NOAH KANTROWITZ

Chef's Two Pass Model 2015-10-09

A common source of bugs in Chef code for new and experienced users alike is Chef’s two-pass (or two-phase) execution model. I would like to provide a quick overview of this system and how it can impact your code.

Phases

Chef’s loading process can be broadly split into four phases:

  1. Load
  2. Compile
  3. Converge
  4. Cleanup

Of these, the compile and converge phases make up the bulk of the work and are “execution phases”. When we talk about the two-pass or two-phase system, it is these two phases we are talking about, but let’s look at each phase in turn.

Load

During the load phase Chef syncs all the needed cookbooks with the Chef Server if one is being used. Each of the five types of support files are loaded in order:

  1. libraries/
  2. attributes/
  3. resources/
  4. providers/
  5. definitions/

All of the files of each type are loaded for all cookbooks before moving on to the next type. Loading happens in sorted (topographic) order with respect to cookbook dependencies and the run list order, and alphabetical order within each cookbook.

This means if cookbook B depends on cookbook A, we might see a load order like:

  1. a/libraries/default.rb (library files are first, A is sorted before B because of dependency)
  2. b/libraries/default.rb (then B’s library files)
  3. a/attributes/default.rb (attribute files are next, again A is first)
  4. b/attributes/default.rb (and B is second because B depends on A)
  5. b/resources/first.rb (A has no resources files so we skip to B, first is alphabetically before second)
  6. b/resources/second.rb

By the time the loading phase is complete, the node object is fully populated, all custom resources are available for use, and we can move on to the compile phase.

Compile

The compile phase is first of the execution phases. The goal of the compile phase is to go from recipe source code to in-memory representations of resource objects. At this point we have taken the node’s run list and fully expanded it, so any roles are replaced by their constituent recipes until all we have is an ordered list of recipes to run. We will run each of those recipes in order, and if no errors are raised we’ll move on to the next phase.

This is where the major “gotcha” of the execution model comes in. Take some example recipe code like:

file '/foo' do
  content 'bar'
end

Assuming that recipe is in the run list, or is run via include_recipe from something else being compiled, that code will run. However running the code does not actually write any content to file. All it does is create a Chef::Resource::File object with all of the data expressed in the recipe code and add that object to the resource collection. This is true of all resources in the DSL, running (compiling) the recipe file is just queuing up resources in the resource collection. No changes to the system should happen until the converge phase. This also means that any Ruby code in the file not explicitly delayed (ruby_block, lazy, not_if/only_if) is run when the file is run, during the compile phase.

Converge

Once the compile phase completes we have a resource collection that is fully loaded and ready to go. This is an array of resource objects that represent the data from our recipes. The majority of the converge phase can be seen as:

resource_collection.each do |resource|
  resource.run_action(resource.action)
end

There is a little more complexity to deal with things like notifications, but that is the heart of it; loop over each resource and run the requested action. This is where provider classes get used, run_action creates a provider instance internally and runs the action code from the provider. Those methods are what actually do all the interesting things like writing files, installing packages, etc.

Cleanup

With the compile phase finished, we just have a few cleanup steps left to process. This includes things like running handler plugins, saving the node state back up to the Chef Server, and sending data to the Chef Analytics server if being used.

Bad Code

A concrete example of the kind of errors that can creep in due to the execution model:

file '/foo' do
  content 'bar'
end

if File.exist?('/foo')
  execute 'myapp /foo'
end

Here we have two resources and an if statement. The bug happens because the File.exist? check will run at compile time, and even though it is after the file resource, at that point in the execution the file hasn’t actually been written yet. The solution in this case is to use an only_if guard like:

file '/foo' do
  content 'bar'
end

execute 'myapp /foo' do
  only_if { File.exist?('/foo') }
end

As only_if guard clauses (and lazy property values and the block on ruby_block) are run during the converge phase, this will more often behave as the author expects.

tl;dr

First all recipes on the run list are compiled in to resources in the resource collection, then all resources in the collection are converged.

Any code outside lazy, only_if/not_if, or a ruby_block’s block property is run at compile time, which is before any resource runs its actions.


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