tasker

tasker is a library for running tasks in parallel with an arbitrary dependency specification.

Usage

Setup a tasker to run with only up to 5 tasks at a time:

tr := tasker.NewTasker(5)

or, if you're feeling ambitious, configure it without that limitation:

tr := tasker.NewTasker(-1)

Add a few tasks, making sure that all dependencies are also added. The ordering of these calls doesn't matter.

tr.Add("d", nil,                func() error { fmt.Println("d"); return nil })
tr.Add("a", []string{"b", "c"}, func() error { fmt.Println("a"); return nil })
tr.Add("b", nil,                func() error { fmt.Println("b"); return nil })
tr.Add("c", []string{"d"},      func() error { fmt.Println("c"); return nil })

Putting it all together:

package main

import (
	"log"
	"github.com/perriv/go-tasker"
)

func print_task(msg string) tasker.Task {
	return func() error {
		log.Println(msg)
		return nil
	}
}

func main() {
	tr, err := tasker.NewTasker(-1)
	if err != nil {
		log.Fatal(err)
	}

	err = tr.Add("d", nil, print_task("d"))
	if err != nil {
		log.Fatal(err)
	}

	err = tr.Add("a", []string{"b", "c"}, print_task("a"))
	if err != nil {
		log.Fatal(err)
	}

	err = tr.Add("b", nil, print_task("b"))
	if err != nil {
		log.Fatal(err)
	}

	err = tr.Add("c", []string{"d"}, print_task("c"))
	if err != nil {
		log.Fatal(err)
	}

	if err = tr.Run(); err != nil {
		log.Fatal(err)
	}
}

This program's output would be something like:

2016/08/03 13:03:08 d
2016/08/03 13:03:08 c
2016/08/03 13:03:08 b
2016/08/03 13:03:08 a

Although c and b could have just as easily been switched:

2016/08/03 13:03:09 d
2016/08/03 13:03:09 b
2016/08/03 13:03:09 c
2016/08/03 13:03:09 a

In case you were wondering, log is used above instead of fmt because log is thread-safe.

Invalid Dependency Graphs

Since dependencies don't have to be defined in a call to Add, two problems can arise that the user must be mindful of before calling Run.

The first problem is that all dependencies might not have been added. This was mentioned earlier, but it worth repeating here. Run checks for this and doesn't run unless all dependencies are defined.

The second problem is that there might be a dependency cycle among the tasks. Add takes care of the obvious self-dependent cycle of "a" depending on "a", but cycles involving more than one task can't be detected as easily. Instead, Run checks for these multi-task cycles using Tarjan's Algorithm for linear performance. It's been modified slightly, however, to return multi-task cycles only: tasks with no dependencies are just as valid as any other task, and those obnoxious tasks that depend on themselves are squelched by Add.

Documentation

Source code documentation can be found on godoc.org.