no app loaded when starting Flask via uwsgi

Question

I know this is a common issue, but due to the fact that I've been searching for 3 hours now, and given that there must be a quantum singularity involved, I had to post this.

I keep getting this familiar error:

[uWSGI] getting INI configuration from uwsgi.ini,
*** Starting uWSGI 2.0.18-debian (64bit) on [Mon May 25 15:56:04 2020] ***,
compiled with version: 8.2.0 on 10 February 2019 02:42:46,
os: Linux-5.6.12-1-MANJARO #1 SMP PREEMPT Sun May 10 14:36:43 UTC 2020,
nodename: a43c6a7bbb2c,
machine: x86_64,
clock source: unix,
pcre jit disabled,
detected number of CPU cores: 8,
current working directory: /srv/myapp,
detected binary path: /usr/bin/uwsgi-core,
setgid() to 33,
setuid() to 33,
your memory page size is 4096 bytes,
detected max file descriptor number: 1048576,
lock engine: pthread robust mutexes,
thunder lock: disabled (you can enable it with --thunder-lock),
uwsgi socket 0 bound to UNIX address /tmp/uwsgi.socket fd 3,
your server socket listen backlog is limited to 100 connections,
your mercy for graceful operations on workers is 60 seconds,
mapped 437520 bytes (427 KB) for 5 cores,
*** Operational MODE: preforking ***,
*** no app loaded. going in full dynamic mode ***,
*** uWSGI is running in multiple interpreter mode ***,
!!!!!!!!!!!!!! WARNING !!!!!!!!!!!!!!,
no request plugin is loaded, you will not be able to manage requests.,
you may need to install the package for your language of choice, or simply load it with --plugin.,
!!!!!!!!!!! END OF WARNING !!!!!!!!!!,
spawned uWSGI master process (pid: 25),
spawned uWSGI worker 1 (pid: 28, cores: 1),
spawned uWSGI worker 2 (pid: 29, cores: 1),
spawned uWSGI worker 3 (pid: 30, cores: 1),
spawned uWSGI worker 4 (pid: 31, cores: 1),
spawned uWSGI worker 5 (pid: 32, cores: 1),

However, I don't see anything wrong with this very simple setup. My setup:

core/__init__.py

from flask import Flask

def create_app():

    app = Flask(__name__)

    @app.route('/')
    def hello_world():
        return 'Hello, World!'

    return app

app.py

from core import create_app

app = create_app()

if __name__ == '__main__':
    app.run()

Dockerfile

FROM python:3.6-slim

RUN apt-get clean \
    && apt-get -y update
RUN apt-get -y install uwsgi nginx

COPY . /srv/myapp
COPY nginx.conf /etc/nginx
WORKDIR /srv/myapp

RUN pip install -r requirements.txt

RUN chmod +r uwsgi.ini
RUN chmod +x ./start.sh
CMD ["./start.sh"]

start.sh

#!/usr/bin/env bash
service nginx start
uwsgi --ini uwsgi.ini

uwsgi.ini

[uwsgi]
module = app:app
uid = www-data
gid = www-data
master = true
processes = 5
socket = /tmp/uwsgi.socket
chmod-sock = 664
vacuum = true
die-on-term = true

nginx.conf

user www-data;
worker_processes auto;
pid /run/nginx.pid;events 
{
    worker_connections 1024;
    use epoll;
    multi_accept on;
}
http {
    access_log /dev/stdout;
    error_log /dev/stdout;    
    sendfile            on;
    tcp_nopush          on;
    tcp_nodelay         on;
    keepalive_timeout   65;
    types_hash_max_size 2048;    
    include             /etc/nginx/mime.types;
    default_type        application/octet-stream;    
    index   index.html index.htm;    
    server {
        listen       80 default_server;
        listen       [::]:80 default_server;
        server_name  localhost;
        root         /var/www/html;        
        location / 
        {
            include uwsgi_params;
            uwsgi_pass unix:/tmp/uwsgi.socket;
        }
    }
}

If I add the python3 plugin RUN apt-get -y install uwsgi nginx uwsgi-plugin-python3 in DOCKERFILE and run uwsgi via uwsgi --ini uwsgi.ini --plugin python3 I get:

Traceback (most recent call last):
  File "./app.py", line 1, in <module>
    from core import create_app
  File "./core/__init__.py", line 1, in <module>
    from flask import Flask
ModuleNotFoundError: No module named 'flask'

I don't know if this may be a hint. Any help is appreciated! Thanx!

Answer 1

After 8 hours of sleep I found my mistake:

Installing uwsgi via apt-get will not work. It is required to install uwsgi via pip install . (My guess is, that these are two different uwsgi instances / services)