Retrying Failed Requests

Retrying failed HTTP requests can improve the reliability of your Angular application. This guide covers the basics of implementing retry logic using HTTP interceptors and RxJS operators.

Setting Up HTTPClientModule

First, import the HttpClientModule into your app module:

// app.module.ts
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';
import { AppComponent } from './app.component';
import { DataService } from './data.service';
import { HomeComponent } from './home/home.component';
import { RetryInterceptor } from './retry.interceptor';

@NgModule({
  declarations: [AppComponent, HomeComponent],
  imports: [BrowserModule, HttpClientModule],
  providers: [
    DataService,
    { provide: HTTP_INTERCEPTORS, useClass: RetryInterceptor, multi: true }
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

Creating an Interceptor

Create an interceptor to manage retry logic using the Angular CLI command:

$ ng generate interceptor retry

This command generates a new interceptor file named retry.interceptor.ts.

Implementing the Interceptor

In the interceptor file, implement the HttpInterceptor interface and define the intercept method to retry failed requests using the retry operator:

// retry.interceptor.ts
import { Injectable } from '@angular/core';
import { HttpEvent, HttpInterceptor, HttpHandler, HttpRequest } from '@angular/common/http';
import { Observable } from 'rxjs';
import { retry, catchError } from 'rxjs/operators';

@Injectable()
export class RetryInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest, next: HttpHandler): Observable> {
    return next.handle(req).pipe(
      retry(3), // Retry failed requests up to 3 times
      catchError((error) => {
        console.error('Request failed', error);
        throw error;
      })
    );
  }
}

Using the Interceptor

HTTP interceptors are automatically applied to all HTTP requests made using the HTTPClient. You don't need to do anything special in your components to use them. Here's an example component that makes HTTP requests:

// home.component.ts
import { Component, OnInit } from '@angular/core';
import { DataService } from '../data.service';

@Component({
  selector: 'app-home',
  template: `
    
Posts
    

      
{{ post.title }}
    
  `
})
export class HomeComponent implements OnInit {
  posts: any[] = [];

  constructor(private dataService: DataService) {}

  ngOnInit() {
    this.dataService.getPosts().subscribe(data => {
      this.posts = data;
    });
  }
}

Handling Retry Limit

You can customize the retry logic to handle specific error conditions or retry limits:

// retry.interceptor.ts
import { Injectable } from '@angular/core';
import { HttpEvent, HttpInterceptor, HttpHandler, HttpRequest, HttpErrorResponse } from '@angular/common/http';
import { Observable, throwError } from 'rxjs';
import { retryWhen, scan, delay, catchError } from 'rxjs/operators';

@Injectable()
export class RetryInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest, next: HttpHandler): Observable> {
    return next.handle(req).pipe(
      retryWhen(errors =>
        errors.pipe(
          scan((retryCount, error) => {
            if (retryCount >= 3 || error.status === 404) {
              throw error;
            }
            return retryCount + 1;
          }, 0),
          delay(1000)
        )
      ),
      catchError((error: HttpErrorResponse) => {
        console.error('Request failed', error);
        return throwError('Something went wrong');
      })
    );
  }
}

Key Points

Conclusion

Retrying failed HTTP requests is a powerful technique for enhancing the reliability of your Angular applications. By setting up interceptors, you can efficiently manage retry logic and provide a better user experience. Happy coding!