Work around Azure App Gateway timeout by returning a response as a stream that sends newline character periodically and then the response JSON. The response body to the client is a bunch of newlines followed by the JSON, which is still valid JSON. Only use if you for some reason can't get the gateway timeout limit increased.

workaround-azure-gateway-timeout-for-a-slow-response.ts

import { finalize, firstValueFrom, timeout } from 'rxjs';

const KEEP_ALIVE_INTERVAL = 3 * 1000;
const KEEP_ALIVE_TIMEOUT = 2 * 60 * 1000;

export interface ErrorInHttpResponse {
  errorMsg: string;
  status: number;
  name?: string;
}

// app is the ExpressJS app object. You have to create it elsewhere.
app.post('my-endpoint', (_req, res) => {
  res.writeHead(200);
  // Send something periodically to prevent Azure App Gateway from terminating
  // the request with 504 Gateway Time-out after 20 seconds. Extra whitespace
  // before JSON doesn't change the parsed result, and newline was chosen over
  // space because it is printed immediately by cURL when testing. This uses
  // chunked transfer encoding which is built into NodeJS and browsers.
  // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Transfer-Encoding
  // https://nodejs.org/api/http.html#responsewritechunk-encoding-callback
  const handle = setInterval(() => res.write('\n'), KEEP_ALIVE_INTERVAL);
  let result: unknown[] | ErrorInHttpResponse;
  try {
    result = await firstValueFrom(
      someSlowObservable$.pipe(
        timeout(KEEP_ALIVE_TIMEOUT),
        finalize(() => clearInterval(handle)),
      ),
    );
  } catch (e) {
    // Cannot change the HTTP response code since the stream already started
    // as 200 OK, so put any error inside the response body:
    if (e instanceof Error) {
      result = { errorMsg: e.message, status: 500, name: e.name };
    } else {
      result = { errorMsg: String(e), status: 500 };
    }
  }
  res.write(JSON.stringify(result));
  res.end();
}