Sequential vs parallel async execution in Node.js - Visual Side-by-Side Comparison

Concept Flow - Sequential vs parallel async execution

Start

↓

Call async task 1

↓

Wait for task 1 to finish

↓

Call async task 2

↓

Wait for task 2 to finish

↓

End

↓

Start

↓

Call async task 1 and task 2 together

↓

Wait for both tasks to finish

↓

End

Shows two ways to run async tasks: one after another (sequential) or both at the same time (parallel).

Execution Sample

Node.js

async function sequential() {
  await task1();
  await task2();
}

async function parallel() {
  await Promise.all([task1(), task2()]);
}

Runs two async tasks sequentially and then runs them in parallel.

Execution Table

Step	Action	Task 1 Status	Task 2 Status	Notes
1	Call task1()	Started	Not started	Sequential: task1 starts
2	Wait for task1 to finish	Finished	Not started	Sequential: waiting for task1
3	Call task2()	Finished	Started	Sequential: task2 starts after task1
4	Wait for task2 to finish	Finished	Finished	Sequential: waiting for task2
5	Sequential done	Finished	Finished	Both tasks done one after another
6	Call task1() and task2() together	Started	Started	Parallel: both tasks start at same time
7	Wait for both tasks to finish	Finished	Finished	Parallel: waiting for both tasks
8	Parallel done	Finished	Finished	Both tasks done together

💡 Execution stops after both tasks finish in each approach.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	After Step 7	Final
task1Status	Not started	Started	Finished	Finished	Started	Finished	Finished
task2Status	Not started	Not started	Started	Finished	Started	Finished	Finished

Key Moments - 2 Insights

Why does task2 only start after task1 finishes in the sequential example?

Because the code uses 'await' on task1 before calling task2, so it waits for task1 to finish first (see execution_table rows 1-3).

How can both tasks run at the same time in the parallel example?

Because Promise.all starts both tasks together without waiting, then waits for both to finish (see execution_table rows 6-8).

Visual Quiz - 3 Questions

Look at the execution table, at which step does task2 start in the sequential execution?

A Step 3

B Step 2

C Step 6

D Step 1

💡 Hint

Check the 'Action' and 'Task 2 Status' columns in execution_table rows 2 and 3.

In the parallel execution, when do both tasks start?

A Step 1

B Step 6

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column and 'Task 1 Status' and 'Task 2 Status' at step 6.

If we remove 'await' before task1 in sequential code, what changes in the execution table?

A Both tasks never start

B Task2 waits for task1 to finish

C Task2 starts immediately after task1 starts

D Task1 never starts

💡 Hint

Think about how 'await' controls waiting shown in variable_tracker and execution_table.

Concept Snapshot

Sequential async: await each task one by one
Parallel async: start all tasks together with Promise.all
Sequential waits for each task to finish before next
Parallel runs tasks at same time, waits for all
Use parallel to save time when tasks don't depend on each other

Full Transcript

This lesson shows two ways to run asynchronous tasks in Node.js. Sequential execution waits for the first task to finish before starting the second. Parallel execution starts both tasks at the same time and waits for both to finish. The execution table traces each step, showing when tasks start and finish. Variable tracking shows task status changes. Key moments clarify why sequential waits and parallel runs together. The quiz tests understanding of when tasks start and how 'await' affects flow. Remember, use sequential when tasks depend on each other, and parallel to save time when tasks are independent.

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