Angular/RxJS: Limiting Concurrent HTTP Requests

Neil Menon · July 2, 2022

In a recent project, I was working with handling parallel file uploads from the UI. In some cases, there would be 50+ upload HTTP requests to process. Initially, I implemented the parallel upload handler using Promise.all(). However, this method fires all the requests at once which would crash the browser. I wanted a way to control how many uploads could be going at once.

The Solution

The solution required putting the list of requests through a pipe with mergeMap, which has a concurrency parameter. Then, I could extract the results of each request using tap and catch any individual errors with catchError. Once one request finishes, the next request starts immediately. Here is a sample implementation:

import { HttpClient, HttpErrorResponse, HttpRequest } from "@angular/common/http";
import { from, of } from "rxjs";
import { catchError, mergeMap, tap } from "rxjs/operators";

// constructor, other things, etc...

ngOnInit(): void {
    this.uploadLimitConcurrency().then(res => {
        if (res instanceof Error) {
        // throw error
        } else {
        // your results

async uploadLimitConcurrency(): Promise<any> {
    const uploadRequests: Array<HttpRequest> = [...] // your many HTTP requests
    const CONCURRENT_UPLOADS = 3 // # of concurrent uploads to go at once
    let results: Array<any> = []
    let requestsCompleted: number = 0
    const concurrentPipe = await from(uploadRequests).pipe(
        mergeMap(obj => this.httpClient.request(obj), CONCURRENT_UPLOADS), // this allows us to limit concurrent uploads
        tap((res: any) => { 
        if(res?.status == 200) { // do something with the results
            // push the headers of the response to results

            // you can keep track of progress of overall upload
        catchError((err: HttpErrorResponse) => {
        if (err.status == 403) { 
            return of(new Error(`A 403 error ocurred while trying to upload the file: ${err.error}`))
        return of(new Error(`Some other error occurred: ${err.error}`))

    // handle error outside the pipe
    if (concurrentPipe instanceof Error) { 
        console.log(concurrentPipe) // contains error
        return concurrentPipe

    return results

Demo / Conclusion

I’ve created a simple demo of this implementation using a random delay to mock real HTTP requests. View the code and demo below, or here.

That’s all! Any issues or comments, drop me a line in the comments below.

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