Web Workers for parallel processing in Deno - Mini Project: Build & Apply

Web Workers for Parallel Processing in Deno

📖 Scenario: You are building a Deno application that needs to perform a heavy calculation without freezing the main program. To do this, you will use Web Workers to run the calculation in parallel.

🎯 Goal: Create a Deno program that uses a Web Worker to calculate the sum of numbers from 1 to 1,000,000 without blocking the main thread.

📋 What You'll Learn

Create a Web Worker script file with the calculation logic

Create a main Deno script that starts the Worker

Send a message from the main script to the Worker to start calculation

Receive the result from the Worker and display it in the main script

💡 Why This Matters

🌍 Real World

Web Workers allow web and Deno applications to perform heavy tasks like calculations or data processing in the background. This keeps the app responsive and smooth for users.

💼 Career

Understanding Web Workers is important for developers building high-performance web apps or server-side scripts that need parallel processing without freezing the main program.

1

Create the Worker script with calculation logic

Create a file named worker.ts that listens for messages. When it receives a message, calculate the sum of numbers from 1 to 1,000,000 using a for loop and post the result back using self.postMessage.

Deno

// Your code here
// Listen for messages and calculate sum from 1 to 1,000,000
// Post the result back to main thread

Hint

Use self.onmessage to listen for messages and self.postMessage to send results back.

2

Create the main script and start the Worker

In a file named main.ts, create a new Worker instance that loads worker.ts. Store this Worker in a variable called worker.

Deno

// Your code here
// Create a Worker instance loading 'worker.ts' and assign it to 'worker'

Hint

Use new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' }) to create the Worker.

3

Send a message to the Worker to start calculation

In main.ts, send a message to the worker using worker.postMessage to trigger the calculation.

Deno

const worker = new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' });

// Your code here
// Send a message to 'worker' to start the calculation

Hint

Use worker.postMessage('start') to send a message to the Worker.

4

Receive and display the result from the Worker

In main.ts, add an event listener for worker.onmessage. When a message is received, assign the data to a variable called result and log it to the console.

Deno

const worker = new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' });

worker.postMessage('start');

// Your code here
// Add 'worker.onmessage' event listener to receive the result and assign it to 'result'

Hint

Use worker.onmessage = (event) => { result = event.data; console.log(result); } to get and display the result.

Practice

1. What is the main purpose of using Web Workers in Deno?

easy

A. To create a new HTTP server

B. To run tasks in parallel without blocking the main thread

C. To manage file system permissions

D. To handle database connections

Solution

1. Step 1: Understand Web Workers role

   Web Workers allow running scripts in background threads separate from the main thread.

2. Step 2: Identify the benefit

   This prevents the main thread from freezing during heavy tasks, enabling parallel processing.

3. Final Answer:

   To run tasks in parallel without blocking the main thread -> Option B

4. Quick Check:

   Web Workers = Parallel tasks without blocking [OK]

Hint: Web Workers run code in background threads [OK]

Common Mistakes:

• Thinking Web Workers handle HTTP or database tasks directly
• Confusing Web Workers with main thread code
• Assuming Web Workers manage permissions

2. Which of the following is the correct way to send a message from the main thread to a Web Worker in Deno?

easy

A. worker.postMessage('Hello')

B. worker.emit('Hello')

C. worker.send('Hello')

D. worker.message('Hello')

Solution

1. Step 1: Recall message sending method

   In Web Workers, the method to send data to the worker is postMessage.

2. Step 2: Match correct syntax

   Among the options, only worker.postMessage('Hello') is valid and recognized.

3. Final Answer:

   worker.postMessage('Hello') -> Option A

4. Quick Check:

   Send message = postMessage [OK]

Hint: Use postMessage to send data to workers [OK]

Common Mistakes:

• Using send() instead of postMessage()
• Confusing emit() with event emitters
• Trying to call message() which doesn't exist

3. Consider this Deno Web Worker code snippet:

const worker = new Worker(new URL('./worker.ts', import.meta.url).href, { type: 'module' });
worker.onmessage = (e) => console.log('Result:', e.data);
worker.postMessage(5);

And the worker.ts contains:

self.onmessage = (e) => {
  const num = e.data;
  self.postMessage(num * 2);
};

What will be printed in the console?

medium

A. Result: 10

B. Result: 5

C. Result: undefined

D. No output, error occurs

Solution

1. Step 1: Understand message flow

   The main thread sends 5 to the worker. The worker receives it as num.

2. Step 2: Calculate worker response

   The worker multiplies 5 by 2 and sends back 10. The main thread logs 'Result: 10'.

3. Final Answer:

   Result: 10 -> Option A

4. Quick Check:

   5 * 2 = 10 output [OK]

Hint: Worker doubles input and sends back result [OK]

Common Mistakes:

• Assuming worker returns original number
• Confusing onmessage event usage
• Expecting error due to URL usage

4. What is wrong with this Deno Web Worker code snippet?

const worker = new Worker('./worker.ts', { type: 'module' });
worker.postMessage('start');
worker.onmessage = (e) => console.log(e.data);
worker.terminate();

medium

A. Worker path './worker.ts' is invalid in Deno

B. postMessage is called before onmessage, causing error

C. Worker is terminated immediately, so no message is received

D. Missing event listener for error events

Solution

1. Step 1: Analyze termination timing

   The worker is terminated right after sending the message, likely before it can respond.

2. Step 2: Effect on message reception

   Because of immediate termination, the onmessage handler never receives data.

3. Final Answer:

   Worker is terminated immediately, so no message is received -> Option C

4. Quick Check:

   Terminate too soon = no response [OK]

Hint: Terminate workers only after receiving messages [OK]

Common Mistakes:

• Thinking postMessage order causes error
• Ignoring worker path resolution rules
• Forgetting to handle errors but not critical here

5. You want to process a large array of numbers in parallel using Deno Web Workers. Which approach correctly splits the work and collects results efficiently?

hard

A. Process the array in the main thread without workers for simplicity

B. Create one worker and send the entire array repeatedly for each number

C. Use a single worker and process numbers sequentially inside it

D. Create multiple workers, send each a slice of the array, collect partial results, then combine them

Solution

1. Step 1: Understand parallel processing strategy

   Splitting the array into parts and assigning each to a separate worker allows true parallelism.

2. Step 2: Collect and combine results

   Each worker returns partial results; the main thread combines them for the final output.

3. Final Answer:

   Create multiple workers, send each a slice of the array, collect partial results, then combine them -> Option D

4. Quick Check:

   Split work + combine results = efficient parallelism [OK]

Hint: Split data among workers, then merge results [OK]

Common Mistakes:

• Sending full array multiple times wastes resources
• Using one worker loses parallel benefit
• Ignoring main thread processing slows app