Sequential vs parallel async execution in Node.js - Hands-On Comparison

Sequential vs Parallel Async Execution in Node.js

📖 Scenario: You are building a simple Node.js script that fetches data from two different sources asynchronously. You want to learn how to run these fetches one after another (sequentially) and how to run them at the same time (in parallel) to see the difference in execution.

🎯 Goal: Build a Node.js script that first fetches two pieces of data sequentially using async/await, then modify it to fetch them in parallel using Promise.all. You will see how the total time changes depending on the approach.

📋 What You'll Learn

Create two async functions that simulate fetching data with a delay

Create a variable to hold the delay time in milliseconds

Write code to fetch data sequentially using async/await

Write code to fetch data in parallel using Promise.all

💡 Why This Matters

🌍 Real World

Many Node.js applications fetch data from multiple sources like APIs or databases. Knowing how to run these fetches sequentially or in parallel helps optimize performance and user experience.

💼 Career

Understanding async patterns like sequential and parallel execution is essential for backend and full-stack developers working with Node.js to write efficient and responsive applications.

1

Create async fetch functions

Create two async functions called fetchData1 and fetchData2. Each should return a Promise that resolves with the strings 'Data 1' and 'Data 2' respectively after a delay of 1000 milliseconds using setTimeout.

Node.js

# Your code here

Hint

Use new Promise and setTimeout inside each async function to simulate delay.

2

Add delay variable

Create a constant variable called delay and set it to 1000 to represent the delay time in milliseconds.

Node.js

async function fetchData1() {
    return new Promise(resolve => setTimeout(() => resolve('Data 1'), 1000));
}

async function fetchData2() {
    return new Promise(resolve => setTimeout(() => resolve('Data 2'), 1000));
}

// Your code here

Hint

Use const delay = 1000; to store the delay time.

3

Fetch data sequentially

Write an async function called fetchSequential. Inside it, use await to call fetchData1() and store the result in result1. Then await fetchData2() and store the result in result2. Return an array with result1 and result2.

Node.js

const delay = 1000;

async function fetchData1() {
    return new Promise(resolve => setTimeout(() => resolve('Data 1'), delay));
}

async function fetchData2() {
    return new Promise(resolve => setTimeout(() => resolve('Data 2'), delay));
}

// Write fetchSequential function below
// Your code here

Hint

Use await to wait for each fetch before starting the next.

4

Fetch data in parallel

Write an async function called fetchParallel. Inside it, use Promise.all with an array of fetchData1() and fetchData2() calls. Store the result in results and return it.

Node.js

const delay = 1000;

async function fetchData1() {
    return new Promise(resolve => setTimeout(() => resolve('Data 1'), delay));
}

async function fetchData2() {
    return new Promise(resolve => setTimeout(() => resolve('Data 2'), delay));
}

async function fetchSequential() {
    const result1 = await fetchData1();
    const result2 = await fetchData2();
    return [result1, result2];
}

// Write fetchParallel function below
// Your code here

Hint

Use Promise.all to run both fetches at the same time and wait for both to finish.

Practice

1. What is the main difference between sequential and parallel async execution in Node.js?

easy

A. Sequential async is faster than parallel async in all cases.

B. Sequential async waits for each task to finish before starting the next, while parallel async runs tasks at the same time.

C. Sequential async uses callbacks, while parallel async uses promises.

D. Sequential async runs all tasks at once, while parallel async runs them one by one.

Solution

1. Step 1: Understand sequential async execution

   Sequential async means tasks run one after another, waiting for each to complete before starting the next.

2. Step 2: Understand parallel async execution

   Parallel async means tasks run at the same time, without waiting for others to finish first.

3. Final Answer:

   Sequential async waits for each task to finish before starting the next, while parallel async runs tasks at the same time. -> Option B

4. Quick Check:

   Sequential vs parallel async = D [OK]

Hint: Sequential waits, parallel runs together [OK]

Common Mistakes:

• Confusing which runs tasks one by one
• Thinking parallel always uses callbacks
• Assuming sequential is always faster

2. Which of the following is the correct syntax to run two async functions task1() and task2() in parallel using Promise.all?

easy

A. await Promise.all([task1(), task2()]);

B. Promise.all(task1(), task2());

C. await task1(); await task2();

D. task1().then(task2());

Solution

1. Step 1: Recall Promise.all syntax

   Promise.all takes an array of promises and waits for all to complete in parallel.

