Could not connect to Postgres from Symfony 4 + Docker - postgresql

Question

I run into the strange problem. I've created docker-compose file to build php + nginx + postgres services:
version: '2'
services:
db:
image: orchardup/postgresql
ports:
- "5433:5432"
environment:
LC_ALL: C.UTF-8
POSTGRESQL_USER: postgres
POSTGRESQL_DB: db
POSTGRESQL_PASS: postgres
php:
build: .docker/php-fpm
ports:
- "9002:9002"
volumes:
- .:/var/www/symfony:cached
- ./var/log/symfony:/var/www/symfony/var/log:cached
links:
- db
nginx:
build: .docker/nginx
ports:
- "8001:80"
links:
- php
volumes_from:
- php
volumes:
- ./var/log/nginx/:/var/log/nginx:cached
After that I created DB schema by running bin/console doctrine:schema:update --force . The tables and migrations created just fine. Seems like DB connection is ok. I checked this by connecting to db from my machine through psql with credentials from .env, the tables are there.
But when I go to the web page and trying to authorize, I get an error told me the connection is not ok:
Connection refused Is the server running on host "127.0.0.1" and accepting TCP/IP connections on port 5433?"
I checked in both case I have dev environment - from the web page and from the console. I tried 5433 and 5432 ports with no success. I tried everything I could find for 3 hours.
This is the output from the postgres container:
# netstat -tlpn | grep 5432
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
tcp 0 0 0.0.0.0:5432 0.0.0.0:* LISTEN 12/postgres
tcp6 0 0 :::5432 :::* LISTEN 12/postgres
# grep listen /etc/postgresql/9.3/main/postgresql.conf
listen_addresses = '*' # what IP address(es) to listen on;

Answer 1

The only way for containers to talk to each other is through IPs. By linking multiple containers together through --link (or links in docker-compose), docker creates a secure tunnel between those two containers so that we don't need to expose any ports externally.
If you try to connect to your database from your local environment through a database client, you will be able to connect to it from 127.0.0.1:5433 as the port is exposed to your host through the docker-compose file. This is the reason why your schema update command succeeded.
Docker exposes connectivity information for the source container to
the recipient container in two ways:
Environment variables,
Updating the /etc/hosts file.
Ref: https://docs.docker.com/network/links/#communication-across-links
In order to connect to your database (which is running in the db container) from the php container, you will need to get the host of your db container through the environment variable DB_PORT_5432_TCP_ADDR (I might be wrong on this, but type env in your php container's terminal to verify. You will need to SSH into your php container).
Alternatively, you can use the second method, which is just db as the hostname instead of 127.0.0.1 since docker updated the /etc/hosts file in the php container to map your linked container's name to its IP, and in this case, the value mapped to the hostname db is the same as the value stored in the environment variable DB_PORT_5432_TCP_ADDR.