Preventing Multiple OTP Requests In Angular With RxJS

Aug 7, 2025 by Omar Yusuf 54 views

Handling Simultaneous Multiple Requests from the Same User in Angular with RxJS

Hey guys! Ever faced a situation where your users are clicking buttons like crazy, and suddenly your backend is getting hammered with duplicate requests? Yeah, it's a common issue, especially when dealing with forms and asynchronous operations in Angular. Today, we're diving deep into how to tackle this problem, focusing on a scenario where a user might be submitting a form multiple times, causing duplicate verifyOtp requests. Let's get started and ensure our applications are robust and user-friendly!

The Problem: Duplicate Requests

Imagine a scenario: a user is filling out a form to verify their PIN. They click the submit button, but sometimes, due to various reasons like slow internet or a momentary lapse in the application's responsiveness, they might click it again… or even multiple times! Each click triggers a new request to your backend, and if these requests happen simultaneously, it can lead to issues. On the backend, this might result in duplicate processing, incorrect data updates, or even security vulnerabilities. For the user, it could mean a frustrating experience with unexpected errors or delays. So, how do we prevent this from happening? Let's explore some strategies using Angular, TypeScript, and RxJS – the powerful trio that can help us build resilient applications.

Why Duplicate Requests Happen

Before we jump into solutions, let's understand why these duplicate requests occur in the first place. In modern web applications, user interactions often trigger asynchronous operations, such as sending data to a server and waiting for a response. JavaScript, being single-threaded, handles these operations using an event loop. This means that while a request is in flight, the user interface remains responsive, allowing users to interact with the application. If the user clicks a button multiple times before the initial request completes, each click can trigger a new request. Network latency, application performance, and even user behavior can contribute to this issue. Therefore, it's crucial to implement measures to prevent duplicate requests and ensure a smooth user experience.

Understanding the Scenario: VerifyPINForm

Let's paint a clearer picture. We have a verifyPINForm, a typical form in Angular used to verify a user's PIN. When the user submits this form, we need to send a verifyOtp request to the backend. Now, the tricky part is ensuring that this request is sent only once, even if the user clicks the submit button multiple times in quick succession. This is where our expertise in Angular, TypeScript, and RxJS comes into play. We need a solution that is both effective and elegant, one that doesn't compromise the user experience while safeguarding our backend from unnecessary load and potential issues. In the following sections, we'll explore different approaches to tackle this challenge, leveraging the power of RxJS operators and Angular's reactive forms.

Solutions Using RxJS

RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using Observables, making it easier to compose asynchronous or callback-based code. It provides a plethora of operators that can help us manage and control the flow of data, making it an ideal tool for handling our duplicate request problem. Let's look at some specific RxJS operators that can come to our rescue.

1. `exhaustMap`

The exhaustMap operator is a gem when it comes to preventing concurrent requests. It works by subscribing to the source Observable (in our case, the form submission event) and mapping each value to an inner Observable (the verifyOtp request). However, here's the magic: exhaustMap ignores any new source values while the inner Observable is still active. This means that if a verifyOtp request is in flight, any subsequent form submissions are effectively ignored until the first request completes. This is perfect for our scenario, as it ensures that only one verifyOtp request is processed at a time, no matter how many times the user clicks the submit button.

Implementation with `exhaustMap`

To implement this, we'll first need to create an Observable from our form submission event. Then, we'll pipe this Observable through exhaustMap, mapping each submission to our verifyOtp service call. Here’s a snippet of how it might look:

import { fromEvent } from 'rxjs';
import { exhaustMap } from 'rxjs/operators';

// Assuming you have a button element with id 'submitButton'
const submitButton = document.getElementById('submitButton');

// Create an Observable from the button click event
const submitClick$ = fromEvent(submitButton, 'click');

// Use exhaustMap to handle the submit events
submitClick$.pipe(
  exhaustMap(() => this.yourService.verifyOtp(this.verifyPINForm.value))
).subscribe(
  response => {
    // Handle successful response
    console.log('OTP verification successful', response);
  },
  error => {
    // Handle error
    console.error('OTP verification failed', error);
  }
);

In this example, fromEvent creates an Observable that emits a value each time the submit button is clicked. exhaustMap then takes over, ensuring that this.yourService.verifyOtp is called only once at a time. Any subsequent clicks while the request is in progress are ignored. This significantly reduces the risk of duplicate requests hitting your backend.

2. `takeUntil`

Another powerful operator is takeUntil. This operator allows us to continue emitting values from an Observable until another Observable emits a value. This can be incredibly useful for scenarios where we want to cancel ongoing requests when a certain condition is met, such as when a component is destroyed or when the user navigates away from the page. In the context of preventing duplicate requests, we can use takeUntil to unsubscribe from our form submission Observable when a request is successfully completed or if an error occurs.

