Croupier

Croupier is a smart task definition and execution library, which can be used for dataflow programming.

What does it mean

You use Croupier to define tasks. Tasks have:

• An id

• Zero or more input files

• Zero or more output files

• A Proc that consumes the inputs and returns a string

• After the Proc returns data which is saved to the output(s) unless the task has the no_save flag set to true, in which case it's expected to have already saved it.

  Note: the return value for procs depends on several factors, see below.

And here is the fun part:

Croupier will examine the inputs and outputs for your tasks and use them to build a dependency graph. This expresses the connections between your tasks and the files on disk, and between tasks, and will use that information to decide what to run.

So, suppose you have task1 consuming input.txt producing fileA and task2 that has fileA as input and outputs fileB. That means your tasks look something like this:

  graph LR;
      id1(["📁 input.txt"])-->idt1["⚙️ task1"]-->id2(["📁 fileA"]);
      id2-->idt2["⚙️ task2"]-->id3(["📁 fileB"]);

Croupier guarantees the following:

• If task1 has never run before, it will run and create fileA
• If task1 has run before and input.txt has not changed, it will not run.
• If task1 has run before and ìnput.txt` has changed, it will run
• If task1 runs, task2 will run and create fileB
• task1 will run before task2

That's a very long way to say: Croupier will run whatever needs running, based on the content of the dependency files and the dependencies between tasks. In this example it may look silly because it's simple, but it should work even for thousands of tasks and dependencies.

The state between runs is kept in .croupier so if you delete that file all tasks will run.

Further documentation at the doc pages

Notes

Notes about proc return types

• Procs in Tasks without outputs can return nil or a string, it will be ignored.

• Procs with one output and no_save==false should return a string which will be saved to that output.

  If no_save==true then the returned value is ignored.

• Procs with multiple outputs and no_save==false should return an Array(String) which will be saved to those outputs.

  If no_save==true then the returned value is ignored.

No target conflicts

If there are two or more tasks with the same output they will be merged into the first task created. The resulting task will:

• Depend on the combination of all dependencies of all merged tasks
• Run the procs of all merged tasks in order of creation

Tasks without output

A task with no output will be registered under its id and is not expected to create any output files. Other than that, it's just a regular task.

Tasks with multiple outputs

If a task expects the TaskManager to create multiple files, it should return an array of strings.

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     croupier:
       github: your-github-user/croupier
   

2. Run shards install

Usage

This is the example described above, in actual code:

require "croupier"

b1 = Croupier::TaskProc.new{
  puts "task1 running"
  File.read("input.txt").downcase
}

Croupier::Task.new(
  output: "fileA",
  inputs: ["input.txt"],
  proc: b1
)

b2 = Croupier::TaskProc.new{
  puts "task2 running"
  File.read("fileA").upcase
}

Croupier::Task.new(
  output: "fileB",
  inputs: ["fileA"],
  proc: b2
)

Croupier::Task.run_tasks

If we create a index.txt file with some text in it and run this program, it will print it's running task1 and task2 and produce fileA with that same text in upper case, and fileB with the text in lowercase.

The second time we run it, it will do nothing because all tasks dependencies are unchanged.

If we modify index.txt or fileA then one or both tasks will run, as needed.

Development

Let's try to keep test coverage good :-)

Other than that, anything is fair game. In the TODO.md file there is a section for things that were considered and decided to be a bad idea, but that is conditional and can change when presented with a good argument.

Contributing

1. Fork it (https://github.com/your-github-user/croupier/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Contributors

• Roberto Alsina - creator and maintainer