Fiber for cooperative concurrency in Ruby - Step-by-Step Execution

Concept Flow - Fiber for cooperative concurrency

Create Fiber

↓

Start Fiber: runs until yield

↓

Yield: pause Fiber, return control

↓

Main resumes, can do other work

↓

Resume Fiber: continues from yield

↓

Fiber finishes or yields again

↓

Repeat resume/yield until done

↓

Fiber ends, control back to main

A Fiber is created and started, runs until it yields control back to main, which can then resume the Fiber later. This allows cooperative multitasking.

Execution Sample

Ruby

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

fiber.resume
fiber.resume

This code creates a Fiber that prints 'Step 1', yields control, then prints 'Step 2' when resumed again.

Execution Table

Step	Action	Fiber State	Output	Control Flow
1	Create Fiber	Created (not started)		Main holds control
2	fiber.resume	Running	Step 1	Fiber runs until yield, then pauses
3	Fiber.yield	Paused		Fiber yields control back to main
4	Main continues	Paused		Main can do other work
5	fiber.resume	Running	Step 2	Fiber resumes after yield
6	Fiber block ends	Terminated		Fiber finishes, control back to main

💡 Fiber ends after second resume, no more code to run

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 5	Final
fiber	nil	Fiber object created	Fiber paused after yield	Fiber running	Fiber terminated

Key Moments - 3 Insights

Why does the Fiber stop after the first resume even though the block has more code?

Because the Fiber calls Fiber.yield which pauses execution and returns control to main (see execution_table step 3). The Fiber resumes only when fiber.resume is called again.

What happens if we call fiber.resume after the Fiber has finished?

Calling resume on a terminated Fiber raises a FiberError because the Fiber has no more code to run (see execution_table step 6).

Does the main program run while the Fiber is paused?

Yes, after Fiber.yield pauses the Fiber (step 3), main regains control and can do other work until fiber.resume is called again (step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the Fiber state after the first fiber.resume call?

A Running

B Terminated

C Paused

D Created

💡 Hint

Check execution_table row 2 and 3 where Fiber runs then yields and pauses

At which step does the Fiber yield control back to main?

A Step 2

B Step 3

C Step 5

D Step 6

💡 Hint

Look at execution_table step 3 where Fiber.yield is called

If we remove Fiber.yield from the code, what happens to the output?

A 'Step 1' and 'Step 2' print immediately on first resume

B Only 'Step 1' prints

C No output prints

D Fiber never terminates

💡 Hint

Without yield, Fiber runs entire block on first resume (see execution flow)

Concept Snapshot

Fiber.new { ... } creates a Fiber.
fiber.resume starts or continues it.
Fiber.yield pauses Fiber, returns control to main.
Main and Fiber cooperate by yielding and resuming.
Fiber ends when block finishes.
Useful for cooperative concurrency.

Full Transcript

This example shows how Ruby Fibers work for cooperative concurrency. First, a Fiber is created with Fiber.new and a block of code. When fiber.resume is called, the Fiber starts running and prints 'Step 1'. Then it calls Fiber.yield, which pauses the Fiber and returns control to the main program. The Fiber is now paused. The main program can do other work or call fiber.resume again to continue the Fiber. On the second resume, the Fiber prints 'Step 2' and finishes. Trying to resume a finished Fiber causes an error. This cooperative switching allows the program to manage multiple tasks by explicitly yielding and resuming Fibers.