in_parallel and in_serial

gistfile1.md

Parallelization in Chef Recipes

Chef presently runs the resources in a recipe serially, one after the next. In this proposal, user-selected groups of resources will run their actions in parallel.

MVP (Minimum Viable Product) Features

in_parallel

To run a group of resources in parallel, you write it this way:

in_parallel do
  remote_file '/tmp/bigfile.txt' do
    source 'https://a.com/bigfile.txt'
  end
  remote_file '/tmp/bigfile2.txt' do
    source 'https://a.com/bigfile2.txt'
  end
end

Declaring Parallel Safety

In order to run inside an in_parallel block, a resource must declare that it is parallel safe. To do this, it should override the parallel_safe method and return true. If the parallel_safe method is missing or returns false, the recipe will fail to compile.

Notifications

If a resource in a parallel group notifies another resource, behavior will depend on whether the notified resource is parallel-safe. If it is parallel-safe, the notification will be queued to run as part of the existing parallel group. If it is not parallel-safe, the notification will be queued to run after the entire parallel group completes.

Failure Handling

If a resource action in a parallel group fails, the parallel group will run all other actions to completion (or failure) before exiting the recipe.

Future Features

Thread pool configuration

The Chef::Config.concurrency parameter, and --concurrency argument to chef-client, limits the number of concurrent parallel resources globally. To limit them specifically, you add parameters to the in_parallel directive like so:

in_parallel :concurrency => 10 do
  ...
end

in_serial

To run a group of actions serially, but inside a parallel grouping, you write this:

in_parallel do
  in_serial do
    directory '/dir1'
    file '/dir1/blah.txt' do
      content 'hi'
    end
  end
  in_serial do
    directory '/dir2'
    file '/dir2/blah.txt' do
      content 'hi'
    end
  end
end

Custom parallelization

Some resource types (such as packages) handle parallelization internally. We will create a directive allowing multiple resources to collaborate and run a single parallel thing to handle multiple actions. For example, if you wrote 10 package directives inside in_parallel, they would cooperate and run a single package installation command, passing all the packages to it.

I'm willing to trade my first born if this is implement.