How to connect to services running in different docker stack? - php

Question

Can a services running in two different docker stack communicate ?
This is what I have tried and not able to achieve it.
Created a stack (stack1) running nginx and php-fpm its running great.
Created another stack (stack2) running mysql database.
Now I want to make the stack1 service able to communicate with stack2 such that it can access the database service.
I though this might help and created a external network and trying to
add the service in stack1 and stack2 to it such that they can
communicate with each others too
Mystack1 docker-compose file
version: "3.4"
networks:
apps-net:
db-net:
external:
name: db-net
services:
web:
image: nginx:latest
ports:
- "9080:80"
volumes:
- ./code:/code
- ./site.conf:/etc/nginx/conf.d/default.conf
depends_on:
- php
networks:
- apps-net
- db-net
deploy:
mode: replicated
replicas: 1
php:
image: php:7-fpm
volumes:
- ./code:/code
networks:
- apps-net
- db-net
deploy:
mode: replicated
replicas: 1
MY stack2 docker-compose file
version: '3.3'
networks:
db-net:
external:
name: db-net
services:
db:
image: mysql:5.7
volumes:
- db_data:/var/lib/mysql
restart: always
environment:
MYSQL_ROOT_PASSWORD: password
MYSQL_DATABASE: wordpress
MYSQL_USER: root
MYSQL_PASSWORD: password
networks:
- db-net
volumes:
db_data:
I created a swarm scope network with docker network create db-net command
OUTPUT:
The nginx and php is working fine but I added the database connection
codes in the index.php which resulted in error message.Is the error
because they are not connected? I have installed php-mysql extensions
too but it has the error. How can I make sure the services are
communicating successfully.
nginx and php working
Fatal error: Uncaught Error: Call to undefined function mysqli_connect() in /code/index.php:11 Stack trace: #0 {main} thrown in /code/index.php on line 11

Answer 1

Instead of networks (which I often see asked here on stack as not working like expected) try using external_links which is well explained here.
Try removing all custom network configurations and simply modify your application compose file like this:
version: "3.4"
# removed all custom networks configuration
services:
web:
image: nginx:latest
external_links:
- mysql_1:mysql
[..]
php:
image: php:7-fpm
external_links:
- mysql_1:mysql
[..]
where mysql_1 is a actual container name create by your latter compose file, and mysql is a alias by which your service will be available inside php and web containers
Links are a legacy option in v3 and Docker suggests using networks instead.
I'll post a edit about swarm deployment as approach would be totally different because Docker ignores links when deploying swarm.