Fibers for concurrency in PHP - Step-by-Step Execution

Concept Flow - Fibers for concurrency

Start main program

↓

Create Fiber

↓

Start Fiber

↓

Fiber runs until yield

↓

Main resumes, does work

↓

Resume Fiber

↓

Fiber continues or ends

↓

Main finishes

The main program creates and starts a fiber, which runs until it yields control back. The main program can then do other work and resume the fiber later, enabling concurrency.

Execution Sample

PHP

<?php
$fiber = new Fiber(function() {
    echo "Fiber started\n";
    Fiber::suspend();
    echo "Fiber resumed\n";
});
$fiber->start();
echo "Main after start\n";
$fiber->resume();

This code creates a fiber that prints a message, suspends itself, then prints another message when resumed. The main program starts the fiber, prints a message, then resumes the fiber.

Execution Table

Step	Action	Output	Fiber State	Main State
1	Create Fiber		Created	Ready
2	Start Fiber	Fiber started	Suspended (yielded)	Running
3	Main prints	Main after start	Suspended (yielded)	Running
4	Resume Fiber	Fiber resumed	Terminated	Running
5	End		Terminated	Finished

💡 Fiber terminates after finishing its function; main program ends after resuming fiber.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
$fiber state	Not created	Suspended (yielded)	Terminated	Terminated

Key Moments - 2 Insights

Why does the fiber stop after the first echo and not run the second echo immediately?

Because at step 2 in the execution_table, the fiber calls Fiber::suspend(), which pauses the fiber and returns control to the main program before the second echo.

What happens when the main program calls resume() on the fiber?

At step 4, resume() continues the fiber from where it was suspended, so it prints "Fiber resumed" and then the fiber ends.

Visual Quiz - 3 Questions

Look at the execution_table, what is the fiber state after step 2?

A Created

B Suspended (yielded)

C Terminated

D Running

💡 Hint

Check the 'Fiber State' column in row for step 2.

At which step does the main program print its message?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Output' columns in the execution_table.

If Fiber::suspend() was removed, what would happen to the fiber state after start()?

A Fiber would be terminated immediately

B Fiber would be created but not started

C Fiber would be suspended

D Fiber would throw an error

💡 Hint

Without suspend, fiber runs to end on start(), see how suspend affects state in execution_table step 2.

Concept Snapshot

Fibers in PHP allow pausing and resuming code blocks.
Create a Fiber with new Fiber(function() {...}).
Start it with start(), which runs until suspend().
Use suspend() to pause and resume() to continue.
This enables cooperative concurrency without threads.

Full Transcript

This example shows how PHP fibers work for concurrency. The main program creates a fiber that prints "Fiber started" and then suspends itself. The main program continues and prints "Main after start". Later, the main program resumes the fiber, which prints "Fiber resumed" and then ends. The fiber's state changes from created to suspended to terminated as it runs. This allows the main program and fiber to cooperate by yielding control back and forth.

Practice

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

easy

A. To speed up database queries automatically

B. To create permanent background threads

C. To pause and resume code execution for concurrency

D. To replace all loops with asynchronous calls

Solution

1. Step 1: Understand Fiber's role

   Fibers allow pausing and resuming code blocks, enabling concurrency without threads.

2. Step 2: Compare options

   Only To pause and resume code execution for concurrency correctly describes Fibers' purpose; others describe unrelated features.

3. Final Answer:

   To pause and resume code execution for concurrency -> Option C

4. Quick Check:

   Fibers = pause and resume concurrency [OK]

Hint: Fibers let you pause and resume code easily [OK]

Common Mistakes:

• Thinking Fibers create real threads
• Assuming Fibers speed up database automatically
• Confusing Fibers with async loops

2. Which of the following is the correct way to start a Fiber in PHP?

easy

A. $fiber->run();

B. $fiber->execute();

C. $fiber->begin();

D. $fiber->start();

Solution

1. Step 1: Recall Fiber methods

   The Fiber class uses start() to begin execution.

2. Step 2: Check other options

   Methods like run(), begin(), and execute() do not exist for Fiber.