Implementation with `takeUntil`

To use takeUntil, we'll need a notifier Observable that emits a value when we want to stop processing requests. This could be a Subject that we manually trigger, or it could be an Observable that emits when the component is destroyed. Here's an example of how you might use takeUntil in conjunction with a Subject:

import { fromEvent, Subject } from 'rxjs';
import { takeUntil } from 'rxjs/operators';
import { Component, OnDestroy, OnInit } from '@angular/core';

@Component({
  selector: 'app-verify-pin',
  templateUrl: './verify-pin.component.html',
  styleUrls: ['./verify-pin.component.css']
})
export class VerifyPinComponent implements OnInit, OnDestroy {
  private destroy$ = new Subject<void>();

  ngOnInit(): void {
    const submitButton = document.getElementById('submitButton');
    const submitClick$ = fromEvent(submitButton, 'click');

    submitClick$.pipe(
      takeUntil(this.destroy$)
    ).subscribe(() => {
      this.yourService.verifyOtp(this.verifyPINForm.value).subscribe(
        response => {
          // Handle successful response
          console.log('OTP verification successful', response);
          this.destroy$.next(); // Stop further requests after success
          this.destroy$.complete();
        },
        error => {
          // Handle error
          console.error('OTP verification failed', error);
          this.destroy$.next(); // Stop further requests after error
          this.destroy$.complete();
        }
      );
    });
  }

  ngOnDestroy(): void {
    this.destroy$.next();
    this.destroy$.complete();
  }
}

In this example, we create a destroy$ Subject that emits when the component is destroyed or when a request completes (either successfully or with an error). We then use takeUntil(this.destroy$) to ensure that our form submission subscription is automatically unsubscribed when destroy$ emits a value. This prevents memory leaks and ensures that we don't continue processing requests after they are no longer needed. The key here is that we manually trigger this.destroy$.next() after a successful response or an error, effectively stopping any further requests from being processed.

3. `switchMap`

The switchMap operator is another excellent choice for handling scenarios where you want to ensure that only the most recent request is processed. Unlike exhaustMap, which ignores new requests while one is in flight, switchMap cancels the previous inner Observable (the verifyOtp request) when a new source value arrives. This means that if the user clicks the submit button multiple times, only the last click will result in a request being sent to the backend. This is particularly useful when you want to avoid processing outdated information or when the user's intention is best represented by their most recent action.

Implementation with `switchMap`

To implement switchMap, we simply replace exhaustMap with switchMap in our earlier example. Here’s how it looks:

import { fromEvent } from 'rxjs';
import { switchMap } from 'rxjs/operators';

// Assuming you have a button element with id 'submitButton'
const submitButton = document.getElementById('submitButton');

// Create an Observable from the button click event
const submitClick$ = fromEvent(submitButton, 'click');

// Use switchMap to handle the submit events
submitClick$.pipe(
  switchMap(() => this.yourService.verifyOtp(this.verifyPINForm.value))
).subscribe(
  response => {
    // Handle successful response
    console.log('OTP verification successful', response);
  },
  error => {
    // Handle error
    console.error('OTP verification failed', error);
  }
);

In this example, each time the submit button is clicked, switchMap cancels any previous verifyOtp requests and initiates a new one. This ensures that only the most recent request is processed, which can be beneficial in scenarios where the user might be rapidly changing their input or clicking the submit button repeatedly. However, it's important to consider the implications of canceling in-flight requests, as it might not be suitable for all situations. For instance, if the verifyOtp request has side effects that should always be executed, switchMap might not be the best choice.

Choosing the Right Operator

So, which operator should you choose? It depends on your specific requirements:

Use exhaustMap if you want to ignore subsequent requests while one is in progress.
Use takeUntil if you want to cancel requests when a certain condition is met.
Use switchMap if you want to cancel the previous request and process only the latest one.

Additional Strategies

While RxJS operators provide powerful tools for handling duplicate requests, there are other strategies we can employ to further enhance our application's robustness.

1. Disabling the Submit Button

A simple yet effective technique is to disable the submit button after the first click and re-enable it once the request completes (or fails). This visual cue prevents the user from clicking the button multiple times and provides immediate feedback that their action is being processed. This approach is particularly user-friendly, as it directly addresses the issue at the UI level, making it clear to the user that their input is being handled.

