AJAX - Concurrency Limits & Backoff / Retry Strategies

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1. What Is Concurrency in AJAX?

Concurrency means:

How many AJAX requests are sent at the same time from a client (browser/app) to a server.

Example:

  • Page loads

  • 10 API calls fire together (profile, notifications, messages, ads, analytics)

If not controlled → problems occur.

2. What Are Concurrency Limits?

Concurrency limits are restrictions on:

  • How many requests a browser can send at once

  • How many requests a server allows from one client

These limits exist to:

  • Protect servers

  • Prevent overload

  • Ensure fair usage

3. Where Do These Limits Come From?

Browser Limits

  • Browsers limit concurrent connections per domain

  • Prevent network flooding

Server Limits

  • APIs often limit requests per user/IP

  • Example: 100 requests per minute

If exceeded → server responds with:

HTTP 429 – Too Many Requests

4. Problems Without Concurrency Control

If many requests are fired blindly:

❌ Server overload
❌ Requests fail randomly
❌ UI becomes slow or unstable
❌ Users see errors

This is common in:

  • Search autocomplete

  • Infinite scrolling

  • Dashboards

5. What Is Retry Strategy?

A retry strategy means:

Trying the same request again if it fails temporarily.

Used when failure is due to:

  • Network issue

  • Temporary server overload

  • Rate limiting (429)

 Retrying blindly is dangerous.

6. Why Simple Retry Is Bad

Bad approach:

Request fails → retry immediately → fails → retry again

This causes:

  • More server load

  • Infinite loops

  • App crashes

So we use Backoff.

7. What Is Backoff Strategy?

Backoff means:

Waiting for some time before retrying a failed request.

Each retry waits longer than the previous one.

This gives:

  • Server time to recover

  • Network time to stabilize

8. Exponential Backoff (Most Important)

In Exponential Backoff, delay increases exponentially.

Example delays:

1st retry → wait 1 second
2nd retry → wait 2 seconds
3rd retry → wait 4 seconds
4th retry → wait 8 seconds

This is the industry standard.

9. Combining Concurrency Limits + Backoff (Best Practice)

Step-by-step flow:

  1. Send limited number of requests at once

  2. If request fails:

    • Check error type

  3. Retry only if:

    • Error is temporary

  4. Apply exponential backoff

  5. Stop after max retries

10. Simple Conceptual Example (No Heavy Code)

Search Input Case

  • User types quickly

  • Old requests are cancelled

  • Only latest request is allowed

  • If server replies 429:

    • Wait

    • Retry with backoff

This prevents:

  • Server abuse

  • Wrong UI updates

11. HTTP Status Codes You Must Remember (Exams)

Status Code Meaning
429 Too Many Requests
503 Service Unavailable
504 Gateway Timeout

These are retry-eligible errors.

12. When Should You NOT Retry?

❌ 400 Bad Request
❌ 401 Unauthorized
❌ 403 Forbidden
❌ Validation errors

These mean:

  • Client mistake

  • Retrying will not help

13. Exam-Friendly Definitions

  • Concurrency Limit:

    The maximum number of simultaneous AJAX requests allowed by a browser or server.

  • Retry Strategy:

    A controlled mechanism to resend failed requests under specific conditions.

  • Backoff Strategy:

    A technique where retry attempts are delayed progressively to reduce server load.

  • Exponential Backoff:

    A retry method where the waiting time doubles after each failed attempt.

14. Real-World Applications

  • Payment gateways

  • Cloud APIs

  • Search suggestions

  • Social media feeds

  • Mobile apps

Almost every large system uses this.

15. Why This Topic Is Important for Students

  • Frequently asked in AJAX / API theory

  • Shows understanding of scalable systems

  • Important for interviews

  • Prevents common real-world bugs