CORS issue on Google Cloud Platform using Python, Flask & Angular

Question

I am developing a single page web application using Python, Flask & Angular on GCP (google cloud platform) and have run into a stubborn issue related to CORS. When I build and serve the frontend I get the following error:

Access to XMLHttpRequest at 'https://jsonData-devshell.appspot.com/org' from origin 'https://mySPA-devshell.appspot.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

BACKEND

To provision CORS I have imported the flask_cors module and added it to the server file by adding CORS(app) in my server file which is main.py.

main.py

from .entities.entity import Session, eco_env_engine, Base
from .entities.org import Org, OrgSchema
from flask import Flask, jsonify
from flask_cors import CORS


app = Flask(__name__)
# enable cross origin resource sharing
CORS(app)

Base.metadata.create_all(eco_env_engine)

@app.route('/org', methods=['GET'])
def get_org():
    session = Session()
    org_objects = session.query(Org).all()
    # transform into JSON-serializable objects
    schema = OrgSchema(many=True)
    org = schema.dump(org_objects)
    # serialize as json
    session.close()
    response = jsonify(org)
    return response

I have a shell script server.sh that creates an interface that I can access at http://0.0.0.0:5000.

server.sh

# set ../src/main.py as the value of the FLASK_APP environment variable
export FLASK_APP=../src/main.py
# FLASK_ENV=development
# FLASK_DEBUG=1

#activate the virtual environment
source $(pipenv --venv)/bin/activate

#run flask listening on all interfaces
#-h binds to interface
flask run -h 0.0.0.0

When I use curl -I http://0.0.0.0:5000/org I get back

HTTP/1.0 200 OK
Content-Type: application/json
Content-Length: 4766
Access-Control-Allow-Origin: *
Server: Werkzeug/1.0.0 Python/3.7.3
Date: Wed, 01 Apr 2020 01:27:11 GMT

notice the Access-Control-Allow-Origin:* header which in my understanding should address the issue but it does not.

I am writing my code within the GCP Cloud Shell editor and deploying from within the GCP Cloud Shell Terminal. To get things running I deploy my backend with ./server.sh & and frontend with ng serve

CORS variations I have tried:

CORS(app, resource={r'*/*': {'origins': '*/*'}})
CORS(app, resources={r"*": {"origins": ['https://mySPA-devshell.appspot.com', 'https://jsonData-devshell.appspot.com']}})
CORS(app, resources={r'*': {"origins": "*"}})
CORS(app, resources={r"*": {"origins": ['http://localhost:4200', 'http://localhost:5000']}})

FRONTEND

org-api.service.ts

import {Injectable} from '@angular/core';
import {HttpClient, HttpErrorResponse} from '@angular/common/http';
import {HttpHeaders, HttpClientModule} from '@angular/common/http';
import {Observable} from 'rxjs/Observable';
import {API_URL} from '../env';
import {Org} from './org.model';
import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';

@Injectable()
export class OrgApiService {

  //create private variable 'http' of type HttpClient
  constructor(private http: HttpClient) {
  }

  private static _handleError(err: HttpErrorResponse | any) {
    // return Observable.throw(err.message || 'Error: Unable to complete request.');
    return throwError(err.message || 'Error: Unable to complete request.');
  }

  getOrg(): Observable<Org[]> {
    return this.http    
      .get<Org[]>(`${API_URL}/org`)
      .pipe(catchError(OrgApiService._handleError));
  }

}

org.component.ts

import {Component, OnDestroy, OnInit} from '@angular/core';
import {Subscription} from 'rxjs/Subscription';
import {Org} from './org.model';
import {OrgApiService} from './org-api.service';