Implementation

In your component's template, you can bind the disabled property of the button to a boolean variable in your component class. This variable can be toggled based on the state of the request. Here's a basic example:

<button type="submit" [disabled]="isSubmitting">Verify OTP</button>

And in your component class:

import { Component } from '@angular/core';

@Component({
  selector: 'app-verify-pin',
  templateUrl: './verify-pin.component.html',
  styleUrls: ['./verify-pin.component.css']
})
export class VerifyPinComponent {
  isSubmitting = false;

  onSubmit() {
    this.isSubmitting = true;
    this.yourService.verifyOtp(this.verifyPINForm.value).subscribe(
      response => {
        // Handle successful response
        console.log('OTP verification successful', response);
        this.isSubmitting = false; // Re-enable the button
      },
      error => {
        // Handle error
        console.error('OTP verification failed', error);
        this.isSubmitting = false; // Re-enable the button
      }
    );
  }
}

In this example, the isSubmitting flag is initially set to false, enabling the button. When the form is submitted, isSubmitting is set to true, disabling the button. Once the verifyOtp request completes (either successfully or with an error), isSubmitting is set back to false, re-enabling the button. This simple mechanism effectively prevents the user from submitting the form multiple times while a request is in progress.

2. Debouncing User Input

Debouncing is a technique used to limit the rate at which a function is executed. In the context of preventing duplicate requests, we can use debouncing to ensure that a request is sent only after a certain period of inactivity. This is particularly useful for scenarios where the user might be rapidly changing their input, such as typing in a search box or filling out a form with multiple fields. By debouncing the input, we can avoid sending unnecessary requests and reduce the load on our backend.

Implementation

RxJS provides the debounceTime operator, which makes debouncing a breeze. This operator delays the emission of values from an Observable until a specified timespan has elapsed without the emission of another value. Here's how you can use debounceTime to debounce form submissions:

import { fromEvent } from 'rxjs';
import { debounceTime, map } from 'rxjs/operators';

// Assuming you have a form input element with id 'otpInput'
const otpInput = document.getElementById('otpInput');

// Create an Observable from the input event
const otpInput$ = fromEvent(otpInput, 'input').pipe(
  map((event: any) => event.target.value) // Extract the input value
);

// Debounce the input for 300 milliseconds
otpInput$.pipe(
  debounceTime(300)
).subscribe(
  value => {
    // Process the debounced input value
    console.log('Debounced OTP input:', value);
    // You can trigger your verifyOtp request here
  }
);

In this example, we create an Observable from the input event of an OTP input field. We then use debounceTime(300) to delay the emission of input values for 300 milliseconds. This means that if the user types rapidly, the subscribe callback will be invoked only after they stop typing for at least 300 milliseconds. This effectively reduces the number of requests sent to the backend, as only the final input value after the debounce period is processed. You can adjust the debounce time to suit your specific requirements, balancing responsiveness with the need to prevent duplicate requests.

3. Using a Token

Another robust approach is to use a token-based system. Before submitting the form, the client requests a unique token from the backend. This token is then included in the verifyOtp request. The backend can then validate this token, ensuring that the request is legitimate and hasn't been duplicated. This method adds an extra layer of security and ensures that each request is processed only once, even if multiple requests are received.

Implementation

Here’s a high-level overview of how you might implement a token-based system:

Client-Side (Angular):
- Before submitting the form, make a request to a backend endpoint to obtain a unique token.
- Include this token in the verifyOtp request.
Backend-Side:
- Generate a unique token for each request.
- Store the token (e.g., in a database or cache) along with a timestamp.
- When a verifyOtp request is received, validate the token:
  - Check if the token exists and hasn't been used before.
  - If the token is valid, process the request and mark the token as used (or delete it).
  - If the token is invalid or has expired, reject the request.

This approach ensures that each request is uniquely identified and processed only once. The token acts as a one-time password, preventing duplicate requests from being processed. The backend can also implement a timeout mechanism, where tokens expire after a certain period, further enhancing security and preventing replay attacks.

Conclusion

Handling multiple requests from the same user is a common challenge in web development, but with the right tools and techniques, it's a problem we can effectively solve. By leveraging RxJS operators like exhaustMap, takeUntil, and switchMap, along with strategies like disabling the submit button, debouncing user input, and using token-based systems, we can build robust and user-friendly applications. Remember, the key is to choose the approach that best fits your specific needs and to always prioritize the user experience. Happy coding, guys!

The Problem: Duplicate Requests

Why Duplicate Requests Happen

Understanding the Scenario: VerifyPINForm

Solutions Using RxJS

1. exhaustMap

Implementation with exhaustMap

2. takeUntil

Implementation with takeUntil

3. switchMap

Implementation with switchMap

Choosing the Right Operator

Additional Strategies

1. Disabling the Submit Button

Implementation

2. Debouncing User Input

Implementation

3. Using a Token

Implementation

Conclusion

1. `exhaustMap`

Implementation with `exhaustMap`

2. `takeUntil`

Implementation with `takeUntil`

3. `switchMap`

Implementation with `switchMap`