2. Step 2: Check correct usage

   The correct syntax is await Promise.all([task1(), task2()]) to run both tasks in parallel and wait for both.

3. Final Answer:

   await Promise.all([task1(), task2()]); -> Option A

4. Quick Check:

   Promise.all needs array of promises [OK]

Hint: Use Promise.all with array for parallel [OK]

Common Mistakes:

• Missing array brackets in Promise.all
• Awaiting tasks one by one (sequential)
• Calling then() incorrectly without chaining

3. Consider this code snippet:

async function run() {
  const result1 = await task1();
  const result2 = await task2();
  return [result1, result2];
}
run().then(console.log);

What will be the order of execution and output behavior?

medium

A. task1 and task2 run in parallel; output is an array of both results.

B. Both tasks run sequentially but output is only result2.

C. task2 runs first, then task1; output is an array with reversed results.

D. task1 runs first, then task2 starts after task1 finishes; output is an array of both results.

Solution

1. Step 1: Analyze await usage

   Each await pauses execution until the promise resolves, so task1 finishes before task2 starts.

2. Step 2: Understand output array

   Both results are collected in order into an array and returned, so output is [result1, result2].

3. Final Answer:

   task1 runs first, then task2 starts after task1 finishes; output is an array of both results. -> Option D

4. Quick Check:

   Sequential await = B [OK]

Hint: Await pauses; tasks run one after another [OK]

Common Mistakes:

• Assuming tasks run in parallel with sequential await
• Thinking output order is reversed
• Believing output contains only one result

4. Identify the error in this code snippet intended to run two async tasks in parallel:

async function run() {
  const results = await Promise.all(task1(), task2());
  console.log(results);
}

medium

A. Promise.all requires an array of promises, but here arguments are passed separately.

B. Await cannot be used with Promise.all.

C. task1 and task2 must be awaited separately before Promise.all.

D. console.log cannot print arrays.

Solution

1. Step 1: Check Promise.all argument

   Promise.all expects a single array of promises, but here two arguments are passed separately.

2. Step 2: Correct usage

   It should be Promise.all([task1(), task2()]) with square brackets to group promises.

3. Final Answer:

   Promise.all requires an array of promises, but here arguments are passed separately. -> Option A

4. Quick Check:

   Promise.all needs array argument [OK]

Hint: Promise.all needs array, not separate args [OK]

Common Mistakes:

• Passing promises as separate arguments
• Thinking await can't be used with Promise.all
• Misunderstanding console.log capabilities

5. You have three independent async tasks: taskA(), taskB(), and taskC(). You want to run taskA and taskB in parallel, then run taskC only after both finish. Which code correctly implements this?

hard

A. const c = await taskC();
const [a, b] = await Promise.all([taskA(), taskB()]);

B. await taskA(); await taskB(); await taskC();

C. const [a, b] = await Promise.all([taskA(), taskB()]);
const c = await taskC();

D. Promise.all([taskA(), taskB(), taskC()]);

Solution

1. Step 1: Run taskA and taskB in parallel

   Using await Promise.all([taskA(), taskB()]) runs both tasks at the same time and waits for both to finish.

2. Step 2: Run taskC after both finish

   After awaiting both, await taskC() runs taskC sequentially, ensuring it starts only after taskA and taskB complete.

3. Final Answer:

   const [a, b] = await Promise.all([taskA(), taskB()]); const c = await taskC(); -> Option C

4. Quick Check:

   Parallel first, then sequential = A [OK]

Hint: Use Promise.all for parallel, then await next task [OK]

Common Mistakes:

• Running all tasks in parallel ignoring order
• Running tasks sequentially without parallelism
• Starting taskC before taskA and taskB finish