@Component({
  selector: 'org',
   template: `
     <div>
       <ul>
         <li *ngFor="let org of orgList">
           {{org.name}}
         </li>
       </ul>
     </div>
   `,
  styleUrls: ['org.component.css'],
})

export class OrgComponent implements OnInit, OnDestroy {
  orgListSubs: Subscription;
  orgList: Org[];
  authenticated = false;

  constructor(private orgApi: OrgApiService) { }

  ngOnInit() {
    this.orgListSubs = this.orgApi
      .getOrg()
      .subscribe(res => {
          this.orgList = res;
        },
        console.error
      );
    const self = this;
  }

  ngOnDestroy() {
    this.orgListSubs.unsubscribe();
  }
}

Additionally, I came across the following solution and decided to employ it because of why not...to no avail: Create a proxy.conf.json in my frontend directory with the following contents

{
    "/": {
        "target": "http://0.0.0.0:5000",
        "secure": false,
        "logLevel": "debug"
    }
}

then added a proxyConfig option to the serve target.

UPDATE

@sideshowbarker pointed out the 302 response in some cases does not have a 'Access-Control-Allow-Origin' header which in my case is true. Upon further digging I came across this post which mentions chrome cancelling a request upon receiving status code 302. It also put me on to chrome://net-export/ where I discovered my request is being cancelled as well.

There is no apparent/accepted solution for this that I have across yet. If you are reading this and have a suggestion.. please share.

t=131761 [st=  4]      QUIC_CONNECTION_MIGRATION_MODE
                       --> connection_migration_mode = 0
t=131761 [st=  4]     +HTTP_TRANSACTION_SEND_REQUEST  [dt=0]
t=131761 [st=  4]        HTTP_TRANSACTION_QUIC_SEND_REQUEST_HEADERS
                         --> :method: GET
                             :authority: 5000-dot-XXXXXXX-dot-devshell.appspot.com
                             :scheme: https
                             :path: /
                             accept: application/json, text/plain, */*
                             sec-fetch-dest: empty
                             user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.149 Safari/537.36
                             origin: https://4200-dot-XXXXXXX-dot-devshell.appspot.com
                             sec-fetch-site: cross-site
                             sec-fetch-mode: cors
                             referer: https://4200-dot-XXXXXXX-dot-devshell.appspot.com/?authuser=0&environment_name=default
                             accept-encoding: gzip, deflate, br
                             accept-language: en-US,en;q=0.9
                         --> quic_priority = 4
                         --> quic_stream_id = 5
t=131761 [st=  4]     -HTTP_TRANSACTION_SEND_REQUEST
t=131761 [st=  4]     +HTTP_TRANSACTION_READ_HEADERS  [dt=168]
t=131928 [st=171]        HTTP_TRANSACTION_READ_RESPONSE_HEADERS
                         --> HTTP/1.1 302
                             status: 302
                             date: Wed, 01 Apr 2020 06:33:46 GMT
                             content-type: text/html; charset=utf-8
                             content-length: 498
                             location: https://some.url.com
                             via: 1.1 google
                             alt-svc: quic=":443"; ma=2592000; v="46,43",h3-Q050=":443"; ma=2592000,h3-Q049=":443"; ma=2592000,h3-Q048=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,h3-T050=":443"; ma=2592000
t=131929 [st=172]     -HTTP_TRANSACTION_READ_HEADERS
t=131929 [st=172]      HTTP_CACHE_WRITE_INFO  [dt=0]
t=131929 [st=172]      HTTP_CACHE_WRITE_DATA  [dt=0]
t=131929 [st=172]      HTTP_CACHE_WRITE_INFO  [dt=0]
t=131929 [st=172]      NETWORK_DELEGATE_HEADERS_RECEIVED  [dt=1]
t=131930 [st=173]      URL_REQUEST_DELEGATE_RECEIVED_REDIRECT  [dt=0]
t=131930 [st=173]      CANCELLED
t=131930 [st=173] -REQUEST_ALIVE

I have spent days trying to solve this however I have been largely unsuccessful. I feel as though there is a simpler solution to this than everything I have tried. Thank you in advance for your help!

Answer 1

SOLVED!

edited env.ts in frontend/app to

export const API_URL = 'http://localhost:4200'

instead of localhost:5000

API_URL corresponds to the ng serve development deployment