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!
