Derived Attributes in Chef 2014-08-25

Update

I’ve written a helper cookbook to make this much easier. Check it out on GitHub.

As the Chef community has moved more and more towards wrapper cookbooks, derived attributes have become a persistent thorn in the side of recipe authors.

What is a Derived Attribute?

A node attribute which depends on the value of another node attribute. As roles, nodes, and environments must be entirely static, this can currently only happen in cookbooks. A common example is including a version number in a path or URL:

default['version'] = '1.0'
default['url'] = "http://example.com/#{node['version']}.zip"

Why is this a Problem?

Chef builds the attributes for the current node in several stages. First the roles, environment, and node data is deep merged together. Then the attribute files for all cookbooks in the current dependency set are loaded in the order specified by dependencies (topological sort).

What this means is if we make a wrapper cookbook with an updated version attribute, it won’t have the desired effect:

default['version'] = '2.0'

Remembering that attribute files are processed in dependency order, that means that the evaluated code is effectively:

default['version'] = '1.0'
default['url'] = "http://example.com/#{node['version']}.zip"
default['version'] = '2.0'

By the time our wrapper cookbook tries to set a new version, the URL has already been computed. The expedient fix for this is to simply override both version and url in our wrapper cookbook, but this means duplicating a lot of attributes and tends to be fragile.

This can work as expected as long as the override is specified in a role or environment, as those are fully prepared before any cookbook attributes are evaluated. Unfortunately this can mean mixing wrapper cookbooks and roles, possibly leading to confusion and mess.

Delayed Interpolation

My proposed solution to this for cookbooks wanting to be reusable or wrappable is to use lazy evaluation for the derived components. Our earlier attributes would become:

default['version'] = '1.0'
default['url'] = "http://example.com/%{version}.zip"

The %{} and % operators in Ruby allows delaying the string interpolation until later. The %{} defines named placeholders in the string and % binds a given input into the placeholders.

In our recipe code we would then have something like:

remote_file '/tmp/example.zip' do
  source node['url'] % {version: node['version']}
end

By delaying this interpolation until recipe compile time, we leave room for all the wrapper cookbook attributes to have been evaluated.

tl;dr

Don’t use "#{}" in cookbook attributes, use "%{}" instead.

UPDATE: I’ve written a helper cookbook to make this much easier. Check it out on GitHub.

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