Crystal by example: Fibers

A Fiber is a lightweight unit of execution in Crystal, kind of like a thread, but cooperative and managed by Crystal scheduler. You can create fibers using spawn.

Fibers take turns running, so multiple tasks can make progress without stopping the whole program. The scheduler decides the order they run in, so it’s not guaranteed.

The sum method or any other method still runs normally inside a fiber. It doesn’t become asynchronous, but the fiber can give other fibers a chance to run at the same time.

Fiber.yield lets the current fiber pause its turn and let other fibers run. It doesn’t sleep for a fixed time and doesn’t promise exactly when the fiber will run again. It’s just a friendly way of telling the scheduler that someone else can go now.

def saudation(name)
  return "Hello, #{name}!"
end

spawn do
  puts "Fiber 2!"
  sleep 2.seconds
end

spawn do
  puts "Fiber 3!"

  names = ["Lattner", "Ary", "Matz"]
  names.each do |name|
    puts saudation(name)
  end
end

puts "Fiber 1! (Main)"

Fiber.yield

$ crystal run fibers.cr

Fiber 1! (Main)
Fiber 2!
Fiber 3!
Hello, Lattner!
Hello, Ary!
Hello, Matz!