Getting "502 Bad Gateway" with nginx, uwsgi python-flask on ubuntu 16.04

Question

I am following the this to deploy a flask app(simple hello world) on the Ubuntu 16-04. digital Ocean tutorial

Everything works fine till Testing uWSGI Serving. After that I followed the step as described and when I finally reach the bottom and check server IP address then I got:

502 Bad Gateway

Ok fine. I searched and checked my error log, I got this :-

2017/01/16 05:29:27 [crit] 20714#20714: *2 connect() to unix:/home/sajjan/project/project.sock failed (2: No such file or directory) while connecting to upstream, client: xx.9.xxx.xxx, server: 138.xxx.xx.xxx, request: "GET / HTTP/1.1", upstream: "uwsgi://unix:/home/sajjan/project/project.sock:", host: "xx.xx.xx.xx"

So After taking a error log , I created the file project.sock manually. again Go to server ip address and then same error "502 Bad Gateway"

Again checked the error log and found this

2017/01/16 06:07:11 [crit] 20874#20874: *1 connect() to unix:/home/sajjan/project/project.sock failed (13: Permission denied) while connecting to upstream, client: 47.9.237.113, server: XX.XX.XX.XX, request: "GET / HTTP/1.1", upstream: "uwsgi://unix:/home/sajjan/project/project.sock:", host: " XX.XX.XX.XX "

I figured out about the permission issue and change the permission using below command

sudo chmod 666 project.sock

Now I checked the permision( using ls -l filename)

-rw-rw-rw- 1 root root 0 Jan 16 05:31 project.sock

Now I go back to check the server's IP but found the same "502 Bad Gateway". Again I checked the error log and found this :

017/01/16 06:13:31 [error] 20897#20897: *6 connect() to unix:/home/sajjan/project/project.sock failed (111: Connection refused) while connecting to upstream, client: 47.9.237.113, server: XX.XX.XX.XX, request: "GET /favicon.ico HTTP/1.1", upstream: "uwsgi://unix:/home/sajjan/project/project.sock:", host: " XX.XX.XX.XX ", referrer: "http:// XX.XX.XX.XX /"

I googled for above error read a lot in last two days but nothing to seem working for me . I have check these answers but no help stackanswer-1 stackanswer-2 and along with these I checked all the digital-ocean community thread but nothing seems to work.

I am total begineer to servers and don't know much about ubuntu. If you can help me to find out what wrong am I doing or suggest some better tutorial/ways to deploy my flask application, then I would be greatful.

These are my files

1. hello.py

   from flask import Flask
   app = Flask(__name__)
   
   @app.route("/")
   def hello():
       return "<h1 style='color:blue'>Hello There!</h1>"
   
   if __name__ == "__main__":
       app.run(host='0.0.0.0')
   

2. project.ini

   [uwsgi]
   
   module = wsgi:app
   
   master = true
   
   processes = 5
   
   socket = /home/sajjan/project/project.sock
   
   chmod-socket = 660
   
   vacuum = true
   
   die-on-term = true
   

3. wsgi.py

   from hello import app
   
   if __name__ == "__main__":
         app.run()
   

4. Below is file : /etc/nginx/sites-available/project

   server {
       listen 80;
       server_name 138.197.28.107;
   
       location / {
           include uwsgi_params;
           uwsgi_pass unix:/home/sajjan/project/project.sock;
       }
   }
   

When I run the command :

    sudo service uwsgi restart

output:

   Failed to restart wsgi.service: Unit wsgi.service not found. 

while output of

   sudo service nginx status/restart

then this show that nginx is running .

Help me, If anything else that you want to know then let me know. Thanks

EDIT :

I have created a project.service file and its conetent is :

    [Unit]
    Description=uWSGI instance to serve project
    After=network.target

    [Service]
    User=sajjan
    Group=www-data
    WorkingDirectory=/home/sajjan/project
    Environment="PATH=/home/sajjan/project/venv/bin"
    ExecStart=/home/sajjan/project/venv/bin/uwsgi --ini project.ini

    [Install]
    WantedBy=multi-user.target

I figured out I have to run below command :

    sudo systemctl start project

Output is :

    Warning: project.service changed on disk. Run 'systemctl daemon-reload' to reload units.

and when I run

     sudo systemcl reload project 

then output :

    Failed to reload project.service: Job type reload is not applicable for unit project.service.
    See system logs and 'systemctl status project.service' for details.

and when I check the "systemctl status project.service"

     ● project.service - uWSGI instance to serve project
      Loaded: loaded (/etc/systemd/system/project.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Mon 2017-01-16 17:49:29 UTC; 6min ago
      Main PID: 27157 (code=exited, status=203/EXEC)

     Jan 16 17:49:29 learningwithpython systemd[1]: Started uWSGI instance to serve project.
     Jan 16 17:49:29 learningwithpython systemd[1]: project.service: Main process exited, code=exited, status=203/EXEC
     Jan 16 17:49:29 learningwithpython systemd[1]: project.service: Unit entered failed state.
     Jan 16 17:49:29 learningwithpython systemd[1]: project.service: Failed with result 'exit-code'.

Answer 1

Try running myapp/bin/uwsgi --ini myapp.ini to see the actual error thats preventing uwsgi to run.

In my case 5 processes in the .ini configuration file were to much to handle for my cpu this was my error output.

your processes number limit is 3900 your memory page size is 4096 bytes detected max file descriptor number: 1024

If this is your case lowering the number of processes to 2 in your .ini file might work.

Answer 2

I had the same problem using the guide. As far as I've read; 502 bad gateway is a symptom of Nginx not being able to properly connect to the uwsgi. Changing the permissions on the socket solved the issue for me.

sudo chmod 777 /home/sajjan/project/project.sock
sudo systemctl restart nginx

777 is offcourse a bit excessive, but its a quick and dirty way to verify if it is in fact a problem with permissions