Connection refused: Is the server running on host "db" (172.21.0.2) and accepting web_1 TCP/IP connections on port 5432?

Question

I know that it is a typical issue, however I need a community help to resolve it. When I run docker-compose I get Connection refused: Is the server running on host "db" (172.21.0.2) and accepting web_1 TCP/IP connections on port 5432? Web portion of the docker-compose fails, but db runs. I can connect to the db using localhost:5432. It is not a wait issue, because tiangolo/uvicorn-gunicorn:python3.8-alpine3.10 implemented a wait mechanism - dockerize. Does anyone know where the issue is? Or perhaps could anyone just point me the right direction? So my Dockerfile is

FROM tiangolo/uvicorn-gunicorn:python3.8-alpine3.10

# copy requirements file
COPY ./requirements.txt /usr/src/app/requirements.txt

# install dependencies
RUN set -eux \
    && apk add --no-cache --virtual .build-deps build-base \
        libressl-dev libffi-dev gcc musl-dev python3-dev \
        postgresql-dev openssl \
    && pip install --upgrade pip setuptools wheel \
    && pip install -r /usr/src/app/requirements.txt \
    && rm -rf /root/.cache/pip

# copy project
COPY . /app

My docker-compose is:

version: '3.7'

services:
    db:
        image: postgres:13-alpine
        volumes:
            - postgres_data:/var/lib/postgresql/data/
        ports:
            - 5432:5432
        environment:
            - POSTGRES_USER=postgres
            - POSTGRES_PASSWORD=postgres
            - POSTGRES_DB=api

    web:
        build: ./src  
        ports:
            - 80:80
        depends_on:
            - db
        environment:
            - DATABASE_URL=postgresql://postgres:postgres@db/api

volumes:
    postgres_data:

I added a wait using dockerize, however the web portion fails:

Attaching to gunicorn_db_1, gunicorn_web_1
db_1   | The files belonging to this database system will be owned by user "postgres".
db_1   | This user must also own the server process.
db_1   |
db_1   | The database cluster will be initialized with locale "en_US.utf8".
db_1   | The default database encoding has accordingly been set to "UTF8".
db_1   | The default text search configuration will be set to "english".
db_1   |
db_1   | Data page checksums are disabled.
db_1   |
db_1   | fixing permissions on existing directory /var/lib/postgresql/data ... ok
db_1   | creating subdirectories ... ok
db_1   | selecting dynamic shared memory implementation ... posix
db_1   | selecting default max_connections ... 100
db_1   | selecting default shared_buffers ... 128MB
web_1  | 2020/08/30 15:33:51 Waiting for: tcp://db:5432
web_1  | 2020/08/30 15:33:51 Problem with dial: dial tcp 172.21.0.2:5432: connect: connection refused. Sleeping 30s
db_1   | selecting default time zone ... Etc/UTC
db_1   | creating configuration files ... ok
db_1   | running bootstrap script ... ok
db_1   | performing post-bootstrap initialization ... ok
db_1   | syncing data to disk ... initdb: warning: enabling "trust" authentication for local connections
db_1   | You can change this by editing pg_hba.conf or using the option -A, or
db_1   | --auth-local and --auth-host, the next time you run initdb.
db_1   | ok
db_1   |
db_1   |
db_1   | Success. You can now start the database server using:
db_1   |
db_1   |     pg_ctl -D /var/lib/postgresql/data -l logfile start
db_1   |
db_1   | waiting for server to start....2020-08-30 15:33:53.037 UTC [45] LOG:  starting PostgreSQL 13beta3 (Debian 13~beta3-1.pgdg100+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit
db_1   | 2020-08-30 15:33:53.044 UTC [45] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
db_1   | 2020-08-30 15:33:53.059 UTC [46] LOG:  database system was shut down at 2020-08-30 15:33:52 UTC
db_1   | 2020-08-30 15:33:53.066 UTC [45] LOG:  database system is ready to accept connections
db_1   |  done
db_1   | server started
db_1   | CREATE DATABASE
db_1   |
db_1   |
db_1   | /usr/local/bin/docker-entrypoint.sh: ignoring /docker-entrypoint-initdb.d/*
db_1   |
db_1   | 2020-08-30 15:33:53.399 UTC [45] LOG:  received fast shutdown request
db_1   | waiting for server to shut down....2020-08-30 15:33:53.404 UTC [45] LOG:  aborting any active transactions
db_1   | 2020-08-30 15:33:53.407 UTC [45] LOG:  background worker "logical replication launcher" (PID 52) exited with exit code 1
db_1   | 2020-08-30 15:33:53.409 UTC [47] LOG:  shutting down
db_1   | 2020-08-30 15:33:53.438 UTC [45] LOG:  database system is shut down
db_1   |  done
db_1   | server stopped
db_1   |
db_1   | PostgreSQL init process complete; ready for start up.
db_1   |
db_1   | 2020-08-30 15:33:53.530 UTC [1] LOG:  starting PostgreSQL 13beta3 (Debian 13~beta3-1.pgdg100+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit
db_1   | 2020-08-30 15:33:53.531 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
db_1   | 2020-08-30 15:33:53.531 UTC [1] LOG:  listening on IPv6 address "::", port 5432
db_1   | 2020-08-30 15:33:53.539 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
db_1   | 2020-08-30 15:33:53.549 UTC [63] LOG:  database system was shut down at 2020-08-30 15:33:53 UTC
db_1   | 2020-08-30 15:33:53.559 UTC [1] LOG:  database system is ready to accept connections
web_1  | 2020/08/30 15:34:21 Connected to tcp://db:5432
gunicorn_web_1 exited with code 0

Answer 1

Just change the Version of your Docker-Compose to version: "3.5" and that solved the problem

Answer 2

In first look your both files seemed good to me, actually adding a depends on should fix this but, according to documentation that doesn't works that way. it express dependency between containers but that does not mean that a container will wait to other to be ready.

For older versions of Compose we were able to add a health check like this

healthcheck:
  test: ["-U postgres"]
  interval: 3s
  timeout: 30s
  retries: 1

But since it's not supported anymore, best option to add a manual wait/sleep to that service.

There is a widely used bash script (wait for it) that you can use to test and wait on the availability of a TCP host and port. It's also recommended solution from the Compose Documentation. You can copy wait for it into your files and starting using it right away.

web:
    command: /wait-for-it.sh db:5432

Also you might want to check this out: Wait for it usage with docker