3. Final Answer:

   $fiber->start(); -> Option D

4. Quick Check:

   Fiber starts with start() method [OK]

Hint: Use start() to begin Fiber execution [OK]

Common Mistakes:

• Using run() instead of start()
• Trying to call execute() or begin()
• Confusing Fiber methods with thread methods

3. What will be the output of this PHP code using Fiber?

$fiber = new Fiber(function(): void {
    $value = Fiber::suspend('paused');
    echo "Resumed with: $value";
});

echo $fiber->start();
$fiber->resume('data');

medium

A. pausedResumed with: data

B. Resumed with: datapaused

C. paused

D. Resumed with: data

Solution

1. Step 1: Analyze Fiber start()

   The start() runs until Fiber::suspend('paused'), which returns 'paused'. So, echo outputs 'paused'.

2. Step 2: Analyze Fiber resume()

   The resume('data') continues after suspend, echoing "Resumed with: data".

3. Final Answer:

   pausedResumed with: data -> Option A

4. Quick Check:

   start() returns suspend value, resume() continues [OK]

Hint: start() returns suspend value, resume() continues execution [OK]

Common Mistakes:

• Assuming start() returns void
• Thinking resume() returns suspend value
• Mixing order of output strings

4. Identify the error in this Fiber code snippet:

$fiber = new Fiber(function() {
    echo "Start\n";
    Fiber::suspend();
    echo "End\n";
});

$fiber->resume();

medium

A. Fiber::suspend() cannot be called without a value

B. Fiber must be started with start(), not resume() first

C. Fiber constructor requires a return type declaration

D. Fiber cannot echo strings inside its function

Solution

1. Step 1: Check Fiber start method usage

   Fibers must be started with start() before calling resume().

2. Step 2: Validate other options

   Calling Fiber::suspend() without value is allowed; no return type needed; echo is valid inside Fiber.

3. Final Answer:

   Fiber must be started with start(), not resume() first -> Option B

4. Quick Check:

   start() first, then resume() [OK]

Hint: Always start Fiber with start() before resume() [OK]

Common Mistakes:

• Calling resume() before start()
• Thinking suspend() needs a value
• Expecting Fiber function to require return type

5. You want to run two Fibers concurrently to simulate two tasks. Which approach correctly switches between them to print alternating messages?

$fiber1 = new Fiber(function() {
    for ($i = 0; $i < 3; $i++) {
        echo "A$i ";
        Fiber::suspend();
    }
});

$fiber2 = new Fiber(function() {
    for ($i = 0; $i < 3; $i++) {
        echo "B$i ";
        Fiber::suspend();
    }
});

// Which code below correctly alternates execution?

A) {
    $fiber1->start();
    $fiber2->start();
    $fiber1->resume();
    $fiber2->resume();
}

B) {
    $fiber1->start();
    $fiber1->resume();
    $fiber2->start();
    $fiber2->resume();
}

C) {
    $fiber1->start();
    $fiber2->start();
    $fiber2->resume();
    $fiber1->resume();
}

D) {
    $fiber1->start();
    $fiber2->start();
    $fiber1->resume();
    $fiber1->resume();
}

hard

A. Start both fibers, then alternate resume calls: fiber1->resume(), fiber2->resume()

B. Start and resume fiber1 twice before starting fiber2

C. Start both fibers, then resume fiber2 before fiber1

D. Start both fibers, then resume fiber1 twice without resuming fiber2

Solution

1. Step 1: Understand Fiber suspension

   Each Fiber suspends after printing one message, so to alternate, we must resume them in turn.

2. Step 2: Analyze options for alternating resumes

   Start both fibers, then alternate resume calls: fiber1->resume(), fiber2->resume() starts both fibers, then resumes fiber1 and fiber2 alternately, producing alternating output. Others resume one fiber multiple times or in wrong order.

3. Final Answer:

   Start both fibers, then alternate resume calls: fiber1->resume(), fiber2->resume() -> Option A

4. Quick Check:

   Alternate resume calls for concurrency [OK]

Hint: Alternate resume calls to switch between fibers [OK]

Common Mistakes:

• Resuming one fiber multiple times before the other
• Starting fibers but not resuming correctly
• Resuming fibers in wrong order