Fiber for cooperative concurrency in Ruby

Introduction

Fibers let your program do many things by switching tasks yourself. They help you manage multiple jobs without waiting for one to finish first.

When you want to run small tasks one after another without blocking the whole program.

When you need to pause a task and come back to it later.

When you want to write code that looks simple but handles multiple jobs cooperatively.

When you want to avoid complex thread management but still do multitasking.

Syntax

Ruby

fiber = Fiber.new do
  # code to run inside fiber
  Fiber.yield # pause and return control
  # resume here
end

fiber.resume # start or continue fiber

Fiber.new creates a new fiber with a block of code.

Fiber.yield pauses the fiber and returns control to where resume was called.

Examples

This fiber prints "Hello", pauses, then prints "World" when resumed again.

Ruby

fiber = Fiber.new do
  puts "Hello"
  Fiber.yield
  puts "World"
end

fiber.resume  # prints Hello
fiber.resume  # prints World

This fiber runs three steps, pausing after each one until resumed.

Ruby

fiber = Fiber.new do
  3.times do |i|
    puts "Step #{i + 1}"
    Fiber.yield
  end
end

3.times { fiber.resume }

Sample Program

This program shows how a fiber pauses and resumes. It prints messages before, during, and after the fiber runs.

Ruby

fiber = Fiber.new do
  puts "Start task"
  Fiber.yield
  puts "Resume task"
  Fiber.yield
  puts "End task"
end

puts "Before first resume"
fiber.resume
puts "Between resumes"
fiber.resume
puts "After last resume"
fiber.resume

Important Notes

Fibers are not threads; they run one at a time and switch only when you tell them.

Use fibers to simplify code that needs to pause and continue later.

Calling resume on a finished fiber will raise an error.

Summary

Fibers let you pause and resume tasks manually.

They help run multiple jobs cooperatively without real parallelism.

Use Fiber.new, Fiber.yield, and resume to control execution.

Practice

1. What is the main purpose of using Fiber in Ruby?

easy

A. To pause and resume tasks manually for cooperative multitasking

B. To create multiple threads that run in parallel automatically

C. To speed up code by running it on multiple CPU cores

D. To handle database connections asynchronously

Solution

1. Step 1: Understand Fiber's role

   Fibers allow manual pausing and resuming of code execution, enabling cooperative multitasking.

2. Step 2: Differentiate from threads

   Unlike threads, Fibers do not run in parallel automatically; they require explicit control.

3. Final Answer:

   To pause and resume tasks manually for cooperative multitasking -> Option A

4. Quick Check:

   Fiber = manual pause/resume [OK]

Hint: Fibers pause and resume tasks manually, not parallel threads [OK]

Common Mistakes:

• Confusing Fibers with threads or parallelism
• Thinking Fibers run automatically in background
• Assuming Fibers speed up CPU usage

2. Which of the following is the correct way to create a Fiber in Ruby?

easy

A. fiber = Fiber.start { puts 'Hello' }

B. fiber = Fiber.new { puts 'Hello' }

C. fiber = Fiber.create { puts 'Hello' }

D. fiber = Fiber.run { puts 'Hello' }

Solution

1. Step 1: Recall Fiber creation syntax

   Ruby uses Fiber.new with a block to create a new Fiber.

2. Step 2: Check other options

   Methods like create, start, or run do not exist for Fiber creation.

3. Final Answer:

   fiber = Fiber.new { puts 'Hello' } -> Option B

4. Quick Check:

   Fiber creation = Fiber.new [OK]

Hint: Use Fiber.new with a block to create a Fiber [OK]

Common Mistakes:

• Using non-existent Fiber methods like create or start
• Forgetting to pass a block to Fiber.new
• Confusing Fiber with Thread syntax

3. What will be the output of the following Ruby code?

fiber = Fiber.new do
  puts 'Step 1'
  Fiber.yield
  puts 'Step 2'
end

fiber.resume
puts 'Between'
fiber.resume

medium

A. Step 2
Between
Step 1

B. Step 1
Step 2
Between

C. Between
Step 1
Step 2

D. Step 1
Between
Step 2

Solution

1. Step 1: Trace first resume call

   Calling fiber.resume runs until Fiber.yield, printing 'Step 1'.

