Why Async batch processing in Rest API? - Purpose & Use Cases

The Big Idea

What if your app could juggle hundreds of tasks at once without breaking a sweat?

The Scenario

Imagine you have to send 100 emails one by one through a web service. You wait for each email to finish sending before starting the next. This takes a long time and keeps your app stuck.

The Problem

Doing tasks one after another is slow and wastes time. If one task is slow or fails, everything waits or breaks. It's hard to keep track and your app feels unresponsive.

The Solution

Async batch processing lets you send many requests at once without waiting for each to finish. Your app can keep working while tasks run in the background, making everything faster and smoother.

Before vs After

✗ Before

for email in emails:
    send_email(email)  # waits for each email to finish

✓ After

send_emails_async(emails)  # sends all emails at once without waiting

What It Enables

You can handle many tasks quickly and keep your app responsive, even when working with slow or many operations.

Real Life Example

A social media app sending notifications to thousands of users at once without freezing or slowing down the user experience.

Key Takeaways

Manual sequential calls are slow and block progress.

Async batch processing runs many tasks together efficiently.

This keeps apps fast and responsive under heavy work.

Practice

1. What is the main benefit of using async batch processing in REST APIs?

easy

A. It allows sending multiple requests at once to save time.

B. It makes each request slower but more secure.

C. It processes requests one by one to avoid errors.

D. It disables parallel processing to reduce server load.

Solution

1. Step 1: Understand async batch processing purpose

   Async batch processing sends many requests together instead of one at a time.

2. Step 2: Identify the benefit

   Sending requests together saves time and improves efficiency.

3. Final Answer:

   It allows sending multiple requests at once to save time. -> Option A

4. Quick Check:

   Async batch processing = multiple requests at once [OK]

Hint: Async batch sends many requests together to save time [OK]

Common Mistakes:

• Thinking async batch slows down requests
• Believing requests are processed one by one
• Confusing async batch with disabling parallelism

2. Which of the following is the correct way to start an async batch request in JavaScript using fetch?

easy

A. fetch('/batch', { method: 'GET', body: requests })

B. fetch('/batch', 'POST', requests)

C. fetch('/batch', { method: 'POST', body: JSON.stringify(requests) })

D. fetch('/batch', { method: 'POST', body: requests })

Solution

1. Step 1: Check fetch syntax for POST with JSON body

   fetch requires method and body as JSON string for POST requests.

2. Step 2: Identify correct option

   fetch('/batch', { method: 'POST', body: JSON.stringify(requests) }) uses method 'POST' and JSON.stringify to convert requests to string.

3. Final Answer:

   fetch('/batch', { method: 'POST', body: JSON.stringify(requests) }) -> Option C

4. Quick Check:

   fetch POST with JSON body = fetch('/batch', { method: 'POST', body: JSON.stringify(requests) }) [OK]

Hint: Use JSON.stringify for POST body in fetch [OK]

Common Mistakes:

• Passing body without JSON.stringify
• Using GET method with body
• Wrong fetch parameter order

3. Given this async batch code snippet, what will be the output?

const batchRequests = [
  fetch('/api/data1'),
  fetch('/api/data2'),
  fetch('/api/data3')
];

Promise.all(batchRequests)
  .then(responses => responses.map(r => r.status))
  .then(statuses => console.log(statuses));

medium

A. [Promise, Promise, Promise]

B. [200, 200, 200]

C. [undefined, undefined, undefined]

D. Error: Cannot read property 'status' of undefined

Solution

1. Step 1: Understand Promise.all with fetch

   Promise.all waits for all fetch requests to complete and returns an array of responses.

2. Step 2: Map responses to their status codes

   Each response has a status property, usually 200 if successful.

3. Final Answer:

   [200, 200, 200] -> Option B

4. Quick Check:

   Promise.all fetch statuses = [200, 200, 200] [OK]

Hint: Promise.all returns array of resolved values [OK]

Common Mistakes:

• Expecting promises instead of resolved responses
• Trying to access status before Promise.all resolves
• Confusing map on promises vs responses

4. Identify the error in this async batch processing code snippet:

async function batchFetch(urls) {
  const results = [];
  urls.forEach(async url => {
    const res = await fetch(url);
    results.push(await res.json());
  });
  return results;
}

medium

A. Using forEach with async does not wait for all fetches to finish.

B. Missing JSON.stringify on fetch body.

C. results array is declared inside the loop.

D. fetch should not be awaited inside async function.

Solution

1. Step 1: Analyze async inside forEach

   forEach does not wait for async callbacks, so results may be empty on return.

2. Step 2: Identify correct fix

   Use for...of loop with await or Promise.all to wait for all fetches.

3. Final Answer:

   Using forEach with async does not wait for all fetches to finish. -> Option A

4. Quick Check:

   forEach async does not await = Using forEach with async does not wait for all fetches to finish. [OK]

Hint: Avoid async inside forEach; use for...of or Promise.all [OK]

Common Mistakes:

• Assuming forEach waits for async callbacks
• Not returning or awaiting promises properly
• Misplacing results array declaration

5. You want to send 100 API requests in batches of 10 asynchronously to avoid overloading the server. Which approach correctly implements this in JavaScript?

hard

A. Send requests one by one using a for loop with await inside.

B. Send all 100 requests at once with Promise.all without batching.

C. Use a forEach loop with async fetch calls without awaiting.

D. Split URLs into chunks of 10, then use Promise.all on each chunk sequentially.

Solution

1. Step 1: Understand batch size control

   Sending all 100 requests at once may overload the server; batching limits concurrency.

2. Step 2: Implement batch processing

   Split URLs into groups of 10, then await Promise.all for each batch sequentially.

3. Final Answer:

   Split URLs into chunks of 10, then use Promise.all on each chunk sequentially. -> Option D

4. Quick Check:

   Batch requests in chunks with Promise.all = Split URLs into chunks of 10, then use Promise.all on each chunk sequentially. [OK]

Hint: Batch requests in small groups, await each batch [OK]

Common Mistakes:

• Sending all requests at once causing overload
• Not awaiting async calls properly
• Sending requests one by one losing async benefits