Nginx: 502 Bad Gateway within docker stack - docker

Question

I have docker stack running 2 containers, first is Nginx, second - application.
The problem is that nginx shows Bad Gateway error:
Here is nginx conf:
upstream example {
server mystack_app1;
# Also tried with just 'app1'
# server mystack_app2;
keepalive 32;
}
server {
listen 80;
server_name example;
location / {
proxy_pass http://example;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_connect_timeout 150;
proxy_send_timeout 100;
proxy_read_timeout 100;
proxy_buffers 4 32k;
client_max_body_size 8m;
client_body_buffer_size 128k;
}
}
Here is docker-compose.yml
version: "3"
services:
app1:
image: my-app:latest
ports:
- "9000:9000"
networks:
- webnet
web:
image: my-web:latest
ports:
- "81:80"
networks:
- webnet
deploy:
restart_policy:
condition: on-failure
networks:
webnet:
I use following command to deploy docker stack:
docker stack deploy -c docker-compose.yml mystack
So I can access application from host's browser by localhost:9000 - it works ok.
Also, from the nginx container, I can ping mystack_app1.
But when accessing localhost:81, nginx shows 502 Bad Gateway
Please help.

Answer 1

It looks like your upstream definition is not correct. It's trying to connect to port 80 instead of port 9000.
Try
upstream example {
server mystack_app1:9000;
# Also tried with just 'app1'
# server mystack_app2;
keepalive 32;
}
Btw, I suggest you to use the container_name in your docker-compose file.