2. Step 2: Trace code after first resume

   After yield, control returns to main, printing 'Between'.

3. Step 3: Trace second resume call

   Second fiber.resume resumes after yield, printing 'Step 2'.

4. Final Answer:

   Step 1 Between Step 2 -> Option D

5. Quick Check:

   Fiber.yield pauses, resume continues [OK]

Hint: Fiber.yield pauses; resume continues after yield [OK]

Common Mistakes:

• Assuming all code runs at first resume
• Confusing order of prints around yield
• Thinking yield stops Fiber permanently

4. Identify the error in this Fiber usage:

fiber = Fiber.new do
  puts 'Start'
  Fiber.yield
  puts 'End'
end

fiber.yield
fiber.resume

medium

A. Calling Fiber.yield outside a Fiber block causes an error

B. Fiber.resume must be called before Fiber.yield

C. Fiber.new requires a parameter for yield count

D. Fiber cannot print inside its block

Solution

1. Step 1: Check where Fiber.yield is called

   The code calls fiber.yield outside the Fiber block, which is invalid.

2. Step 2: Understand Fiber.yield usage

   Fiber.yield is a class method to pause current Fiber, not an instance method on Fiber objects.

3. Final Answer:

   Calling Fiber.yield outside a Fiber block causes an error -> Option A

4. Quick Check:

   Fiber.yield only inside Fiber block [OK]

Hint: Fiber.yield is called inside Fiber block only [OK]

Common Mistakes:

• Calling fiber.yield instead of Fiber.yield
• Expecting Fiber.resume before yield call
• Thinking Fiber.new needs parameters for yield

5. Given two Fibers that alternate printing numbers, what is the output of this code?

fiber1 = Fiber.new do
  3.times do |i|
    puts "Fiber1: #{i}"
    Fiber.yield
  end
end

fiber2 = Fiber.new do
  3.times do |i|
    puts "Fiber2: #{i}"
    Fiber.yield
  end
end

while fiber1.alive? || fiber2.alive?
  fiber1.resume if fiber1.alive?
  fiber2.resume if fiber2.alive?
end

hard

A. Fiber2: 0
Fiber1: 0
Fiber2: 1
Fiber1: 1
Fiber2: 2
Fiber1: 2

B. Fiber1: 0
Fiber1: 1
Fiber1: 2
Fiber2: 0
Fiber2: 1
Fiber2: 2

C. Fiber1: 0
Fiber2: 0
Fiber1: 1
Fiber2: 1
Fiber1: 2
Fiber2: 2

D. Fiber1: 0
Fiber2: 0
Fiber2: 1
Fiber1: 1
Fiber1: 2
Fiber2: 2

Solution

1. Step 1: Understand Fiber alternation

   Each Fiber prints a number then yields, pausing itself.

2. Step 2: Trace the while loop

   The loop resumes fiber1 then fiber2 alternately while they are alive.

3. Step 3: Follow output order

   Output alternates: Fiber1:0, Fiber2:0, Fiber1:1, Fiber2:1, Fiber1:2, Fiber2:2.

4. Final Answer:

   Fiber1: 0 Fiber2: 0 Fiber1: 1 Fiber2: 1 Fiber1: 2 Fiber2: 2 -> Option C

5. Quick Check:

   Fibers alternate resume and yield [OK]

Hint: Fibers alternate resume and yield to interleave output [OK]

Common Mistakes:

• Assuming one Fiber runs completely before the other
• Mixing order of Fiber resumes
• Forgetting Fiber.yield pauses execution