(I've also tried using default, force_default, override, and force_override. Made no difference.)

cookbooks/my_cookbook/recipes/default.rb

...
include_recipe 'blah-sdk'
...

Output:

==> default: blah version: 24.4.1

==> default: blah download_url: http://dl.company.com/blah/blah-sdk_r24.4-linux.tgz

Code demonstrating the issue and coderanger's suggested solution (if you can fix the third party cookbook): https://github.com/marc-swingler/stackoverflow_question

Not the best solution, but due to the order in which attributes are loaded, dropping the version into a role or environment works too. https://christinemdraper.wordpress.com/2014/10/06/avoiding-the-possible-pitfalls-of-derived-attributes/

Community
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Marc Swingler
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  • Use normal precedence attributes is the wrapper cookbook and overide the source cookbook defaults. See example: http://stackoverflow.com/questions/28775225/how-to-use-chef-to-update-alternatives-for-java-using-execute/28775684#28775684 – Mark O'Connor Nov 15 '15 at 03:06
  • Tried that. I also tried force_default, override and force_override. Same result. The download_url attribute is resolved using the version attribute defined in the blah-sdk cookbook rather than the one defined in my wrapper. (BTW, I edited the question to reflect your suggestion. Hopefully makes it a little more clear.) – Marc Swingler Nov 15 '15 at 03:51

1 Answers1

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See https://coderanger.net/derived-attributes/ for an overview of this problem. There is no good solution that doesn't involve modifying the upstream cookbook. Easiest solution is to duplicate the derived attribute in your wrapper as well.

coderanger
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  • That's the answer I was afraid of. I'll be able to stop banging my head against the wall though, thanks. Awesome link BTW. I'll use that when writing my own cookbooks. – Marc Swingler Nov 15 '15 at 04:09